Compound Interest Calculator

What exactly is being projected?

The future value of a starting amount plus a regular monthly contribution, at a fixed annual rate. The tool reports the final balance, the total you actually put in, and the difference between them — the part the interest did.

How often does it compound, and when are contributions added?

Monthly, and contributions land at the end of each month — an ordinary annuity. That matters: a deposit made at the end of a month earns nothing for that month. If your real plan pays in at the start of each month, the true result is slightly higher than shown.

What is the formula?

Two parts added together. The starting amount grows as P × (1 + r)^n. The contributions grow as PMT × ((1 + r)^n − 1) / r. In both, r is the annual rate divided by 12 and n is the number of months. At a rate of zero the second part collapses to PMT × n, which the tool handles rather than dividing by zero.

Why does the balance curve upwards instead of climbing steadily?

Because each period's interest is calculated on a balance that already includes previous interest. Early on, growth is dominated by what you contribute; later, the returns on accumulated returns take over. Set a long horizon in the calculator and watch when the interest line overtakes the contributions line — that crossover is the whole argument for starting early.

Does it account for inflation, tax or fees?

No. The output is a nominal figure. Inflation reduces what that balance can buy, investment tax reduces what you keep, and platform or fund fees are deducted from returns before you see them. A percentage point of annual fees compounds against you exactly the way returns compound for you.

What rate should I use?

Whatever you enter, the tool treats as certain and constant, which no real investment is. A fixed savings rate is knowable; a market return is not, and real sequences include falling years that a smooth average hides. Modelling a pessimistic rate alongside an optimistic one is more informative than a single confident number.

Is my data sent anywhere?

No. The projection runs entirely in your browser.

Can I plan my finances with this?

Use it to understand how compounding behaves, not as a forecast. Actual outcomes depend on markets, taxes, fees and your own behaviour during downturns. Speak to a licensed financial adviser before committing to a plan.