Calculate flat simple interest in seconds, or switch to a declining-balance loan payoff to see monthly payment, total interest, extra-payment savings, and the full schedule.
Two different models, clearly separated. Flat simple interest keeps the original principal constant for the whole term. Loan payoff recalculates interest on the remaining principal after each monthly payment.
Use I = P × r × t. Pick the value you want to solve, then enter the other three.
P × r × t
Model a loan where each month’s interest is charged on the remaining principal. This is different from applying P × r × t to the original balance for the whole term.
Planning model: monthly interest = remaining principal × APR ÷ 12. Actual simple-interest loans may accrue daily based on exact payment dates, late payments, lender rules, and fees. Verify the contract before relying on the estimate.
Enter a loan scenario.
See whether the extra monthly amount materially changes payoff time and interest.
| Month | Payment | Principal | Interest | Balance |
|---|---|---|---|---|
| No payoff schedule yet. | ||||
Keep up to six quick what-if scenarios without an account.
Ready.
Use Flat simple interest when an agreement or classroom problem explicitly uses I = P × r × t on the original principal. Use Loan payoff when payments reduce the balance and interest is charged on what remains. That distinction matters more than adding decorative options.
With a declining balance, each payment reduces principal, so later interest charges are calculated on a smaller amount. Extra principal accelerates that decline. Actual lenders may use daily accrual and exact payment dates, so treat this as a planning estimate and check the contract method.
connects debt payoff, credit-card cost, loan comparison, APR/APY, effective-rate, weighted-rate, and hypothetical consolidation calculations.
Results are deterministic browser-local math from the values you enter. They are useful for scenario comparison, but they are not a lender quote, approval decision, investment forecast, credit recommendation, or substitute for an account agreement.
Use the flat tab for P × r × t, or switch to the payoff tab when payments reduce principal over time. Keep the loan model explicit before comparing results.
Use the tool first, then apply these checks to verify inputs, interpret the result, and hand it off without displacing the primary workflow.
The formula I = P r t applies interest to the original principal over the stated time and does not compound interest on interest. Many installment loans accrue interest on a declining balance and many savings products compound periodically, so their total cost or return will differ. Use this workflow only when simple interest is the intended model or when you are explicitly building a simplified comparison scenario.
An annual percentage rate entered as r must be paired with time expressed in years. Months can be converted to years by dividing by twelve; day-based calculations require a stated day-count basis such as 365 or 360. Do not mix a monthly rate with a year count or silently assume the same day basis as a contract. Record the chosen time unit and basis with the result.
Solving for principal, rate, or time rearranges the same simple-interest equation. A zero principal cannot support a finite rate solution, a zero rate cannot support a finite time solution for positive interest, and negative or contradictory inputs should be reviewed rather than forced through the formula. Keep the substituted equation visible so impossible or unintended scenarios are easier to catch.
Simple interest alone does not include origination fees, late charges, taxes, payment timing, amortization, minimum payments, or lender-specific rounding. The final amount P + I is a model total, not automatically the amount of every payment or an APR disclosure. Compare any decision-relevant result with the actual agreement and use an amortization or loan-payment tool when the balance changes through scheduled payments.
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