Compound Interest Calculator

About this calculator

A free compound interest calculator that projects how a starting balance grows over time at a fixed annual rate. It applies the exact compound formula P*(1+r/n)^(n*t) to your principal, then layers on an optional recurring monthly contribution grown month by month, and breaks the ending balance into total contributions versus interest earned. Everything runs in your browser. The result is an estimate that assumes a constant rate and ignores taxes, fees, and inflation — not investment, tax, or financial advice.

How compounding turns a rate into growth

Compounding is interest earning interest. Each time the account credits a return, that return is added to the balance, and the next period's interest is figured on the larger total. This calculator applies the standard formula P*(1+r/n)^(n*t), where P is the starting principal, r is the annual rate, n is how many times a year interest is credited, and t is the number of years.

Because the exponent is n*t, time and frequency both stack up inside the formula. The balance does not grow in a straight line; it curves upward, slowly at first and then faster, as each period's growth widens the base that the next period works on. That curve is the whole point of starting early rather than contributing more later.

Frequency, contributions, and time horizon

Crediting interest more often raises the effective annual yield above the stated nominal rate, because partial growth starts earning sooner. That gap is small at low rates and grows with the rate — the monthly and annual variant pages below show the same nominal rate producing different ending balances purely from how often it compounds.

A recurring monthly contribution is grown month by month at the per-month rate, so each deposit compounds for however many months remain. The longer the horizon, the more the later interest dwarfs the contributions: a rough shorthand called the rule of 72 says a balance roughly doubles in 72 divided by the rate in years, so 8% doubles in about nine years.

Reading total contributions versus interest earned

The result splits the ending balance into two parts. Total contributions is the money you actually put in — the starting principal plus every monthly deposit. Interest earned is everything above that, the growth compounding added on top. Watching the interest portion overtake the contribution portion over a long horizon is the clearest picture of compounding at work.

Treat the figure as a clean estimate, not a forecast. It assumes one fixed rate for the whole horizon and ignores taxes, account fees, and inflation, which all erode real-world returns. Use it to compare scenarios and see the shape of growth, then apply your own assumptions about a realistic rate.

By variant

• Monthly Compounding
• Annual Compounding

Questions

Is the compound interest calculator free?

Yes. It is free, needs no account, and calculates entirely in your browser — none of the figures you enter are uploaded or stored.

How is compound interest calculated here?

The starting principal grows with the standard compound formula P*(1+r/n)^(n*t), where r is the annual rate, n is compounds per year, and t is years. An optional monthly contribution is added each month and grown at the per-month rate, so the contribution result stays correct no matter which compounding frequency you pick for the principal.

What is the difference between total contributions and interest earned?

Total contributions is the money you actually put in — the starting principal plus every monthly contribution. Interest earned is the future value minus those contributions, i.e. the growth compounding added on top.

Why might the result differ from my actual returns?

The tool assumes a single fixed rate for the whole horizon and does not model taxes, account fees, variable returns, or inflation. Real-world returns fluctuate, so treat the number as a clean estimate of compounding, not a guaranteed outcome.