Compound Interest Calculator
See what a starting pot plus regular monthly saving grows into, and how much of the final number is interest rather than money you put in.
Your details
Added at the start of every month.
Use the nominal rate, not AER. If you only have AER, choose annual compounding below. Returns are assumptions, not guarantees.
Advanced options▾
If you plan to save more as your pay rises.
Updates as you change the figures. No submit button needed.
Result
£31,920
after 10 years. 22% of that is interest you did not have to earn.
How we calculate this
Contributions are added at the start of each month. We divide the nominal annual rate by the selected number of compounding periods, then convert that period rate to an equivalent monthly growth rate. A deposit earns interest only for the time it is invested, including fractional quarters or years. Actual savings-account daily accrual can differ.
The rate is applied before tax. Interest inside a Cash ISA or Stocks and Shares ISA is tax-free. Outside one, the Personal Savings Allowance covers £1,000 of interest for basic-rate taxpayers and £500 for higher-rate, with anything above taxed at your marginal rate. Investment returns are not fixed and can be negative in any given year; the calculator shows a smooth average, which real markets never deliver.
The doubling figure uses the rule of 72: divide 72 by the annual rate for a quick estimate of the years it takes money to double.
This tool is for general guidance only and is not financial or tax advice. Always check your own circumstances, ideally with a qualified adviser or accountant, before making decisions.
Related reading
Investing
What Investing Actually Means (And Why Your Bank Never Explains It Properly)
Cash in a savings account quietly loses value every year, and understanding why is the first step to fixing it.
Investing
Index Funds: The Investment the Fund Management Industry Hopes You Never Fully Understand
Most actively managed funds underperform a simple index tracker after fees, yet they're still the ones being marketed to you.