Compound Interest Calculator
See how savings and investments grow with compound interest and regular contributions.
Results update as you type.
About this calculator
Compound interest is interest earned on both your original money and the interest it has already earned. Because each interest payment starts earning interest of its own, a balance grows along a curve rather than a straight line. This calculator projects the future value of a starting balance plus optional regular contributions, using the formula A = P(1 + r/n)^(nt) for the lump sum and the future-value-of-a-series formula for the ongoing deposits, compounded as often as you choose.
For example, 1,000 invested for 10 years at 7% a year, with 100 added every month, grows to about 19,300. Of that, roughly 13,000 is money you actually paid in (the 1,000 start plus 120 monthly deposits) and about 6,300 is interest the account earned for you — money that came from nothing but time and compounding.
Use the solve-for tabs to run the projection backwards: instead of the end balance, pin a target and solve for the starting amount, the return rate, the monthly contribution or the number of years you need. That makes it a planning tool for a house deposit, a college fund or any long-term savings goal.
Frequently asked questions
How does compounding frequency matter?
The more often interest compounds, the more you earn, because each new interest payment starts earning interest sooner. The effect is small but real over long periods.
What is the compound interest formula?
A = P(1 + r/n)^(nt) for a lump sum, plus the future value of a series for regular contributions. This tool combines both.
What do the solve-for tabs do?
Instead of only computing the end balance, pick a tab to solve for a different unknown — the starting amount, return rate, monthly contribution or number of years needed to reach a target future value. The field being solved is hidden and the target end amount is used instead.
What is the difference between simple and compound interest?
Simple interest is paid only on the original principal, so it grows in a straight line. Compound interest is paid on the principal plus all previously earned interest, so it accelerates over time. Over long horizons the gap between the two becomes very large.
Does contributing at the start of the month help?
Slightly. A deposit made at the start of the month earns one extra period of interest compared with an end-of-month deposit. Over many years the timing choice adds up to a small but real difference — switch it under advanced options.
What is the rule of 72?
It is a shortcut for how long money takes to double: divide 72 by the annual percentage rate. At 7% a year, 72 ÷ 7 ≈ 10 years to double. It is an approximation, but a handy sanity check against the exact figure this calculator gives.
How much difference does the return rate make?
A large one. Because growth compounds, a couple of extra percentage points of return can nearly double the final balance over 20 or 30 years. Try changing only the rate to see how sensitive long-term investing is to it.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/compound-interest/curl
curl "https://calculator.free/api/v1/compound-interest/?target=50000&principal=1000&rate=7&years=10&contribution=100&_mode=end"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/compound-interest/?" + new URLSearchParams({
"target": "50000",
"principal": "1000",
"rate": "7",
"years": "10",
"contribution": "100",
"_mode": "end"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.