The simple-interest formula is I = P·r·t/100. P is the principal, r is the annual rate written as a percentage (5% is just 5 here), and t is the time in years; the total amount is A = P + I. For example, 10,000 at 5% for 3 years earns 10,000 × 5 × 3 / 100 = 1,500 in interest, for a total of 11,500. Unlike compound interest, the interest base never changes, so the same amount is added every year and the growth is linear. AnyTool recomputes this live as you type.

Because I = P·r·t/100 connects four quantities, you can find any one from the other three. To get the principal, use P = 100·I/(r·t); for the rate, r = 100·I/(P·t); and for the time, t = 100·I/(P·r). AnyTool has a solve-for selector so you choose the unknown, enter the three known values, and it returns the missing one along with the total amount, the comparison with compound interest and the accrual table — no algebra needed.

Simple interest applies the rate only to the original principal, so the interest added each year is constant and the balance rises in a straight line. Compound interest applies the rate to the principal plus all interest already earned, so it grows faster and faster — interest on interest. On 10,000 at 5% for 10 years, simple interest gives 5,000 of interest, while annual compounding gives about 6,289 — roughly 1,289 more. AnyTool shows both side by side and the exact extra amount compounding adds, so the gap is obvious.

Most savings accounts, credit cards and long-term loans compound, but simple interest is genuinely used for some short-term and fixed products: many car and personal instalment loans, bridge and bond interest, Treasury bills, per-diem (daily) interest quotes and quick estimates. It is a useful approximation but understates anything that compounds. AnyTool Simple Interest Calculator runs entirely in your browser, so your amounts are never sent to a server and the page works offline.