diff --git a/CHANGELOG.md b/CHANGELOG.md
index fe920851eb..603da63dff 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Fixed the visibility of the asset class, asset sub class, fee and quantity fields of a valuable in the create activity dialog
 - Fixed the missing mapping for Aland Islands in the country weightings of the _Financial Modeling Prep_ service
+- Fixed the net performance percentage of date ranges in the portfolio performance calculation by weighting the average investment by the number of days between the chart dates
 
 ## 3.80.2 - 2026-10-06
 
diff --git a/apps/api/src/app/portfolio/calculator/portfolio-calculator.ts b/apps/api/src/app/portfolio/calculator/portfolio-calculator.ts
index 9b94545fd8..1154f897d1 100644
--- a/apps/api/src/app/portfolio/calculator/portfolio-calculator.ts
+++ b/apps/api/src/app/portfolio/calculator/portfolio-calculator.ts
@@ -362,6 +362,18 @@ export abstract class PortfolioCalculator {
       return chartDate;
     });
 
+    // The chart dates are the same for each holding and each date range. Thus
+    // calculate the number of days until the next chart date one time only.
+    const parsedChartDates = chartDates.map((chartDate) => {
+      return parseDate(chartDate);
+    });
+
+    const daysUntilNextChartDate = parsedChartDates
+      .slice(1)
+      .map((nextChartDate, index) => {
+        return differenceInDays(nextChartDate, parsedChartDates[index]);
+      });
+
     const errors: ResponseError['errors'] = [];
     let hasAnyHoldingPerformanceErrors = false;
 
@@ -438,6 +450,7 @@ export abstract class PortfolioCalculator {
         totalLiabilitiesInBaseCurrency
       } = this.getHoldingPerformance({
         chartDates,
+        daysUntilNextChartDate,
         marketSymbolMap,
         dataSource: item.dataSource,
         end: this.endDate,
@@ -935,6 +948,7 @@ export abstract class PortfolioCalculator {
   protected abstract getHoldingPerformance({
     chartDates,
     dataSource,
+    daysUntilNextChartDate,
     end,
     exchangeRates,
     marketSymbolMap,
@@ -942,6 +956,7 @@ export abstract class PortfolioCalculator {
     symbol
   }: {
     chartDates: string[];
+    daysUntilNextChartDate: number[];
     end: Date;
     exchangeRates: { [dateString: string]: number };
     marketSymbolMap: {
diff --git a/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-btcusd-buy-and-sell-partially.spec.ts b/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-btcusd-buy-and-sell-partially.spec.ts
index 15474a0236..0110f68f27 100644
--- a/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-btcusd-buy-and-sell-partially.spec.ts
+++ b/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-btcusd-buy-and-sell-partially.spec.ts
@@ -112,7 +112,7 @@ describe('PortfolioCalculator', () => {
 
       const performanceByDateRange = await getPerformanceByDateRange({
         portfolioCalculator,
-        dateRanges: ['1d', 'max', 'ytd']
+        dateRanges: ['1d', '2017', 'max', 'ytd']
       });
 
