Compound Interest Calculator

The eighth wonder of the world

Compound interest is often called exactly that, because your money earns growth, and then that growth earns growth of its own. The longer you leave it, the more the curve bends upwards, so the final total is driven as much by time as by how much you put in.

Two habits that make the biggest difference

• Start early. Even small amounts have decades to compound. See pound cost averaging for the simplest way to keep it going.
• Keep contributing. Regular monthly saving quietly does the heavy lifting, as the calculator shows when you raise the monthly figure.

New to investing? Start with our plain-English guide to investing for beginners in the UK. Saving towards a specific target instead of open-ended growth? The savings goal calculator works backwards from the amount you want.

The formula, in plain English

For a single lump sum, compound growth is final = start × (1 + rate) raised to the number of years. Add regular monthly saving and each contribution compounds for however long it stays invested, which is why the calculator works month by month rather than with one tidy sum. The maths matters less than the habit: pay in steadily and leave it alone.

A worked example

Start with £1,000, add £200 a month, and assume 5% a year for 20 years. You pay in £49,000 over that time, and it grows to about £84,900. Roughly £36,000 of the total is growth you never contributed, the compounding doing the heavy lifting. Stretch the same habit to 30 years and the growth comfortably outweighs what you put in.

What this calculator assumes

• A steady average return, compounded monthly. Real investments rise and fall, sometimes sharply, so read the line as a smooth long-run average rather than a straight path.
• The rate is before charges and tax. A platform or fund fee of even 1% a year takes a real bite out of the total over decades.
• If the rate you use is above inflation, treat the projected value as roughly today’s money.

Frequently asked questions