Leave blank the box you want to work out. Free, nothing to install, no sign-up.
Simple interest is interest worked out on the original amount every time, with the interest never earning interest of its own. This calculator solves for whichever box you leave empty: give it the principal, the rate and the time and it gives you the interest, but it works backwards too — what principal you need, what rate that is, how long you have to wait. It runs inside your browser and downloads as a PDF.
Interest = principal × rate × time, with the rate as a decimal (5% is 0.05) and the time in years. All four versions come out of that single line: the principal is interest ÷ (rate × time), the rate is interest ÷ (principal × time) and the time is interest ÷ (principal × rate). That is why the calculator does not have four modes: it has one formula and it solves for whatever is missing.
Two thousand at 5% a year for three years. The rate as a decimal is 0.05, so 2000 × 0.05 × 3 = 300 in interest, and you end with 2,300. Notice that each year produces exactly 100 and not a penny more: the third year earns the same as the first, because it is still being worked out on the original 2,000 and not on the 2,200 you had by then.
That is the only difference, and it is the whole difference. With compound interest the interest is added to the principal and from then on it earns interest too. The same 2,000 at 5% over three years would give 315.25 instead of 300. Over three years that is fifteen; over thirty, the gap between the two becomes enormous. Simple interest is mostly used on short arrangements and on some private loans.
When the term comes in days rather than years you have to divide by the days in a year, and not everyone uses the same number. The commercial year is 360 days and the exact year is 365. With 360 the answer comes out slightly higher, because you are dividing by less. That is why the calculator lets you pick rather than deciding for you: if you are checking a contract, you have to use whichever the contract says.
Work out one year and multiply. If 2,000 at 5% gives 100 a year, three years has to give exactly 300: simple interest is linear, so twice the time is twice the interest. If doubling the time does not double the interest, either you changed something else or you are looking at a compound interest calculator.
The box asks for the percentage as it is written, so 5. It divides by a hundred internally to do the sum. Entering 0.05 would be asking for 0.05%, and the interest would come out a hundred times too small.
Yes, but the formula works in years, so you have to convert: six months is 0.5 years, and 90 days is 90 divided by either 360 or 365, depending on which year you pick with the switch. That switch is there for exactly this.
Whichever the document you are checking uses. Plenty of bank contracts and bills use the commercial 360-day year; other calculations use the exact 365. Neither is "correct" in the abstract — it is a convention, and what matters is matching the other party.
No. Everything is worked out inside your browser: what you type never leaves your device and is not stored anywhere. No account, no email address.
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