Compound interest calculator
See how a starting amount plus regular monthly investing could grow, and how much of that growth fees take. Results are illustrations based on the growth rate you enter, not predictions.
Year by year
| Year | Paid in | Value | Fees cost so far |
|---|
Worked example: £200 a month for 20 years
The calculator opens with this example. Someone invests £1,000 up front and then £200 a month for 20 years. Their investments grow 5% a year before charges, and they pay 0.5% a year in platform and fund fees combined. These are assumptions for illustration, not a forecast.
| Total paid in | £49,000 |
|---|---|
| Value with no fees | £83,814 |
| Value after 0.5% a year in fees | £79,023 |
| Cost of fees | £4,791 |
| Growth after fees | £30,023 |
| Value in today's money (2% inflation) | £53,180 |
Of the £79,023 final value, £49,000 is money paid in and £30,023 is growth. Fees of 0.5% a year cost £4,791 over the period: the charges themselves, plus the growth that money would have earned. Adjusted for 2% a year inflation, the pot would buy about what £53,180 buys today.
Time does more than extra money. Keeping £200 a month going for 30 years instead of 20 gives about £152,484 from £73,000 paid in. Doubling to £400 a month for 20 years gives about £155,645, but needs £97,000 paid in. That's almost the same result for £24,000 more of your own money.
How the calculator works
Your growth rate is treated as an annual figure and applied each month, so 5% a year compounds to exactly 5% over twelve months. Monthly contributions are added at the end of each month. Fees are taken as a percentage of your balance each month, which is how most UK platform and fund charges work, so a 0.5% fee takes 0.5% of your money every year, not 0.5% of your growth.
"Cost of fees" is the difference between the final value with your fees and the final value with no fees at all. It is larger than the fees you would see deducted, because money taken in fees also loses all the growth it would have earned in later years.
"In today's money" divides the final value by the rise in prices over the period at the inflation rate you enter, so you can see roughly what the pot would buy at today's prices.
What is compound interest?
Compounding means earning growth on your earlier growth, not just on the money you put in. In the first year, growth is earned only on your contributions. In later years it is also earned on all the growth that has built up, which is why a balance can rise slowly at first and much faster towards the end. Time matters more than any other input: try changing the years from 20 to 30 and compare the result with doubling your monthly contribution.
Why small fees make a big difference
A fee of 1% a year sounds small next to a 5% growth rate, but it removes a fifth of that growth every year, and the effect compounds. Set fees to 0% and then to 1% to see the gap on your own numbers. Our guide to platform fees explains each type of charge and how to compare them.
What this calculator can't tell you
Real investments do not grow at a steady rate. Markets rise and fall, sometimes sharply, and the order of good and bad years changes the outcome. The calculator does not include tax, which depends on the account you use. ISAs and pensions shelter growth from UK tax; a general investment account may not. If you're just starting out, our beginner's guidecovers what to sort out first. Treat the results as a way to compare scenarios, not as a forecast of what you will get.
Capital at risk. Investments can fall as well as rise, and you may get back less than you put in. Our guides are general information and education, not personal financial advice. If you are unsure whether an investment is right for you, speak to a regulated financial adviser.