Simple interest calculator with steps
Calculate interest on a fixed principal with one annual percentage rate. Choose a term in years, months or days, or calculate between two dates. The result shows the total simple interest, original principal, final amount and the year fraction used in the formula.
Simple interest does not add earned interest to the balance for later interest calculations. This tool assumes the principal stays unchanged throughout the selected term. It is useful for classroom problems, fixed-balance scenarios and checking a single accrual interval. A repayment loan with a changing balance needs a schedule, such as the existing Loan Calculator.
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For optional arithmetic checks away from the browser, browse financial calculators (paid link). Check the model’s supported functions; no purchase is needed.
How to calculate simple interest
- Enter the principal in USD, GBP, CAD, AUD or EUR. Currency labels identify amounts; they do not change the formula or perform a currency conversion.
- Enter the annual simple interest rate as a percentage, such as 5 for 5% per year.
- Select a term and unit, or choose two dates.
- For days or dates, choose Actual/365 Fixed or Actual/360 to match the assumption you intend to test.
- Calculate, then inspect the interest, final amount and steps before copying, downloading or printing.
This simple interest calculator uses months ÷ 12 for a monthly term and actual elapsed days ÷ the chosen denominator for a daily term. Enter an annual rate even when the term is monthly or daily. A monthly rate of 1% is not the same input as an annual rate of 1%.
Simple interest formula: I = P × r × t
A = P + I
P is the fixed principal, r is the annual rate divided by 100, t is the term measured in years, I is total interest and A is the principal plus interest. For example, USD 1,000 at 5% per year for 3 years earns USD 150 in simple interest, giving a final amount of USD 1,150.
To calculate simple interest monthly, use t = months ÷ 12. For USD 2,000 at 6% annually over 9 months, I = 2,000 × 0.06 × 9/12 = USD 90. The final amount is USD 2,090. This is interest accumulated over the term, not a monthly installment or an EMI.
Optional consumer math workbooks (paid link) can help practice percentage conversions and the distinction between simple and compound growth. For a growing balance that earns interest on prior interest, use the Compound Interest Calculator.
Simple interest between two dates and in days
Date mode counts actual elapsed calendar days. The start date is excluded and the end date is included: January 1 to January 2 is one day. Dates are evaluated at UTC midnight so daylight-saving changes do not create a 23-hour or 25-hour calculation day. An end date before the start date is rejected.
Actual/365 Fixed divides elapsed days by 365, including in a leap year. Actual/360 divides actual days by 360. This tool does not implement Actual/Actual, 30/360 or business-day adjustments. A 360 denominator does not mean each calendar month is treated as 30 days.
From January 1, 2024 to January 1, 2025 there are 366 elapsed days. At USD 1,000 and 5% per year, Actual/365 Fixed produces about USD 50.14; Actual/360 produces about USD 50.83. Both follow their stated convention. A term entered as exactly 1 year instead produces USD 50.00 because its year fraction is exactly 1.
Optional calculation notebooks (paid link) can help record both dates and the denominator so repeated checks use the same assumptions. The existing Date Difference Calculator is useful when you only need the calendar interval.
Assumptions, rounding and limits
The principal must be greater than zero and no more than 1 trillion currency units, with up to two decimal places. Annual rates range from 0% to 1,000%, with up to eight decimal places. Zero rates and zero-length terms return zero interest. Year terms support up to 200 years; month terms up to 2,400 months; day or date spans up to 73,050 days. Dates must be real calendar dates within 1900–2200. Use plain numbers without currency symbols, commas or scientific notation.
Decimal and time fractions use exact rational arithmetic before results are rounded half-up to two decimal places. Intermediate equation values may be shortened for display; the calculation keeps the full fraction. Fees, taxes, withholding, late charges, changing rates, contributions, partial repayments and compounding are excluded.
Some lenders call daily interest on a changing unpaid balance “simple interest.” This fixed-principal calculator covers one unchanged-balance interval; it does not reproduce a sequence of payments or a payoff quote. Keep the agreement and statements that define your rate, balance and timing. Optional document organizers (paid link) can help organize those records.
For scheduled repayments, use the Amortization Calculator. For historical purchasing-power comparisons, use the Inflation Calculator.
Simple Interest Calculator FAQs
Sources and methodology
The formula is a fixed-principal educational model. Day-count conventions are explicit implementation choices; they should be matched to the agreement you are checking. Page reviewed October 20, 2026.