       const investments = portfolioCalculator.getInvestments();
@@ -162,7 +162,8 @@ describe('PortfolioCalculator', () => {
             netPerformancePercentage: new Big('42.41978276196153750666'),
             netPerformancePercentageWithCurrencyEffectMap: {
               '1d': new Big('-0.04016229506406263535'),
-              max: new Big('41.72313811883729606471'),
+              '2017': new Big('43.14843430282283692638'),
+              max: new Big('41.65910103572163194783'),
               ytd: new Big('-0.04016229506406263535')
             },
             netPerformanceWithCurrencyEffectMap: {
@@ -258,11 +259,20 @@ describe('PortfolioCalculator', () => {
           totalInvestmentValueWithCurrencyEffect: 318.54266729999995,
           valueWithCurrencyEffect: 13298.425356
         },
+        '2017': {
+          date: '2017-12-31',
+          netPerformance: 26957.033439,
+          netPerformanceInPercentage: 44.109822737327406,
+          netPerformanceInPercentageWithCurrencyEffect: 43.14859823223876,
+          netPerformanceWithCurrencyEffect: 27081.23736,
+          totalInvestmentValueWithCurrencyEffect: 318.54266729999995,
+          valueWithCurrencyEffect: 13854.86868
+        },
         max: {
           date: '2018-01-01',
           netPerformance: 26458.9121202,
-          netPerformanceInPercentage: 42.50435329547954,
-          netPerformanceInPercentageWithCurrencyEffect: 41.72313811883715,
+          netPerformanceInPercentage: 42.43911719562077,
+          netPerformanceInPercentageWithCurrencyEffect: 41.65910103572173,
           netPerformanceWithCurrencyEffect: 26516.2087014,
           totalInvestmentValueWithCurrencyEffect: 318.54266729999995,
           valueWithCurrencyEffect: 13298.425356
@@ -278,5 +288,66 @@ describe('PortfolioCalculator', () => {
         }
       });
     });
+
+    it.only('with BTCUSD buy and sell partially and fewer chart items', async () => {
+      jest.useFakeTimers().setSystemTime(parseDate('2018-01-01').getTime());
+
+      const environmentConfigurationService = new ConfigurationService();
+
+      // Fewer chart items give fewer chart dates, which must not change the
+      // average investment. The range 2017 starts with a gross performance
+      // and ends with the sell.
+      jest.spyOn(configurationService, 'get').mockImplementation((key) => {
+        return key === 'MAX_CHART_ITEMS'
+          ? 50
+          : environmentConfigurationService.get(key);
+      });
+
+      const { activities, userCurrency } = loadActivitiesFromExportFile(
+        'btcusd-buy-and-sell-partially.json'
+      );
+
+      const portfolioCalculator = portfolioCalculatorFactory.createCalculator({
+        activities,
+        calculationType: PerformanceCalculationType.ROAI,
+        currency: userCurrency,
+        usePortfolioSnapshotCache: false,
+        userId: userDummyData.id
+      });
+
+      const portfolioSnapshot = await portfolioCalculator.computeSnapshot();
+
+      const performanceByDateRange = await getPerformanceByDateRange({
+        portfolioCalculator,
+        dateRanges: ['2017', 'max']
+      });
+
+      expect(
+        portfolioSnapshot.positions[0]
+          .netPerformancePercentageWithCurrencyEffectMap
+      ).toMatchObject({
+        '2017': new Big('43.14843430282283692638'),
+        max: new Big('41.65910103572163194783')
+      });
+
+      // Other weights give another rounding of the floating point numbers
+      expect(
+        performanceByDateRange['2017'].netPerformanceInPercentage
+      ).toBeCloseTo(44.109822737327406, 10);
+
+      expect(
+        performanceByDateRange['2017']
+          .netPerformanceInPercentageWithCurrencyEffect
+      ).toBeCloseTo(43.14859823223876, 10);
+
+      expect(performanceByDateRange.max.netPerformanceInPercentage).toBeCloseTo(
+        42.43911719562077,
+        10
+      );
+
+      expect(
+        performanceByDateRange.max.netPerformanceInPercentageWithCurrencyEffect
+      ).toBeCloseTo(41.65910103572173, 10);
+    });
   });
