Free tool
The Compound Interest Visualiser
The most expensive money mistake in Britain isn't a bad purchase — it's waiting. Move the sliders and watch what starting early actually does. The teal line is you starting now; the copper line is you putting it off.
Start now
Start later
This is an illustration of how compounding works, not a prediction or advice. Real-world savings rates and investment returns vary, investments can fall as well as rise, and inflation quietly shrinks what future pounds buy. Nobody can promise you a growth rate — but the shape of these curves is mathematics, not marketing: whatever growth you get, time multiplies it. For free, impartial guidance on savings and pensions, MoneyHelper (the government-backed service) is a good place to start; for personal recommendations, speak to an FCA-regulated adviser.
What you're looking at
Compound interest is growth on your growth. Year one, your money earns a return. Year two, you earn a return on the original money and on last year's return. For ages, this does almost nothing visible — the first decade of the teal line looks flat and disappointing. Then the curve wakes up, because the growth itself starts out-earning your contributions. That late steepness is why the earliest pounds you ever save end up the most valuable ones — they're the pounds with the most compounding runway.
Notice something else in the chart: the copper "start later" line never catches up — not because the late starter saves less per month (they don't), but because their money never gets those missing years back. The gap between the lines is the price of waiting, and it's usually far bigger than the missed contributions alone.
If this has put you in the mood to find some money to start with: the ten-minute subscription audit typically frees £15–£40 a month, and the £1,000 emergency fund is the sensible first destination before anything longer-term.
Compound interest FAQs
Why does the line curve upwards instead of going straight?
A straight line is what you'd get with no growth — just your contributions stacking up (the dashed line). The curve is the compounding: each year's growth joins the pile and earns its own growth. The longer it runs, the steeper it gets.
What growth rate is realistic?
Honest answer: nobody knows in advance. Cash savings rates and investment returns both vary over time, and investments can lose money. Illustrations commonly use moderate single-digit rates, but treat any rate — including this tool's default — as a "what if", not a forecast. The early-start lesson holds at any positive rate.
Does this account for inflation?
No — the figures are in future pounds, which will buy less than today's. One common approach is to use a lower "real" growth rate (growth minus expected inflation) to see the answer in roughly today's money.
I'm older — is it too late to bother?
No. The best compounding decade is always the next one you actually use. Move the "wait" slider to 1 and compare 15 years against 14 — the difference is still real money. Late beats never by a wide margin.
Now find the money to compound
Our free printable budget planner helps you find the monthly amount — and gives it a job before it wanders off.