Compound interest calculator

Growth on your growth. In year one your money earns a return; in year two the return earns its own return, and so on. Early on the effect looks unremarkable - the default example above earns more in its final five years than in its first ten. That back-loading is why starting early and staying invested matter more than picking the perfect fund.

We treat your growth rate as an annual figure and convert it to an exact monthly equivalent, then add your contributions at the end of each month. So 5% a year means precisely 5% after twelve months - unlike calculators that divide by twelve, which quietly overstate the outcome.

Diversified investment portfolios have historically returned around 5-7% a year before inflation and fees over long periods; equity-heavy portfolios sat at the top of that range with bigger swings along the way. Cash savings accounts return far less, especially after inflation. Whatever you pick, it’s an assumption, not a promise - try a range and look at the spread.

Because £107,694 in twenty years won’t buy what £107,694 buys today. At 2.5% inflation it buys what £65,722 buys now - still excellent, but a very different number to plan around. Most calculators only show the big future figure; we think you should see both.

Compounding works on costs too. £300 a month for 30 years at 7% grows to £350,836 with no fees - but £292,354 if 1% a year goes in charges. That’s £58,482, roughly 16 years of those contributions, for nothing you’d ever notice on a statement. Set the fees field to your fund’s real number and look.

Yes. A stocks and shares ISA is a wrapper around exactly this kind of investing, with growth and withdrawals free of UK tax. The annual ISA allowance is £20,000 (about £1,666 a month), so most of the scenarios this calculator models fit entirely inside one. Above that, pensions and general accounts enter the picture.

It raises your monthly contribution once a year, the way a pay rise naturally could. It’s quietly powerful: £300 a month at 7% for 30 years reaches £350,836 flat, but £481,438 if the contribution rises just 3% a year - an extra £130,000 for increases you’d barely feel.

No. The calculator runs entirely in your browser and nothing you type is sent to our servers. If you choose to email yourself the results, we send your email address and the link to your scenario - that’s the only data that leaves the page, and only when you ask.