BriskCal

Compound interest calculator

Starting with £1,000 and saving £200 a month at 4.5%, after 20 years you would have £80,371 — of which £49,000 is money you paid in and £31,371 is interest. Change any of it below.

% a year
years
After 20 years you would have £80,371 £49,000 paid in · £31,371 interest · interest is 39% of the total
YearPaid inInterestBalance
1£3,400.00£105.26£3,505.26
2£5,800.00£325.60£6,125.60
3£8,200.00£666.32£8,866.32
4£10,600.00£1,132.95£11,732.95
5£13,000.00£1,731.28£14,731.28
6£15,400.00£2,467.34£17,867.34
7£17,800.00£3,347.47£21,147.47
8£20,200.00£4,378.29£24,578.29
9£22,600.00£5,566.72£28,166.72
10£25,000.00£6,920.00£31,920.00
11£27,400.00£8,445.71£35,845.71
12£29,800.00£10,151.76£39,951.76
13£32,200.00£12,046.45£44,246.45
14£34,600.00£14,138.44£48,738.44
15£37,000.00£16,436.79£53,436.79
16£39,400.00£18,950.99£58,350.99
17£41,800.00£21,690.94£63,490.94
18£44,200.00£24,667.03£68,867.03
19£46,600.00£27,890.08£74,490.08
20£49,000.00£31,371.47£80,371.47

How this is worked out

The balance is grown one month at a time rather than by a formula. Each month the contribution is added first, then interest at a twelfth of the annual rate is calculated on the resulting balance and added, rounded to the penny. That order matters — a contribution paid at the start of the month earns that month's interest, and one paid at the end does not. Working backwards from a target uses the same simulation rather than an inverted formula, searching for the smallest monthly amount that actually reaches the goal, so the two answers on the page are guaranteed to agree with each other.

£1,000 at 12% a year, with nothing added, for three months

  1. Month 1: 1% of £1,000 is £10.00, so the balance becomes £1,010.00.
  2. Month 2: 1% of £1,010.00 is £10.10 — more than last month, because the interest is now earning too. Balance £1,020.10.
  3. Month 3: 1% of £1,020.10 is £10.20, giving £1,030.30.
  4. Simple interest would have paid £10.00 three times, for £1,030.00. The extra 30p is the compounding, and over decades it is most of the result.

A test recomputes that example on every build, and separately checks the whole simulation against the standard closed-form future value formula across a range of plans. Two different methods, one answer.

Why there is nothing to cite here

The rate is one you supply, not one anybody publishes, so this calculator cannot go out of date and has no external source to lean on — it shows its method instead. Two figures mentioned below are published, and those are linked: the FSCS deposit protection limit and the starting rate for savings.

What catches people out

  • This is arithmetic, not a forecast. It works out what a rate you state would produce — it does not predict one. Easy-access savings rates change whenever the provider likes, and a fixed bond only holds its rate for its term, so a twenty-year projection at today's rate is a thought experiment rather than a plan.
  • Interest is compounded monthly here, on a twelfth of the annual rate. A quoted AER already accounts for compounding, so an account that genuinely pays once a year will come out slightly behind this. It is the convention every savings comparison uses, but it is a convention rather than a law.
  • Contributions are treated as landing at the start of each month and earning that month's interest. Paying in on the last day instead costs you a month of interest on every single payment, which over twenty years is a visible amount. Most calculators do not say which they assume.
  • Inflation is not modelled, and it matters more than the rate. £10,000 growing at 4% while prices rise at 3% is gaining 1% in what it will actually buy. The figure this produces is in today's pounds only in the sense that it ignores the question entirely.
  • Interest above your allowances is taxable. The Personal Savings Allowance covers £1,000 of interest for a basic rate taxpayer and £500 for a higher rate one, with none at all at the additional rate, and there is a separate starting rate for savings of up to £5,000 if your other income is under £17,570. Interest inside an ISA is outside all of that.
  • Money in a UK-authorised bank, building society or credit union is protected by the FSCS up to £120,000 per person per institution — a limit that rose from £85,000 on 1 December 2025. The limit applies across every account you hold with the same banking licence, and several familiar brands share one.

Questions

What actually is compound interest?

Interest paid on interest already earned. In the first month you earn interest on what you put in; in the second you earn it on what you put in plus what you earned, and so on. The balance grows faster the longer it runs, which is why the last few years of a long plan add far more than the first few.

The year-by-year table underneath the result shows the split, so you can see the point where interest starts contributing more than your payments do.

Why is my bank's figure slightly different?

Almost always because of when interest is credited and when contributions land. This compounds monthly and pays contributions at the start of the month; your account may pay interest annually, or on an anniversary, or calculate daily. Over a year the gap is small. Over twenty it is visible without either being wrong.

The other common cause is that AER already assumes compounding, so applying a twelfth of it every month is very slightly generous.

Should I be saving or paying off debt?

The arithmetic side of that is usually straightforward — debt interest is typically several times savings interest, so a pound against a credit card at 22% does more than a pound into an account at 4%. The debt payoff calculator works out the first half of that comparison.

What this site will not do is tell you which to do. Having accessible savings matters for reasons arithmetic cannot see, and an emergency fund is the thing that stops the next unexpected bill becoming more debt.

Does this account for tax on the interest?

No. It shows the gross figure. Whether any of it is taxable depends on your other income, which allowances you have left and whether the account is an ISA — none of which a rate and a term can tell it.

If your interest sits inside the Personal Savings Allowance, or the account is an ISA, the gross figure is what you keep.

The published figures mentioned above

Debt payoff calculator