 });
diff --git a/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-googl-fee-and-dividend.spec.ts b/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-googl-fee-and-dividend.spec.ts
index 18929a680c..bb6385d18d 100644
--- a/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-googl-fee-and-dividend.spec.ts
+++ b/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-googl-fee-and-dividend.spec.ts
@@ -190,9 +190,12 @@ describe('PortfolioCalculator', () => {
       expect(performanceByDateRange).toMatchObject({
         max: {
           dividendInBaseCurrency: 5.62,
-          dividendInPercentageWithCurrencyEffect: 0.01882242615044535,
+          dividendInPercentageWithCurrencyEffect: expect.closeTo(
+            0.01882242615044535,
+            10
+          ),
           netPerformance: 18.87,
-          netPerformanceInPercentage: 0.06319914260834586
+          netPerformanceInPercentage: expect.closeTo(0.06319914260834586, 10)
         }
       });
     });
@@ -282,9 +285,12 @@ describe('PortfolioCalculator', () => {
       expect(performanceByDateRange).toMatchObject({
         max: {
           dividendInBaseCurrency: 5.62,
-          dividendInPercentageWithCurrencyEffect: 0.01882242615044535,
+          dividendInPercentageWithCurrencyEffect: expect.closeTo(
+            0.01882242615044535,
+            10
+          ),
           netPerformance: 19.87,
-          netPerformanceInPercentage: 0.06654832875611194
+          netPerformanceInPercentage: expect.closeTo(0.06654832875611194, 10)
         }
       });
     });
@@ -373,9 +379,12 @@ describe('PortfolioCalculator', () => {
       expect(performanceByDateRange).toMatchObject({
         max: {
           dividendInBaseCurrency: 0.62,
-          dividendInPercentageWithCurrencyEffect: 0.002076495411614967,
+          dividendInPercentageWithCurrencyEffect: expect.closeTo(
+            0.002076495411614967,
+            10
+          ),
           netPerformance: 14.87,
-          netPerformanceInPercentage: 0.049802398017281556
+          netPerformanceInPercentage: expect.closeTo(0.049802398017281556, 10)
         }
       });
     });
@@ -458,11 +467,17 @@ describe('PortfolioCalculator', () => {
         },
         max: {
           dividendInBaseCurrency: 1.3,
-          dividendInPercentageWithCurrencyEffect: 0.004353941992095899
+          dividendInPercentageWithCurrencyEffect: expect.closeTo(
+            0.004353941992095899,
+            10
+          )
         },
         ytd: {
           dividendInBaseCurrency: 0.68,
-          dividendInPercentageWithCurrencyEffect: 0.002002886512915673
+          dividendInPercentageWithCurrencyEffect: expect.closeTo(
+            0.002002886512915673,
+            10
+          )
         }
       });
     });
diff --git a/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-msft-buy-with-dividend.spec.ts b/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-msft-buy-with-dividend.spec.ts
index 0fdd0b3000..2b89ddd31b 100644
--- a/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-msft-buy-with-dividend.spec.ts
+++ b/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator-msft-buy-with-dividend.spec.ts
@@ -177,10 +177,16 @@ describe('PortfolioCalculator', () => {
         max: {
           date: '2023-07-10',
           dividendInBaseCurrency: 0.62,
-          dividendInPercentageWithCurrencyEffect: 0.002076495411614967,
+          dividendInPercentageWithCurrencyEffect: expect.closeTo(
+            0.002076495411614967,
+            10
+          ),
           netPerformance: 14.87,
-          netPerformanceInPercentage: 0.049802398017281556,
-          netPerformanceInPercentageWithCurrencyEffect: 0.049802398017281556,
+          netPerformanceInPercentage: expect.closeTo(0.049802398017281556, 10),
+          netPerformanceInPercentageWithCurrencyEffect: expect.closeTo(
+            0.049802398017281556,
+            10
+          ),
           netPerformanceWithCurrencyEffect: 14.87,
           totalInvestmentValueWithCurrencyEffect: 298.58,
