Compound Interest Calculator
See how savings grow with compound interest and monthly payments. Free, with the formula and a worked example.
New: please check your results.
- Final balance
- £17,175.24
- Total paid in
- £13,000.00
- Interest earned
- £4,175.24
What it does
Work out what your savings or investments will be worth once interest starts earning interest of its own. Enter what you have now, what you'll add each month and the rate, and see the final balance and how much of it is interest.
How to use it
- Starting amount
What you have saved or invested today. Use 0 if you're starting from nothing.
- Monthly contribution
What you'll add each month. Payments are assumed at the end of each month.
- Annual interest rate
The yearly rate your bank or provider quotes. If it's an AER, set compounding to Annually.
- Time period
How long you'll leave the money to grow.
- Interest compounds
How often interest is added. Your account terms will say; monthly is the most common.
Common uses
- Checking how much a cash ISA or savings account will grow.
- Projecting a pension pot.
- Seeing what inflation does to the value of savings.
The formula
A = P(1 + r)ⁿ + C × [(1 + r)ⁿ − 1] ÷ rWhere:
- A
- final balance
- P
- starting amount
- C
- monthly contribution
- r
- interest rate per month
- n
- number of months
For annual, quarterly or daily compounding, r is the equivalent monthly rate.
Worked example
You start with £1,000, add £100 a month and earn 5% a year, compounded monthly, for 10 years.
- Monthly rate: 5% ÷ 12 = 0.42%
- Your starting amount grows to £1,647.01 over 120 months
- Your monthly payments grow to £15,528.23
Final balance: £17,175.24, of which £4,175.24 is interest.
Questions people ask
What's the difference between AER and the interest rate?
AER (annual equivalent rate) shows what you'd earn in a year with compounding included, so it's the fairest way to compare accounts. If you enter an AER, choose Annually so compounding isn't counted twice.
Does this include tax?
No. Interest in an ISA is tax-free. Outside an ISA, interest above your Personal Savings Allowance is taxed. Results here are before tax.
How long does it take to double my money?
Roughly 72 divided by the interest rate: at 6% a year, about 12 years. This is the rule of 72, a quick estimate rather than an exact figure.
Last checked 24 October 2026