Simple Interest Calculator

How to use the Simple Interest Calculator

What this calculator does

This calculator applies the simple interest identity I = P × r × t and the ending-balance identity A = P + I. It can solve for the ending balance, original principal, term, or simple interest rate when the other three values are known. It is useful for a fixed-principal estimate in which interest does not compound and no deposits, withdrawals, fees, taxes, or repayments change the principal during the term. It does not calculate an amortized payment plan or an account whose interest is added back to the balance. OpenStax’s simple interest formula and examples explain the same principal-rate-time relationship and show how months or days can be expressed as fractions of a year.

When to use it

Use this tool to estimate a fixed-term deposit that explicitly pays simple interest, compare a quoted flat-rate borrowing scenario, back-solve the starting principal needed to reach a target amount, or determine the rate or term implied by a known principal and ending balance. For real auto loans, the Consumer Financial Protection Bureau notes that common “simple interest” contracts usually calculate interest on the changing outstanding balance rather than holding principal fixed, so review the contract before treating this fixed-principal model as a payoff schedule.

How to calculate

1. Under Calculate, choose Balance, Principal, Term, or Rate. The selected value becomes read-only and is solved from the other inputs.
2. Enter the required money values in End balance and Principal. U.S. dollar signs and correctly grouped commas are accepted.
3. Enter Interest rate as a percentage, then choose per year, per month, per week, or per day in Interest rate period.
4. Enter Term, then choose years, months, weeks, or days in Term unit. The calculator converts the two time bases before applying the formula.
5. Read the live result, breakdown, charts, and schedule. Select Reset to restore the default $20,000 principal, 3% annual rate, and 10-year term.
6. Select Download Excel to create a validated workbook containing the current assumptions, results, and complete schedule.

Input guide

Calculate is a required four-option control. Choose Balance to find the ending amount, Principal to find the starting amount, Term to find elapsed time, or Rate to find the periodic percentage. A common mistake is trying to edit the selected target; switch to a different target first.

End balance is a nonnegative U.S. dollar amount, required unless Balance is selected. A realistic example is $26,000.00. Raising it increases the solved principal, rate, or term, depending on the selected target. When solving Rate or Term with a nonnegative rate, the end balance cannot be below principal.

Principal is the original fixed amount, required unless Principal is selected. A realistic example is $20,000.00. A higher principal raises total interest and ending balance when rate and term are fixed. Do not use a changing loan balance here; this model assumes the principal remains unchanged throughout the calculation.

Interest rate is a nonnegative percentage, required unless Rate is selected. A realistic example is 3.00%. The adjacent Interest rate period is also required and defines whether that percentage applies per year, month, week, or day. For example, 3% per month is not the same as 3% per year; it annualizes to 36% under this calculator’s simple frequency conversion. Do not enter 0.03 when you mean 3%.

Term is a nonnegative number, required unless Term is selected, with up to 1,000 selected periods supported in the detailed schedule. A realistic example is 10. The adjacent Term unit defines years, months, weeks, or days. Longer terms increase interest linearly. When solving Term, the principal and rate must be above zero; when solving Rate, principal and term must be above zero.

Output guide

Primary result displays the selected target as currency, a percentage with its rate period, or a number with its term unit. End balance is principal plus total simple interest. Total interest is the exact model identity A − P and is driven by principal, converted rate, and converted term. Zero interest means the rate, term, or principal is zero. Interest per term period is the constant amount for one selected term unit; a partial final period is prorated. Annualized simple rate converts the selected periodic rate to an annual simple-rate equivalent without compounding.

The summary pills repeat Principal, Rate, Term, and Interest from the same model. In Balance accumulation, the Principal series stays flat and the Balance series rises linearly; the chart is replaced with a compact message when fewer than three schedule points or no meaningful change exists. In Ending balance composition, Principal and Total interest are the two parts of the final amount; a zero-interest scenario is shown as a single-value summary instead of a misleading one-slice chart.

The Schedule columns are Period, the row number; Elapsed, the cumulative time in the selected term unit; Interest, the amount generated during that row’s period; and Balance, principal plus cumulative interest through that row. The final schedule balance must equal th e displayed end balance.

Worked example

With Calculate set to Balance, Principal at $20,000.00, Interest rate at 3.00% per year, and Term at 10 years, the decimal rate is 0.03 and time is 10 years. Total interest is $20,000 × 0.03 × 10 = $6,000.00. The end balance is $20,000 + $6,000 = $26,000.00. Each yearly schedule row adds $600.00, so Year 1 ends at $20,600.00 and Year 10 ends at $26,000.00. These values match the live cards, charts, final schedule row, and downloaded workbook.

Learn more

Simple interest differs from compound interest. The SEC’s Investor.gov glossary defines compound interest as interest paid on principal and accumulated interest. That distinction matters because this calculator deliberately keeps principal fixed. For a practical lending contrast, read the CFPB’s explanation of simple-interest and precomputed-interest auto loans, which explains how actual loan interest can depend on the outstanding balance and payment timing.