Compound Interest Explained for UK Savers

What compound interest actually is

Compound interest is interest calculated on the original principal plus the interest that has already accumulated. In plain English: you earn interest on your interest.

The difference between that and simple interest (which only pays on the original principal) sounds trivial for one year. Over a 40-year pension pot it transforms the outcome — and that's why every serious UK wealth-building vehicle, from Cash ISAs to workplace pensions, is built on compounding. Use our Compound Interest calculadora to plug in real numbers.

The formula you only need to remember once

Future Value (FV) = P × (1 + r/n)^(n × t)

Where:

• P — starting principal (£)
• r — annual interest rate as a decimal (4% = 0.04)
• n — compounding periods per year (monthly = 12, daily = 365)
• t — time in years

Worked example — £10,000 at 5% for 10 years

Simple interest: £10,000 × 5% × 10 = £5,000 of interest. Total = £15,000.

Annually compounded: 10,000 × (1.05)^10 = £16,288.95.

Monthly compounded at 5% AER: 10,000 × (1 + 0.05/12)^120 = £16,470.09.

The £1,470 gap between simple and monthly compound is the "interest on interest" part — and it's why the rate (5%) alone doesn't tell the whole story.

The rule of 72 — a 5-second sanity check

Divide 72 by your annual rate to get the number of years for money to double.

At 3% AER → 24 years.

At 6% AER → 12 years.

At 10% AER → 7.2 years.

It's close enough to be useful in your head, and it instantly exposes inflation-era problems: with cash yielding 1% real (4% nominal minus 3% inflation), your real money doubles every 72 years — in other words, barely.

UK wrappers that supercharge compounding

Stocks & Shares ISA — £20,000 annual allowance (2026/27). Gains and dividends are UK tax-free, so compounding isn't dragged down by Capital Gains Tax or dividend tax.

Workplace pension — employer contributions plus 20% tax relief at source means £100 into the pot can cost a basic-rate taxpayer £80 of take-home. The match alone multiplies your effective return.

Lifetime ISA (LISA) — 25% government bonus up to £4,000/year. Only useable for a first-time-buyer property under £450,000 or from age 60 for retirement.

Regular Saver — time-limited high-rate accounts (often 6–8%) but capped monthly contributions and only one year typically — the effective interest earned is less than the headline.

Regular contributions — the compounding multiplier

Most real savers don't drop a lump sum once; they drip-feed. The formula for FV with regular monthly contributions PMT is:

FV = P × (1 + r/n)^(n t) + PMT × [((1 + r/n)^(n t) − 1) / (r/n)]

Example: £100/month for 25 years at 6% AER = £69,299. Of that, £30,000 is your own contributions and £39,299 is compound interest earned.

The two biggest killers of compound returns

1) Fees. A 1.5% annual fund charge versus a 0.2% charge looks small in year one, but over 30 years eats roughly a third of your pot. The calculadora on this page lets you add a fee rate and see the bite.

2) Inflation. If inflation runs at 3% and your savings earn 3%, your money stays the same in real terms. The compound maths still works, but only nominally. Always compare the real return (rate minus inflation), especially for long-dated plans.

Frequent-compounding myth — does daily beat monthly?

Only marginally. The difference between monthly and daily compounding at 5% over ten years on £10,000 is under £10. What actually matters is the AER (Annual Equivalent Rate) — that's the rate banks are required to quote so different products can be compared fairly.

Focus on AER, contribution frequency and fees. Compounding frequency inside that AER is noise.

Putting it into practice

• Start early — a £100/month pension started at 25 vs 35 ends up ~£60,000 richer at 65 for the same 6% AER
• Automate contributions so you don't rely on willpower
• Choose low-cost index funds — every basis point of fee compounds against you
• Re-invest dividends inside tax wrappers, not out of them
• Run our Compound Interest calculadora with realistic rates and a fee bar — not a best-case scenario