           valueWithCurrencyEffect: 331.83
@@ -190,8 +196,11 @@ describe('PortfolioCalculator', () => {
           dividendInBaseCurrency: 0,
           dividendInPercentageWithCurrencyEffect: 0,
           netPerformance: -7.6800000000000015,
-          netPerformanceInPercentage: -0.022620835910577015,
-          netPerformanceInPercentageWithCurrencyEffect: -0.022620835910577015,
+          netPerformanceInPercentage: expect.closeTo(-0.022620835910577015, 10),
+          netPerformanceInPercentageWithCurrencyEffect: expect.closeTo(
+            -0.022620835910577015,
+            10
+          ),
           netPerformanceWithCurrencyEffect: -7.6800000000000015,
           totalInvestmentValueWithCurrencyEffect: 298.58,
           valueWithCurrencyEffect: 331.83
diff --git a/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator.spec.ts b/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator.spec.ts
index 99bee2c21d..5f8d9cdc44 100644
--- a/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator.spec.ts
+++ b/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator.spec.ts
@@ -1,3 +1,186 @@
+import { PortfolioCalculator } from '@ghostfolio/api/app/portfolio/calculator/portfolio-calculator';
+import { userDummyData } from '@ghostfolio/api/app/portfolio/calculator/portfolio-calculator-test-utils';
+import { PortfolioCalculatorFactory } from '@ghostfolio/api/app/portfolio/calculator/portfolio-calculator.factory';
+import { CurrentRateService } from '@ghostfolio/api/app/portfolio/current-rate.service';
+import { CurrentRateServiceMock } from '@ghostfolio/api/app/portfolio/current-rate.service.mock';
+import { RedisCacheService } from '@ghostfolio/api/app/redis-cache/redis-cache.service';
+import { RedisCacheServiceMock } from '@ghostfolio/api/app/redis-cache/redis-cache.service.mock';
+import { ConfigurationService } from '@ghostfolio/api/services/configuration/configuration.service';
+import { ExchangeRateDataService } from '@ghostfolio/api/services/exchange-rate-data/exchange-rate-data.service';
+import { PortfolioSnapshotService } from '@ghostfolio/api/services/queues/portfolio-snapshot/portfolio-snapshot.service';
+import { PortfolioSnapshotServiceMock } from '@ghostfolio/api/services/queues/portfolio-snapshot/portfolio-snapshot.service.mock';
+import { HistoricalDataItem } from '@ghostfolio/common/interfaces';
+import { PerformanceCalculationType } from '@ghostfolio/common/types/performance-calculation-type.type';
+
+jest.mock('@ghostfolio/api/app/portfolio/current-rate.service', () => {
+  return {
+    CurrentRateService: jest.fn().mockImplementation(() => {
+      return CurrentRateServiceMock;
+    })
+  };
+});
+
+jest.mock(
+  '@ghostfolio/api/services/queues/portfolio-snapshot/portfolio-snapshot.service',
+  () => {
+    return {
+      PortfolioSnapshotService: jest.fn().mockImplementation(() => {
+        return PortfolioSnapshotServiceMock;
+      })
+    };
+  }
+);
+
+jest.mock('@ghostfolio/api/app/redis-cache/redis-cache.service', () => {
+  return {
+    RedisCacheService: jest.fn().mockImplementation(() => {
+      return RedisCacheServiceMock;
+    })
+  };
+});
+
+function getHistoricalDataItem({
+  date,
+  investment,
+  netPerformance
+}: {
+  date: string;
+  investment: number;
+  netPerformance: number;
+}): HistoricalDataItem {
+  return {
+    date,
+    netPerformance,
+    dividendInBaseCurrency: 0,
+    netPerformanceWithCurrencyEffect: netPerformance,
+    totalInvestment: investment,
+    totalInvestmentValueWithCurrencyEffect: investment,
+    value: investment + netPerformance,
+    valueWithCurrencyEffect: investment + netPerformance
+  };
+}
+
 describe('PortfolioCalculator', () => {
-  test.skip('Skip empty test', () => 1);
+  let configurationService: ConfigurationService;
+  let currentRateService: CurrentRateService;
+  let exchangeRateDataService: ExchangeRateDataService;
+  let portfolioCalculator: PortfolioCalculator;
+  let portfolioCalculatorFactory: PortfolioCalculatorFactory;
+  let portfolioSnapshotService: PortfolioSnapshotService;
