Compound interest calculator
See how your money could grow over time with regular contributions and compound interest. Enter a starting amount, monthly contribution, interest rate, and time horizon to project the final value.
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See how your money could grow over time with regular contributions and compound interest. Enter a starting amount, monthly contribution, interest rate, and time horizon to project the final value.
Compound interest means you earn returns on your accumulated balance — not just on the money you put in. In the early years, most of the growth comes from your contributions. Over time, the interest-on-interest effect accelerates and can eventually dwarf what you personally saved.
For example, putting in £200 per month with no starting lump sum at 5% annual interest for 20 years gives you roughly £83,000 — but your total contributions are only £48,000. The remaining £35,000 is compound growth. The longer the time horizon, the more dramatic the compounding effect becomes.
Enter a starting amount, a monthly contribution, an annual interest rate, and a time horizon in years. Click Calculate to see the projected final value alongside a breakdown of how much you contributed and how much came from growth.
The chart shows year-by-year how the total value and your cumulative contributions diverge over time — the gap between the two lines is your compounded growth.
The right rate to enter depends on where your money is held. A cash savings account rate is typically set by the provider and fluctuates with the Bank of England base rate. A stocks and shares ISA or investment account does not have a guaranteed rate — historical equity returns have averaged around 5–8% per year in real terms, but past performance is not a guide to future results and the value of investments can go down as well as up.
Try a range of rates to see how sensitive the outcome is to your assumptions. Small differences in the rate — especially over long horizons — have a large effect on the final figure.
Monthly contributions are summed to an annual amount and added at the start of each year. The annual interest rate is then applied to the full balance (starting amount plus that year's contribution) to produce the year-end total. This is an approximation: true monthly compounding would apply one-twelfth of the rate twelve times per year, producing a marginally higher result. The difference is small at typical savings rates but grows at higher rates or longer horizons.
No inflation adjustment, no fees or charges, and no tax are applied. The projection assumes a constant rate throughout the time horizon; real returns will vary year to year.
Compound interest is calculated by applying a growth rate to your balance at the end of each period — so you earn returns on your returns, not just on your original amount. This calculator adds your annual contribution at the start of each year, then applies the annual interest rate to the total. The result compounds year-on-year over your chosen time horizon.
Monthly compounding applies the interest rate twelve times per year on a twelfth of the annual rate, producing a slightly higher end result than annual compounding at the same stated rate. This calculator uses annual compounding: your monthly contributions are summed to an annual amount added at the start of each year, and the full annual rate is applied once per year. The difference is modest for typical savings rates (under 0.1% per year at 5%) but grows with the rate and time horizon.
No — it projects nominal (not inflation-adjusted) growth. The figures shown are in today's pounds without adjusting for future price rises. To estimate real purchasing power, you can subtract an expected inflation rate from your interest rate before entering it (for example, entering 3% instead of 5% if you expect 2% annual inflation).
It works for any savings or investment vehicle: a cash savings account, a stocks and shares ISA, a general investment account, or any other account that compounds over time. The calculator does not apply tax — if your account is not tax-sheltered, the effective rate after tax may be lower than the headline rate, so you may want to adjust the rate you enter.
Want to shelter your growth from tax? Use our ISA calculator to project tax-free compound growth within your annual ISA allowance. Targeting financial independence? The FIRE calculator shows how long until your pot reaches your target spending level. For retirement savings specifically, use the pension calculator to project your pot with employer contributions and tax relief. To see how much of your salary is available to save, try the salary after tax calculator.
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