Compound Interest Calculator
Enter a starting amount, a monthly contribution and an interest rate to see how your savings could grow, and how much of that growth comes from interest.
Your savings details
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Your results will appear here
Enter how much you'll start with, what you'll add each month, the interest rate and how long you'll save, then press Calculate to see how your savings grow.
How compound interest works
With simple interest, you only earn interest on the money you put in. With compound interest, the interest you earn is added to your balance, and from then on it earns interest too. Over time you earn interest on your interest, so your savings grow faster each year.
A = P × (1 + r ÷ n)ⁿᵗ
The calculator also adds your regular monthly contributions. Each one starts earning interest from the month after it's paid in, so money added early in the term has the longest time to grow.
Worked example
Say you put £10,000 into savings paying 5% a year and leave it for 10 years.
- With interest compounded annually, you would end up with £16,288.95.
- Compounded monthly, you would have £16,470.09, because interest starts earning interest sooner.
- Adding £100 a month on top would grow your savings to £31,998.32.
Open this example in the calculator to see how your contributions and interest build up year by year.
Understanding your results
- Final balance: how much you would have at the end of the time period.
- Total contributed: your initial deposit plus all your monthly contributions.
- Total interest: everything your money earned on top of what you paid in.
- Effective annual rate (AER): the rate you would get if interest were paid once a year. It lets you compare accounts that compound at different frequencies: 5% compounded monthly works out at an AER of 5.12%.
Frequently asked questions
What is the difference between the interest rate and the AER?
The interest rate is the headline annual rate. The AER (Annual Equivalent Rate) shows what that rate is worth once compounding is taken into account, as if interest were paid once a year. When interest is compounded more often than yearly, the AER is a little higher than the headline rate.
How often is interest compounded?
It depends on the account or investment. Many savings accounts pay interest monthly or annually. Choose the option that matches yours; if you are not sure, monthly is a common choice.
When are my monthly contributions added?
The calculator assumes each contribution is paid in at the end of the month. Interest is spread across the months at the rate that matches your chosen compounding frequency, which is exact for monthly compounding and a close approximation for the others.
Does this take tax or inflation into account?
No. The results show growth before any tax on interest and before inflation. Depending on your circumstances, some of your interest may be taxable, and inflation reduces what your savings will buy in the future.
How long will it take to double my money?
A quick estimate is the Rule of 72: divide 72 by the interest rate. At 5%, your money would double in roughly 14.4 years. Use the calculator for an exact figure with your own rate and contributions.