+  let redisCacheService: RedisCacheService;
+
+  beforeEach(() => {
+    PortfolioSnapshotServiceMock.reset();
+    RedisCacheServiceMock.reset();
+
+    configurationService = new ConfigurationService();
+
+    currentRateService = new CurrentRateService(null, null, null);
+
+    exchangeRateDataService = new ExchangeRateDataService(
+      null,
+      null,
+      null,
+      null
+    );
+
+    portfolioSnapshotService = new PortfolioSnapshotService(null, null);
+
+    redisCacheService = new RedisCacheService(null, null);
+
+    portfolioCalculatorFactory = new PortfolioCalculatorFactory(
+      configurationService,
+      currentRateService,
+      exchangeRateDataService,
+      portfolioSnapshotService,
+      redisCacheService
+    );
+
+    portfolioCalculator = portfolioCalculatorFactory.createCalculator({
+      activities: [],
+      calculationType: PerformanceCalculationType.ROAI,
+      currency: 'USD',
+      userId: userDummyData.id
+    });
+  });
+
+  describe('calculate performance percentages for date range', () => {
+    it('with a buy followed by a gap between chart dates', () => {
+      const performancePercentagesByDate = portfolioCalculator[
+        'calculatePerformancePercentagesForDateRange'
+      ]({
+        historicalDataItems: [
+          getHistoricalDataItem({
+            date: '2017-01-01',
+            investment: 100,
+            netPerformance: 0
+          }),
+          // Buy, followed by a gap of 21 days until the next chart date
+          getHistoricalDataItem({
+            date: '2017-01-02',
+            investment: 200,
+            netPerformance: 0
+          }),
+          // Sell
+          getHistoricalDataItem({
+            date: '2017-01-23',
+            investment: 100,
+            netPerformance: 44
+          })
+        ]
+      });
+
+      // The investment of each chart date applies to each day until the next
+      // chart date. The investment of the last chart date counts for this day
+      // only: (1 * 100 + 21 * 200 + 1 * 100) / 23 = 4400 / 23
+      expect(
+        performancePercentagesByDate['2017-01-23'].netPerformanceInPercentage
+      ).toBeCloseTo(0.23, 10);
+
+      expect(
+        performancePercentagesByDate['2017-01-23']
+          .netPerformanceInPercentageWithCurrencyEffect
+      ).toBeCloseTo(0.23, 10);
+    });
+
+    it('with an additional chart date of the same investment', () => {
+      const performancePercentagesByDate = portfolioCalculator[
+        'calculatePerformancePercentagesForDateRange'
+      ]({
+        historicalDataItems: [
+          getHistoricalDataItem({
+            date: '2017-01-01',
+            investment: 100,
+            netPerformance: 0
+          }),
+          getHistoricalDataItem({
+            date: '2017-01-02',
+            investment: 200,
+            netPerformance: 0
+          }),
+          // An additional chart date splits the gap but must not change the
+          // average investment
+          getHistoricalDataItem({
+            date: '2017-01-12',
+            investment: 200,
+            netPerformance: 20
+          }),
+          getHistoricalDataItem({
+            date: '2017-01-23',
+            investment: 100,
+            netPerformance: 44
+          })
+        ]
+      });
+
+      expect(
+        performancePercentagesByDate['2017-01-23'].netPerformanceInPercentage
+      ).toBeCloseTo(0.23, 10);
+
+      expect(
+        performancePercentagesByDate['2017-01-23']
+          .netPerformanceInPercentageWithCurrencyEffect
+      ).toBeCloseTo(0.23, 10);
+    });
+  });
 });
diff --git a/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator.ts b/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator.ts
index a1e41b53a2..0115b4a57c 100644
--- a/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator.ts
+++ b/apps/api/src/app/portfolio/calculator/roai/portfolio-calculator.ts
@@ -29,7 +29,6 @@ import {
   isBefore,
   isThisYear
 } from 'date-fns';
-import { sum } from 'lodash-es';
 
