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Compound interest calculator

Starting amount, monthly top-ups, a rate and some years: see the balance, the chart of contributions versus interest and the year-by-year table.

Guide

How to use it

  1. Enter the starting amount and any monthly contribution (zero is fine).
  2. Set the annual rate and the years. Monthly compounding matches most savings accounts.
  3. Read the tiles, then the chart: gold is money you paid in, blue is interest stacking on top.
  4. Open the year-by-year table to see when interest starts outearning your contributions.
  5. Working towards a number instead? The savings goal page solves for the monthly amount.

Examples

Worked examples

£5,000 + £200 a month, 4.5%, 20 years

The defaults in the tool: about £89,000, of which £53,000 was paid in and £36,000 is interest. Change one input and watch which tile moves most.

Rule of 72 check

£10,000 at 6% with no contributions: 72 ÷ 6 says doubling in ~12 years. The table shows £20,122 at year 12, the rule holds.

Starting ten years earlier

£200 a month at 5% for 20 years beats £400 a month for 10 years, £82k against £62k, despite identical contributions. Time is the biggest input.

Method

How it works

The balance is simulated period by period: multiply by (1 + rate ÷ periods per year), then add that period's contributions at the end. That matches the closed formula FV = P(1+r÷n)ⁿᵗ + PMT×((1+r÷n)ⁿᵗ−1)÷(r÷n) and also produces the honest year-by-year table.

Assumptions worth repeating: constant rate, contributions at month end, no fees, tax or inflation. It is arithmetic, not advice, and nothing you enter leaves your browser.

FAQ

Frequently asked questions

What is compound interest?

Interest paid on interest. Each period's growth joins the balance, so the next period grows from a bigger base. Over decades that curve, not the contributions, does most of the work.

How much difference does the compounding frequency make?

Less than people expect: £10,000 at 5% for 10 years is £16,289 compounded yearly and £16,470 monthly. The rate and the time matter far more than the frequency.

What is the rule of 72?

Divide 72 by the annual rate for the rough doubling time. At 6%, money doubles in about 12 years; at 3%, about 24. It is a head check for the calculator's answer.

Are the contributions assumed at the start or end of the month?

End of the month, the conservative convention. Contributions at the start would earn slightly more; real accounts sit in between depending on your payday.

Does this account for inflation or tax?

No. It projects the nominal balance at a constant rate with no fees, tax or inflation. Real purchasing power grows more slowly, and returns outside ISAs and pensions can be taxed.

Is this financial advice?

No, it is arithmetic on the numbers you type. Rates are not guaranteed, and decisions about products belong with a regulated adviser.

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Skip to the tool

Compound interest calculator

Starting amount, monthly top-ups, a rate and some years: see the balance, the chart of contributions versus interest and the year-by-year table.

£89,902 balance after 20 years
£53,000 total paid in
£36,902 interest earned

Each bar is one year: gold is money paid in, blue is interest on top.

Balance compounds monthly at 4.5% with £200 added monthly · FV = P(1+r÷n)ⁿᵗ + PMT×((1+r÷n)ⁿᵗ−1)÷(r÷n)

Year-by-year table
Year Paid in Interest Balance 1 £7,400 £280 £7,680 2 £9,800 £683 £10,483 3 £12,200 £1,214 £13,414 4 £14,600 £1,881 £16,481 5 £17,000 £2,688 £19,688 6 £19,400 £3,643 £23,043 7 £21,800 £4,751 £26,551 8 £24,200 £6,021 £30,221 9 £26,600 £7,460 £34,060 10 £29,000 £9,075 £38,075 11 £31,400 £10,874 £42,274 12 £33,800 £12,866 £46,666 13 £36,200 £15,060 £51,260 14 £38,600 £17,465 £56,065 15 £41,000 £20,091 £61,091 16 £43,400 £22,947 £66,347 17 £45,800 £26,045 £71,845 18 £48,200 £29,396 £77,596 19 £50,600 £33,011 £83,611 20 £53,000 £36,902 £89,902

Projections assume a constant rate, contributions at month end and no fees, tax or inflation. This is arithmetic, not financial advice.

Everything runs in your browser. Nothing you enter is sent to a server.

Guide

How to use it

  1. Enter the starting amount and any monthly contribution (zero is fine).
  2. Set the annual rate and the years. Monthly compounding matches most savings accounts.
  3. Read the tiles, then the chart: gold is money you paid in, blue is interest stacking on top.
  4. Open the year-by-year table to see when interest starts outearning your contributions.
  5. Working towards a number instead? The savings goal page solves for the monthly amount.

Examples

Worked examples

£5,000 + £200 a month, 4.5%, 20 years

The defaults in the tool: about £89,000, of which £53,000 was paid in and £36,000 is interest. Change one input and watch which tile moves most.

Rule of 72 check

£10,000 at 6% with no contributions: 72 ÷ 6 says doubling in ~12 years. The table shows £20,122 at year 12, the rule holds.

Starting ten years earlier

£200 a month at 5% for 20 years beats £400 a month for 10 years, £82k against £62k, despite identical contributions. Time is the biggest input.

Method

How it works

The balance is simulated period by period: multiply by (1 + rate ÷ periods per year), then add that period's contributions at the end. That matches the closed formula FV = P(1+r÷n)ⁿᵗ + PMT×((1+r÷n)ⁿᵗ−1)÷(r÷n) and also produces the honest year-by-year table.

Assumptions worth repeating: constant rate, contributions at month end, no fees, tax or inflation. It is arithmetic, not advice, and nothing you enter leaves your browser.

FAQ

Frequently asked questions

What is compound interest?

Interest paid on interest. Each period's growth joins the balance, so the next period grows from a bigger base. Over decades that curve, not the contributions, does most of the work.

How much difference does the compounding frequency make?

Less than people expect: £10,000 at 5% for 10 years is £16,289 compounded yearly and £16,470 monthly. The rate and the time matter far more than the frequency.

What is the rule of 72?

Divide 72 by the annual rate for the rough doubling time. At 6%, money doubles in about 12 years; at 3%, about 24. It is a head check for the calculator's answer.

Are the contributions assumed at the start or end of the month?

End of the month, the conservative convention. Contributions at the start would earn slightly more; real accounts sit in between depending on your payday.

Does this account for inflation or tax?

No. It projects the nominal balance at a constant rate with no fees, tax or inflation. Real purchasing power grows more slowly, and returns outside ISAs and pensions can be taxed.

Is this financial advice?

No, it is arithmetic on the numbers you type. Rates are not guaranteed, and decisions about products belong with a regulated adviser.

More tools

Related tools