Free calculator
Compound interest calculator
See how a starting amount plus regular monthly contributions could grow over time with compound interest, at an assumed rate of return. An illustration to picture the power of compounding — not a guaranteed return or a forecast.
See how money left to grow could add up over time — a starting pot, a bit added each month, and the magic of returns building on returns.
This calculator, the result and the breakdown are free for everyone, no account needed.
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How it works
- 1Enter a starting amount and how much you'd add each month.
- 2Choose how many years to project over — longer horizons show compounding more clearly.
- 3Set an assumed annual rate of return. Returns aren't guaranteed, so this is an assumption you can change.
- 4The calculator compounds the balance each month and shows the projected value, how much you put in, and how much is growth on top — with a chart over time.
Worked example
A £5,000 starting amount, £200 saved each month, over 20 years, at a 5% assumed return. The calculator projects the value after 20 years, splitting it into what you put in and the growth on top.
Compounding means growth earns its own growth over time, so most of the gain arrives in the later years. The return is an assumption, not a promise — real returns vary and can fall as well as rise.
Frequently asked questions
How does compound interest work?
You earn a return on your original money and on the returns it has already made, so growth accelerates over time. This calculator compounds monthly and shows how the balance builds.
How much will £200 a month grow to?
It depends on the return and the number of years. Enter £200 a month, your time horizon and an assumed return to see an illustrative projected value and how much of it is growth.
Is the return guaranteed?
No. The rate of return is an assumption you set, not a promise. Real investment returns vary year to year and can fall as well as rise, so treat the result as an illustration.
Should this money go in an ISA?
Where you hold savings or investments affects the tax on any growth — an ISA shelters returns up to the annual allowance. This calculator shows the growth itself; whether an ISA fits depends on your wider situation.