Money Tips & Guides · September 2026
Compound Interest Explained: How Growth Builds Over Time
Capital at risk. The value of investments can fall as well as rise, and you may get back less than you put in. Past performance is not a guide to future returns.
FSCS protects eligible deposits up to £120,000 per person, per authorised institution.
Compound interest explained
A plain-English UK guide to simple vs compound interest, why time and top-ups matter, how AER fits in, and how savings compounding differs from investments.
Last updated: .
Compound interest means earning interest on your original balance and on interest already added. Over time that can make a bigger difference than “simple” interest on the starting sum alone.
Quick answer
- Compound interest = interest on interest (if left in).
- Time and regular additions matter more than obsessing over tiny day-one differences.
- AER is the UK comparison tool that reflects compounding assumptions — see AER explained.
- Savings interest is not the same as investment growth; capital is at risk on investments.
Key decision routes
Related reading: Money Tips & Guides, Savings, AER explained, Inflation explained, and our compound interest calculator under Money Tools.
Simple vs compound interest
| Idea | Plain meaning |
|---|---|
| Simple interest | Interest calculated on the original amount only |
| Compound interest | Interest calculated on principal plus interest already credited (if left in the account) |
| Paid away | Interest sent to another account — may not compound in the savings pot unless you put it back |
Most UK savings accounts credit interest into the balance (or offer a choice). Compounding only works on money that stays in to earn again.
Why time matters
In the early years, compounding often looks small in pounds. Over longer horizons, the “interest on interest” layer can grow if the rate and balance hold and you keep adding.
Regular top-ups amplify the effect more than waiting for a perfect rate — that is an educational habit, not personal advice.
Withdrawals shrink the base that future interest can grow on.
Even compounding savings can lose real purchasing power if inflation outpaces the rate — see Inflation explained.
AER and how often interest is added
AER
AER (Annual Equivalent Rate) illustrates the gross savings rate as if compounded annually. It is the comparison yardstick on many UK savings ads.
Deep dive: AER explained. Compare AER with AER, not AER with a random gross figure from another advert.
Frequency of compounding
Interest may be calculated or paid daily, monthly, quarterly or yearly depending on the product.
More frequent compounding can push the AER slightly above the gross rate. AER already folds that into the yearly illustration.
If interest is paid away to another account, frequency can help with income timing — but that pot will not compound unless the interest comes back in.
Interest paid in vs paid away
How this differs from investment returns
Savings interest (subject to terms) is typically a stated rate on cash. Eligible deposits may have FSCS protection up to £120,000 per person per authorised firm from 1 December 2025 (source: FSCS). That protection is about firm failure — not inflation or rate changes.
Investments (shares, funds and similar) can rise or fall. Past performance is not a guide to future results. Capital is at risk.
“Compounding” language appears in investing too (for example reinvested dividends), but that is not the same as a savings AER promise.
This page does not recommend moving cash into investments to “beat” compounding, and it does not pick products.
Worked example
Example 1: interest on a growing balance
Start with £1,000. Assume an illustrative 4% a year, with interest left in the pot (a simple yearly table — real products may compound more often):
| End of year | Approximate balance (illustrative) | Rough interest that year |
|---|---|---|
| 1 | £1,040 | £40 on £1,000 |
| 2 | £1,082 | about £42 on £1,040 |
| 5 | about £1,217 | Interest each year is calculated on a larger base |
Year-one interest looks modest. By year five, more of the growth comes from interest already earned — if you leave it in and the illustrative rate holds.
Variable rates change and fixed terms end.
Example 2: top-ups vs a lump sum
Same starting £1,000 and illustrative 4%. One path adds nothing; another adds a small monthly top-up.
The topped-up path usually ends higher because more money sits earning — not because of magic. Outcomes are not guaranteed.
Try your own numbers on the compound interest calculator — still illustrations, not forecasts.
The catch
Common mistakes
- Expecting huge pound growth in month one
- Comparing gross on one account with AER on another
- Assuming an illustrative rate lasts forever (variable rates change; fixed terms end)
- Ignoring inflation when celebrating nominal growth
- Treating investment charts like guaranteed compound savings
- Paying interest away then wondering why the pot did not compound
- Using borrowing to “feed” a savings compound story — this guide stays on savings education only
When to check MoneyHelper and your provider
Use MoneyHelper for independent savings and budgeting explainers.
Read your provider’s key information for how often interest is paid and whether it is added to the balance.
For purchasing-power context, see ONS data and Inflation explained. More on everyday savings products: Savings.
This page is general information. For large sums or investment decisions, consider regulated advice.
Summary / key takeaways
- Compound interest = interest on interest (if left in).
- Time and regular additions matter more than obsessing over tiny day-one differences.
- AER is the UK comparison tool that reflects compounding assumptions — see AER explained.
- Savings interest is not the same as investment growth; capital is at risk on investments.
- Illustrations are teaching tools; no guaranteed returns.
- Check MoneyHelper and provider documents.
- Tools and more reading: compound interest calculator, Money Tools, Money Tips & Guides.
FAQ
Is compound interest free money?
No. It is interest calculated on a growing balance when interest is left in. Rates can change, and inflation can still reduce real buying power.
Does AER already include compounding?
AER illustrates the rate as if compounded annually, so it helps you compare accounts with different payment frequencies. See AER explained.
Will my savings definitely grow like the table?
No. The table is illustrative. Variable rates change; fixed deals end; you may withdraw money.
Is investing the same as compound interest on savings?
No. Investments can fall as well as rise. Capital is at risk. Do not treat investment charts as a guaranteed savings-style compound story.
Does FSCS guarantee my compound growth?
No. FSCS is about eligible deposits if a firm fails (£120,000 from 1 December 2025). It does not lock in a rate or protect against inflation.
Where can I try the maths myself?
Use our compound interest calculator for scenarios — not as a personal forecast.
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