 export class RoaiPortfolioCalculator extends PortfolioCalculator {
   protected calculateOverallPerformance(
@@ -210,16 +209,21 @@ export class RoaiPortfolioCalculator extends PortfolioCalculator {
   }: {
     historicalDataItems: HistoricalDataItem[];
   }): { [date: string]: PerformancePercentages } {
-    const averageInvestmentValues: number[] = [];
-    const averageInvestmentValuesWithCurrencyEffect: number[] = [];
     let grossPerformanceAtStartDate: number;
     let grossPerformanceWithCurrencyEffectAtStartDate: number;
+    let previousDate: Date;
+    let sumOfWeightedInvestments = 0;
+    let sumOfWeightedInvestmentsWithCurrencyEffect = 0;
+    let totalInvestmentDays = 0;
+    let totalInvestmentDaysWithCurrencyEffect = 0;
 
     const performancePercentagesByDate: {
       [date: string]: PerformancePercentages;
     } = {};
 
     for (const [index, historicalDataItem] of historicalDataItems.entries()) {
+      const date = parseDate(historicalDataItem.date);
+
       // Take the values at the start date from the first day of the date
       // range
       if (index === 0) {
@@ -229,34 +233,53 @@ export class RoaiPortfolioCalculator extends PortfolioCalculator {
         grossPerformanceWithCurrencyEffectAtStartDate =
           historicalDataItem.valueWithCurrencyEffect -
           historicalDataItem.totalInvestmentValueWithCurrencyEffect;
+      } else {
+        // The chart dates are not evenly spaced, and the investment changes on
+        // a chart date only. Thus the investment of the previous chart date
+        // applies to each day until this chart date.
+        const previousHistoricalDataItem = historicalDataItems[index - 1];
+
+        const daysSincePreviousChartDate = differenceInDays(date, previousDate);
+
+        if (previousHistoricalDataItem.totalInvestment > 0) {
+          sumOfWeightedInvestments +=
+            (previousHistoricalDataItem.totalInvestment +
+              grossPerformanceAtStartDate) *
+            daysSincePreviousChartDate;
+
+          totalInvestmentDays += daysSincePreviousChartDate;
+        }
+
+        if (
+          previousHistoricalDataItem.totalInvestmentValueWithCurrencyEffect > 0
+        ) {
+          sumOfWeightedInvestmentsWithCurrencyEffect +=
+            (previousHistoricalDataItem.totalInvestmentValueWithCurrencyEffect +
+              grossPerformanceWithCurrencyEffectAtStartDate) *
+            daysSincePreviousChartDate;
+
+          totalInvestmentDaysWithCurrencyEffect += daysSincePreviousChartDate;
+        }
       }
 
