Compound interest calculator

How to use this calculator

Enter what you are starting with, the interest rate, and how long you are leaving the money alone. If you add to your savings regularly, set the amount and how often. The result updates as you type — there is no button to press.

The chart underneath separates what you put in from what the interest added, so you can see the point where the interest starts to matter more than the contributions. The year-by-year table below it can be copied out or downloaded if you want to keep the numbers.

The rate takes two decimal places

Real deposit rates are 4.35% or 2.75%, not round numbers, and most free calculators only let you pick whole percentages. Here the rate is typed, not stepped, so you can enter the rate you were actually offered.

If your keyboard writes decimals with a comma, that works too — 4,35 and 4.35 are read the same way.

What compounding frequency actually changes

Compounding is how often earned interest starts earning interest itself. Moving from yearly to monthly compounding at the same nominal rate makes a difference, but a smaller one than most people expect. On a lump sum left alone at 4%, with no contributions, monthly compounding gives about 0.7% more than yearly after ten years — noticeable, but not a transformation.

That gap widens with both the rate and the term: about 2% over thirty years at 4%, and about 4% over ten years at 10%. Going further, from monthly to daily or continuous, is worth well under a tenth of a percent at these rates.

Where it matters more is when your contributions and the compounding fall on different schedules. Paying in monthly to an account that compounds quarterly is handled here by converting the rate properly rather than dividing the annual figure by twelve, which is the shortcut that makes some calculators disagree with the bank.

A range instead of one number

A single figure ten years out reads as more certain than it is. Turn on the variance band and the calculator runs the same arithmetic at a lower and a higher rate as well, and shows the three results together.

That is a what-if, not a forecast. It shows how much of the answer depends on the rate holding — nothing more.

Why it runs in your browser

What you earn, what you have saved, and what you are saving for are nobody else’s business. This page does the arithmetic on your own device and sends nothing anywhere, so there is no account to make, no server holding your figures, and nothing to delete afterwards.

Why is my bank's figure different from this one?

Usually because the bank quotes a different rate. Advertised savings rates are often effective annual rates, which already include the effect of compounding, while this calculator takes the nominal rate and applies the compounding frequency you choose. Tax on interest and account fees are not included here either.

Does it matter whether I add money at the start or the end of the month?

More than people expect. A contribution made at the start of each period earns interest for that period too, and over twenty years that difference can add up to roughly a year's worth of contributions. You can set it either way; end of period is the more common arrangement.

Is the high and low range a prediction?

No. It is the same calculation run three times at three rates you choose, so you can see how sensitive the result is to the rate. It is not a forecast, it does not model markets, and no return is being suggested.

Are the amounts I type sent anywhere?

No. The calculation runs in your browser and nothing is uploaded, which is also why the page keeps working if you disconnect from the internet after it has loaded.