Compound Interest Calculator
Project how savings grow when interest compounds and you contribute regularly.
Runs entirely in your browser — nothing you enter is uploaded.
The result is arithmetic from the figures you entered, under the assumptions listed below. It is not regulated financial advice and not an offer. Speak to a qualified adviser before making a decision.
How to use Compound Interest Calculator
- Enter your starting balance — enter 0 if you're beginning from nothing.
- Enter how much you'll add each period in "Regular contribution".
- Enter your expected annual return.
- Enter the period in years.
- Choose how often interest compounds — monthly, quarterly or annually.
- Check the chart to see when growth overtakes your own contributions.
How this works
Compounding means interest is calculated on the interest you already earned, not just on your original deposit. The calculation runs in two parts: the starting balance grows on its own, and each regular contribution grows for however many periods remain after it is paid in. A contribution made in year one compounds far longer than one made in year nineteen, which is why the separation between deposits and growth widens sharply toward the end.
FV = P(1 + r/n)^(nt) + C × [ ((1 + r/n)^(nt) − 1) ÷ (r/n) ]
Assumptions
- The rate of return is constant every period. Real investment returns vary and can be negative in any given year.
- Contributions are made at the end of each period, at the same amount throughout.
- Figures are nominal. Inflation is not deducted, so the buying power of the final balance will be lower than the number shown.
- Platform fees, fund charges and taxes on interest, dividends or gains are excluded.
Worked example
£5,000 to start, £200 added monthly for 20 years at 6% compounded monthly.
- Starting balance
- £5,000
- Monthly contribution
- £200
- Annual rate
- 6%
- Period
- 20 years
- Result
- £108,959
Of that, £53,000 is money you actually paid in (£5,000 plus 240 × £200) and £55,959 is growth. Growth only edges past your own contributions in year 20 — for the entire nineteen years before that, most of the balance is simply your own deposits.
How to read the result
Look at the split between contributions and growth rather than the headline total. For most of a twenty-year run the balance is largely your own money and the compounding looks disappointing; the curve only becomes steep once there is enough capital for the returns to matter. That slow start is the honest case for beginning early — not because the early years produce much, but because they are what makes the later years work.
Limitations
- A projection under fixed assumptions, not a forecast. Actual returns will vary year to year and may be negative.
- Past performance does not indicate future returns.
- Because inflation is excluded, do not read the final figure as today's buying power.
- This is a calculation, not regulated investment advice.
Frequently asked questions
- Should I use a real or nominal rate of return?
- The calculator treats the rate you enter as nominal — it does not subtract inflation. If you want a rough estimate in today's purchasing power, enter your expected return minus your expected inflation rate instead.
- Why does the balance grow so slowly at first?
- Compounding needs a base to compound on. Early in the period, most of the balance is your own deposits and there simply hasn't been enough time for growth to accumulate — the worked example below shows growth only overtaking contributions in year 20 of a 20-year run.
- Does this account for fees or tax?
- No. Platform fees, fund charges, and tax on interest, dividends or capital gains are all excluded, as stated in the assumptions above — they would reduce the final figure shown here.