       // Add the gross performance at the start date of the range to the
       // investment of each day. Thus the range starts with the value of its
       // first day, and subsequent buy and sell activities stay included.
-      if (historicalDataItem.totalInvestment > 0) {
-        averageInvestmentValues.push(
-          historicalDataItem.totalInvestment + grossPerformanceAtStartDate
-        );
-      }
-
-      if (historicalDataItem.totalInvestmentValueWithCurrencyEffect > 0) {
-        averageInvestmentValuesWithCurrencyEffect.push(
-          historicalDataItem.totalInvestmentValueWithCurrencyEffect +
-            grossPerformanceWithCurrencyEffectAtStartDate
-        );
-      }
-
-      const averageInvestmentValue =
-        averageInvestmentValues.length > 0
-          ? sum(averageInvestmentValues) / averageInvestmentValues.length
-          : 0;
+      const averageInvestmentValue = this.getAverageInvestment({
+        grossPerformanceAtStartDate,
+        sumOfWeightedInvestments,
+        totalInvestmentDays,
+        investment: historicalDataItem.totalInvestment
+      });
 
       const averageInvestmentValueWithCurrencyEffect =
-        averageInvestmentValuesWithCurrencyEffect.length > 0
-          ? sum(averageInvestmentValuesWithCurrencyEffect) /
-            averageInvestmentValuesWithCurrencyEffect.length
-          : 0;
+        this.getAverageInvestment({
+          grossPerformanceAtStartDate:
+            grossPerformanceWithCurrencyEffectAtStartDate,
+          investment: historicalDataItem.totalInvestmentValueWithCurrencyEffect,
+          sumOfWeightedInvestments: sumOfWeightedInvestmentsWithCurrencyEffect,
+          totalInvestmentDays: totalInvestmentDaysWithCurrencyEffect
+        });
 
       performancePercentagesByDate[historicalDataItem.date] = {
         dividendInPercentageWithCurrencyEffect:
@@ -274,6 +297,8 @@ export class RoaiPortfolioCalculator extends PortfolioCalculator {
               averageInvestmentValueWithCurrencyEffect
             : 0
       };
+
+      previousDate = date;
     }
 
     return performancePercentagesByDate;
@@ -282,6 +307,7 @@ export class RoaiPortfolioCalculator extends PortfolioCalculator {
   protected getHoldingPerformance({
     chartDates,
     dataSource,
+    daysUntilNextChartDate,
     end,
     exchangeRates,
     marketSymbolMap,
@@ -289,6 +315,7 @@ export class RoaiPortfolioCalculator extends PortfolioCalculator {
     symbol
   }: {
     chartDates: string[];
+    daysUntilNextChartDate: number[];
     end: Date;
     exchangeRates: { [dateString: string]: number };
     marketSymbolMap: {
@@ -642,17 +669,28 @@ export class RoaiPortfolioCalculator extends PortfolioCalculator {
           break;
         }
 
+        const isLastChartDateOfRange =
+          i === chartDates.length - 1 || chartDates[i + 1] > rangeEndDateString;
+
+        // The chart dates are not evenly spaced, and the investment changes
+        // on a chart date only. Thus the investment of a chart date applies to
+        // each day until the next chart date. The investment of the last chart
+        // date of the range counts for this day only.
+        const investmentDays = isLastChartDateOfRange
+          ? 1
+          : daysUntilNextChartDate[i];
+
         if (
           investmentValuesAccumulatedWithCurrencyEffect[date] instanceof Big &&
           investmentValuesAccumulatedWithCurrencyEffect[date].gt(0)
         ) {
           average = average.add(
-            investmentValuesAccumulatedWithCurrencyEffect[date].add(
-              grossPerformanceAtDateRangeStartWithCurrencyEffect
-            )
+            investmentValuesAccumulatedWithCurrencyEffect[date]
+              .add(grossPerformanceAtDateRangeStartWithCurrencyEffect)
+              .mul(investmentDays)
           );
 
-          dayCount++;
+          dayCount += investmentDays;
         }
       }
 
@@ -753,4 +791,29 @@ export class RoaiPortfolioCalculator extends PortfolioCalculator {
   protected getPerformanceCalculationType() {
     return PerformanceCalculationType.ROAI;
   }
+
+  private getAverageInvestment({
+    grossPerformanceAtStartDate,
+    investment,
+    sumOfWeightedInvestments,
+    totalInvestmentDays
+  }: {
+    grossPerformanceAtStartDate: number;
+    investment: number;
+    sumOfWeightedInvestments: number;
+    totalInvestmentDays: number;
+  }) {
+    // The investment of the current chart date counts for this day only
+    if (investment > 0) {
+      return (
+        (sumOfWeightedInvestments +
+          (investment + grossPerformanceAtStartDate)) /
+        (totalInvestmentDays + 1)
+      );
+    }
+
+    return totalInvestmentDays > 0
+      ? sumOfWeightedInvestments / totalInvestmentDays
+      : 0;
+  }
 }