InterestExtensions 3.0.0

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

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.