Growth

Compound interest calculator

The useful output of a compound interest calculator is not the final balance. It is the split: how much of that balance you put in, and how much arrived on its own.

Your plan

Currency

Changes the symbol on your figures. It does not convert them — there is no exchange rate here.

Starting amount

Added each month

Annual return

For how many years

Compounded

How often the interest is added. Monthly is the usual answer for a fund or a savings account.

After that time you would have

—

You put in

—

Interest earned

—

Interest as a share

—

Year by year
Year	You put in	Interest	Balance

This calculator needs JavaScript to recalculate. The figures above are a worked example, and every assumption behind them is written out below.

What this assumes

Contributions land at the end of each period, which is the convention your bank and every other compound calculator uses. Our FIRE calculator deliberately uses a mid-year convention instead, because a forty-year projection is sensitive to the half-period in a way a savings account is not.

The return is nominal here — inflation is not removed. A balance of 1,000,000 in forty years will not buy what a million buys today. For a figure in today’s money, use the inflation calculator on the result, or the FIRE calculator, which works in real terms throughout.

Your monthly figure stays a monthly figure whichever compounding frequency you pick — switching to annual does not quietly change how much you are saving.

The share that is not yours

Over ten years at a normal return, most of what you end up with is money you put in. Over thirty, most of it is not. The crossover is the entire argument for starting early, and it is worth watching the third statistic above as you change the number of years rather than watching the balance.

This is also why the last decade before retirement carries so much weight, and why a market fall at 60 hurts far more than the same fall at 30 — there is simply more money exposed to it.

Nominal returns flatter everything

A projection that ignores inflation produces impressive numbers that mean less than they appear to. At 2.5% inflation, money halves in purchasing power roughly every 28 years. A balance that looks like comfortable wealth in 2056 may be an ordinary amount by the standards of the time.

The fix is not to be gloomy about it but to be consistent: either work in today’s money throughout, as the FIRE calculator does, or convert the final figure back at the end.

Common questions

What is the difference between APR and APY?

APY (annual percentage yield) already includes the effect of compounding, so a 5% APY account genuinely returns 5% over a year. APR is a nominal rate quoted as twelve equal monthly periods, which is the convention for credit cards and loans — 22% APR means 1.8333% a month, and compounds to slightly more than 22% over the year.

Does compounding frequency make much difference?

Less than people expect. On a 7% nominal rate, monthly compounding beats annual by about 0.23 percentage points a year. Over twenty years on a lump sum that is a few per cent of the total — real, but far smaller than the effect of changing the rate itself or adding a year.

What return should I use?

For a diversified equity portfolio, 6 to 8% before inflation reflects long-run history. For a savings account, use the rate you are actually being paid. The single most common error on these calculators is entering an equity return for money that is sitting in cash.

The app this comes from

The monthly figure is the one you control

You cannot set the return, and the number of years is mostly decided for you. What you can change is how much goes in each month — and that comes out of knowing where the rest of it currently goes.

Not out yet. TLDR Money International is in build for the United States and the United Kingdom first, on iPhone and Android. There is nothing to download and nothing to sign up to — when there is, this page will say so.

Last reviewed 20 August 2026. The arithmetic behind this page is covered by automated tests that run on every deploy.