InterestExtensions

Small, dependency-free extension methods for simple and compound interest calculations.

Installation

dotnet add package InterestExtensions

Compatibility

Targets .NET Standard 2.0 and .NET 8. Tested on .NET 8 and .NET 10.

Calculation contract

• principal is a non-negative decimal amount.
• interestRate is a non-negative nominal annual rate expressed as a fraction: use 0.05m for 5% per year.
• period is a non-negative number of whole years.
• compoundingPeriodCount is the total number of times interest is applied.
• compoundingPeriodsPerYear is a positive frequency used to derive the periodic rate.
• Yearly, Monthly, and Daily compound 1, 12, and 365 times per year respectively.
• Daily compounding uses a fixed 365-day year; it is not a date-based day-count convention.
• Results are returned without implicit rounding. Choose the scale and midpoint rule required by your currency and domain.
• Invalid inputs throw ArgumentOutOfRangeException; calculations outside the range of decimal throw OverflowException.

The formulas are:

simple total   = principal × (1 + annual rate × years)
periodic rate  = annual rate / periods per year
compound total = principal × (1 + periodic rate)^period count

Usage

using InterestExtension;
using InterestExtension.Enums;

decimal principal = 1_000m;
decimal annualRate = 0.05m; // 5% per year
int years = 3;

decimal simpleTotal = principal.CalculateSimpleInterest(annualRate, years);
decimal simpleInterest = principal.CalculateSimpleInterestAmount(annualRate, years);

decimal annualTotal = principal.CalculateCompoundInterest(annualRate, years);
decimal annualInterest = principal.CalculateCompoundInterestAmount(annualRate, years);

decimal monthlyTotal = principal.CalculateCompoundInterest(
    annualRate,
    years,
    InterestPeriod.Monthly);

// 18 monthly compounding periods, equivalent to 18 months.
decimal eighteenMonthTotal = principal.CalculateCompoundInterestForPeriods(
    annualRate,
    compoundingPeriodCount: 18,
    compoundingPeriodsPerYear: 12);

decimal eighteenMonthInterest = principal.CalculateCompoundInterestAmountForPeriods(
    annualRate,
    compoundingPeriodCount: 18,
    compoundingPeriodsPerYear: 12);

// InterestExtensions does not choose a financial rounding policy for you.
decimal displayAmount = decimal.Round(monthlyTotal, 2, MidpointRounding.ToEven);

API overview

Method	Result
CalculateSimpleInterest	Principal plus simple interest
CalculateSimpleInterestAmount	Simple interest only
CalculateCompoundInterest	Principal plus compound interest for whole years
CalculateCompoundInterestAmount	Compound interest only for whole years
CalculateCompoundInterestForPeriods	Principal plus compound interest for a custom period count and frequency
CalculateCompoundInterestAmountForPeriods	Compound interest only for a custom period count and frequency

Scope

InterestExtensions provides deterministic simple and compound interest calculations using decimal. Date-based calculations and product-specific financial rules are intentionally out of scope.

Contributing

Contributions are welcome. See the contribution guide.