How Much Will My Savings Grow?

By the HomeMathCalc Team · Updated 24 October 2026

Short answer: £10,000 at 4% interest compounded monthly grows to £14,908.33 in 10 years and £22,225.82 in 20 years. Saving £200 a month at 4% gives £29,449.96 after 10 years, of which £5,449.96 is interest. The tables below cover lump sums and monthly saving at common rates.

How compound interest works

With compound interest, each month's interest is added to your balance, and the next month's interest is paid on the bigger balance. In the first year the effect is small, but over 10 or 20 years the interest earned on earlier interest becomes a large part of the total.

The figures here assume a fixed rate for the whole period, interest added monthly and no tax taken off. Real savings rates change, so rerun the numbers when your rate does.

How £10,000 grows at different rates

Over 20 years, 5% instead of 2% turns the same £10,000 into £27,126.40 instead of £14,913.28. A rate difference that looks small in a single year adds up over time.

Saving every month at 4%

Look at the interest column: doubling the time from 10 to 20 years more than quadruples the interest, because later years earn interest on a much bigger balance.

How long does it take to double your money?

A quick shortcut is the "rule of 72": divide 72 by the interest rate to get roughly how many years money takes to double. With monthly compounding the exact answer is a little shorter.

Worked example: £10,000 plus £300 a month for 15 years

You start with £10,000, add £300 a month and earn 4.5% for 15 years. You pay in £64,000 in total and end with £96,539.95, so £32,539.95 is interest.

At 4% the same plan ends at £92,030.16, and at 5% it reaches £101,323.72. Half a percentage point either way moves the result by about £4,500 to £4,800, which is why it pays to move savings when a better rate is available.

Tax on savings interest (checked on GOV.UK)

• ISAs: interest in a cash ISA is tax free. In the 2026 to 2027 tax year you can put up to £20,000 in ISAs.
• Personal Savings Allowance: outside an ISA, basic rate taxpayers can earn £1,000 of interest a year tax free, higher rate taxpayers £500 and additional rate taxpayers £0.
• Starting rate for savings: if your other taxable income is less than £17,570, you may be able to earn up to £5,000 of interest tax free on top.

How big a balance uses up the allowance? With interest added monthly, a year's interest reaches £1,000 on about £24,545 at 4%, £19,546 at 5% or £32,877 at 3%. For the £500 higher rate allowance, the balances are about £12,272 at 4%, £9,773 at 5% and £16,439 at 3%. Above these amounts, an ISA keeps the extra interest tax free.

Tips to make savings grow faster

• Save on payday. A standing order the day your salary arrives means you save first and spend what is left.
• Check your rate every few months. Introductory bonuses often end after a year and the rate can drop.
• Use the right account for the goal. Easy access for an emergency fund, fixed rate or notice accounts for money you will not need for a while.
• Think long term for retirement. For decades away, a pension adds tax relief and employer money: see our guide how much will my pension be worth and the pension calculator.
• Clear expensive debt first. A loan or card at a higher rate costs more than savings earn. The loan calculator shows what borrowing really costs.

Try the compound interest and savings calculator

Enter your own numbers below, or open the full compound interest and savings calculator with the formula and more examples.

This guide is general information with example numbers, not financial, tax or building advice. Your costs and results will differ. Terms

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