Simple Interest Calculator

How to use

1. 1Pick what you want to solve for: Find interest, Find principal, Find rate, or Find time. The form swaps the input fields automatically.
2. 2Choose a currency, then enter the three known values (some combination of principal, interest, rate, and term).
3. 3If the rate you have is quoted per month or per day instead of per year, switch the Rate period selector and the tool converts it to an annual basis behind the scenes.
4. 4Enter the term as years plus extra months or as a flat number of days, then pick a day-count convention (Actual / 365 for most personal loans, Actual / 360 for US money-market style, or 30 / 360 for bond-style accrual).
5. 5Read the headline result and the summary rows, then scroll to the accrual schedule for a period-by-period breakdown. Use Copy result or Copy CSV to grab the numbers.

About this tool

Simple Interest Calculator solves the classic equation I = P x r x t for any of its four variables in your browser. Find interest mode multiplies the principal by the annual rate and the term to show interest earned or owed, the total balance after that term, and a year-by-year or month-by-month accrual schedule. Find principal mode rearranges to P = I / (r x t) so you can back out the starting amount that produced a given interest figure (useful when only the headline interest from a quote is given and you need to size the loan or deposit). Find rate mode applies r = I / (P x t) so you can compute the implied simple interest rate of a quote, a short-term loan slip, or a savings offer where only the absolute interest is published. Find time mode applies t = I / (P x r) so you can answer how long money was lent or how long it needs to sit at a given rate to hit a target gain. The rate can be entered as an annual, monthly, or daily figure, and the tool converts to an annual rate internally so the math is always consistent. The term can be entered as years plus months or as a flat number of days, and the day-count convention selector covers the three most widely used market conventions: Actual / 365 (the everyday convention used for many personal loans and UK and Australian bank deposits), Actual / 360 (the US money-market and commercial paper convention used by short-term lenders and the rate basis behind many US Treasury bills), and 30 / 360 (a convention common in US corporate and municipal bonds where every month is treated as 30 days). Eight currencies are supported (USD, EUR, GBP, CAD, AUD, JPY, INR, TRY) and figures use the appropriate currency symbol throughout. The accrual schedule below the result shows interest earned each period, cumulative interest, and total balance through the end of the term, with a one-click Copy CSV button so you can paste the table straight into a spreadsheet. Useful for sizing a short-term personal loan, sanity-checking a savings account quote, computing the implied rate behind a payday-style headline figure, planning the interest cost of a fixed bridge or business loan, comparing simple interest against a compound alternative, or just practicing the I = P x r x t formula for class. Simple interest is distinct from compound interest: every period the rate is applied only to the original principal, never to interest already accrued. For long-term investments that reinvest dividends or for savings accounts that capitalize interest, use the Compound Interest Calculator instead. All math runs locally in your browser. The amounts, rates, and balances you enter never leave your device.

Free to use. Works in your browser. No signup, no login.