Compound Interest Calculator
Visualize how your investments grow over time.
Growth Over Time
The power of compound interest
Compound interest is interest paid on both your starting balance and the interest you have already earned. That creates a snowball effect: the longer you leave the money, the faster it grows.
The formula
A = P(1 + r/n)nt, where A is the final amount, P is the starting balance, r is the annual rate as a decimal, n is how many times a year interest is added, and t is the number of years.
Example: £10,000 at 4.5% compounded monthly for 25 years grows to £30,737, of which £20,737 is interest.
How to use the calculator
- Enter your starting balance.
- Enter the interest rate (use the AER when comparing savings products).
- Enter the time horizon in years.
- Pick the compounding frequency. If you entered an AER, pick Annually, because the AER already includes the effect of compounding.
- Optionally add a monthly contribution (the calculator adds it at the end of each month).
Frequently asked questions
What is the difference from simple interest?
Simple interest is paid only on the starting balance. Compound interest is paid on the balance plus all the interest earned so far, so it grows faster over time.
How long does money take to double?
Use the rule of 72: divide 72 by the interest rate. At 4%, money roughly doubles every 18 years; at 6%, every 12 years.
Is compound interest taxable in the UK?
Inside an ISA, no. Outside an ISA, interest is taxable above your personal savings allowance (£1,000 for basic-rate taxpayers, £500 for higher rate).
For UK ISA examples and the maths in full, read How to Calculate Compound Interest. Saving for a home? Try the Mortgage Calculator and Stamp Duty Calculator, or browse all financial tools.