Enter principal, annual rate and days to calculate simple interest, or use earned interest to find the annual rate. Both calculations use a 365-day year and exclude compounding, fees and taxes.
Runs locally in your browserThe page works in both directions around the same 365-day year. The first panel turns a principal, an annual rate and a term in days into the interest earned; the second turns a principal, a term and an interest amount back into an annualized rate.
Both panels recalculate while you type and never touch the field you are editing. Nothing is sent anywhere: the arithmetic runs in this browser, and the page keeps working offline.
The first panel uses interest = principal x annual rate / 100 x term / 365. The second inverts it: annualized rate = interest x 365 / principal / term x 100. A term of 365 days at 3.65 percent returns exactly 36500 on a principal of 1000000.
The year is fixed at 365 days and the interest is simple: no compounding, no fees and no taxes. A bank quoting a 360-day money-market basis or actual/actual days lands on a slightly different figure, and this page is neither an APY nor an APR.
Amounts are shown to two decimals and rates to four, while the arithmetic keeps the unrounded value. A small number that would round to a flat 0.00 is shown with its significant digits instead, so the interest on 100 at 10 percent for one day reads 0.03 rather than 0.
Values up to 1e15 and exponent notation such as 1e5 are accepted, and the term may be fractional - 1.5 counts as a day and a half, not one day. A comma is reported with a sentence instead of being read as a thousands separator, a negative principal, rate or term is refused, and the second panel refuses a zero principal or term instead of printing Infinity.
The two panels cover the arithmetic behind deposit interest, short-term notes, invoice discounts and quick comparisons between offers: 5000 at 4.5 percent for 90 days earns 55.48, and 2500 of interest on 20000 over 120 days is an annualized 38.0208 percent.
Because the year is fixed at 365 days and compounding is excluded, treat the result as the simple-interest figure it is: a useful check on a statement, not a substitute for the contract or the tax rules that apply to it.