Every example at a glance
3 scenarios, all worked the same way. Tap a row to jump to the full breakdown.
| Plan | Final value | Paid in | Interest earned |
|---|---|---|---|
| £5,000 at 7% for 20 years | £20,194 | £5,000 | £15,194 |
| £10,000 at 5% for 10 years | £16,470 | £10,000 | £6,470 |
| £10,000 plus £100 a month at 5% for 10 years | £31,998 | £22,000 | £9,998 |
How it is worked out
A lump sum left to compound follows the standard growth formula, with monthly compounding:
- P — the starting amount
- r — the monthly rate: the annual rate ÷ 12
- n — the number of months
Where money is added each month, every contribution is its own small lump sum compounding for however many months remain, and the plan's value is the sum of all of them. That is why the contributions row ends up so far ahead: £12,000 of deposits adds £15,528 to the final value, because the early ones have had nearly the full ten years to work.
Time matters more than rate over long runs. £5,000 at 7% for 20 years turns £5,000 into £20,194 — quadrupling it — while £10,000 at 5% for 10 years gains only 65%. Doubling the term does more than adding two percentage points.
Each example in full
Every card below opens in the Compound Interest Calculator with its own numbers already filled in, so you can change one figure and see what moves.
£5,000 at 7% for 20 years
Twenty years at 7% is roughly the long-run equity assumption people use for retirement sums, and it quadruples the money without a single contribution.
Open this one in the Compound Interest Calculator →£10,000 at 5% for 10 years
A lump sum left completely alone is the cleanest way to see compounding on its own, with no contributions muddling where the growth came from.
Open this one in the Compound Interest Calculator →£10,000 plus £100 a month at 5% for 10 years
The same £10,000 with £100 added every month, which is the realistic version — and it shows how quickly regular contributions overtake the starting balance.
Open this one in the Compound Interest Calculator →What these figures do not include
- Inflation. Every figure is in nominal pounds; at 3% inflation the £16,470 in ten years buys roughly what £12,300 buys today.
- Tax on interest or gains, which depends on the wrapper the money sits in.
- Fees and charges, which come out of the return before it compounds.
- Any assumption that the rate holds. A fixed savings rate is knowable; an investment return is not, and a steady percentage is a modelling convenience rather than a forecast.
- Sequence of returns. Two investments averaging the same annual return can end at very different values depending on when the good and bad years fall.
Frequently asked questions
Do contributions matter more than the rate?+
Over shorter horizons, usually yes. Adding £100 a month to the £10,000 plan nearly doubles the ten-year value, from £16,470 to £31,998 — an effect far larger than any realistic rate improvement would deliver over the same period.
Why does the 20-year plan beat the 10-year one so heavily?+
Because compounding accelerates. £5,000 at 7% for 20 years earns £15,194 in interest, three times the original sum, while £10,000 at 5% for 10 years earns £6,470. Late years each add far more than early ones, so the last stretch of a long run does the heavy lifting.
Are these figures adjusted for inflation?+
No, they are nominal. To see what a projection is worth in today’s money, subtract expected inflation from the growth rate before running it — 5% growth with 3% inflation is closer to 2% in real terms.