What Is Compound Interest? The Complete Guide

Compound interest is one of the most important financial concepts to understand — whether you're saving money or paying off debt. Often called "interest on interest," it's the reason small, consistent savings can grow into significant wealth over decades — and also why credit card debt spirals so fast. Here's everything you need to know.

Simple Interest vs Compound Interest: The Key Difference

Simple interest is calculated only on the original amount (the principal). If you deposit £1,000 at 5% simple interest, you earn £50 every year — the same amount each year, forever.

Compound interest is calculated on both the principal and the interest already earned. So in year 1 you earn £50, in year 2 you earn interest on £1,050 (not £1,000), in year 3 on £1,102.50, and so on. The balance snowballs over time.

The Compound Interest Formula

• A = Final amount (principal + interest)
• P = Principal (starting amount)
• r = Annual interest rate (as a decimal, e.g. 5% = 0.05)
• n = Number of times interest compounds per year
• t = Number of years

You don't need to do this by hand — our free compound interest calculator does it instantly for any amount and rate.

How Compounding Frequency Affects Growth

The more often interest compounds, the more you earn. Here's how £10,000 at 5% annual interest grows over 10 years, depending on how often it compounds:

The difference between annual and daily compounding is relatively small, but it adds up over very long timeframes. For most savings accounts, the practical difference is a few hundred pounds over 10 years — meaningful, but not dramatic.

The Rule of 72 — Quick Mental Maths

The Rule of 72 lets you estimate how long it takes to double your money without a calculator. Simply divide 72 by the annual interest rate:

• At 4%: 72 ÷ 4 = 18 years to double
• At 6%: 72 ÷ 6 = 12 years to double
• At 8%: 72 ÷ 8 = 9 years to double
• At 10%: 72 ÷ 10 = 7.2 years to double

This also works for debt. Credit card debt at 25% APR doubles in under 3 years if unpaid — which explains why minimum payments barely touch the balance.

Where You'll Encounter Compound Interest in Real Life

Working in your favour:

• ISAs and savings accounts — interest compounds on your deposits
• Pension funds — investment returns compound over decades
• Stocks and index funds — dividends reinvested compound growth
• Premium Bonds — prizes reinvested compound the effective return

Working against you:

• Credit cards — often compound daily on unpaid balances
• Personal loans — interest front-loaded in repayment schedules
• Buy Now Pay Later — deferred interest can compound if not paid by deadline
• Overdrafts — can compound daily at high rates

How to Make Compound Interest Work For You

• Start early. The most powerful factor in compounding is time. £100/month from age 25 at 7% grows to ~£262,000 by age 65. Starting at 35 gives you only ~£121,000 — less than half, for just 10 fewer years.
• Never interrupt it. Withdrawing savings resets your compounding base. Keep long-term savings locked away.
• Reinvest returns. In investment accounts, always reinvest dividends rather than taking them as cash.
• Pay off high-interest debt first. Compound interest on debt at 20%+ destroys wealth faster than almost any investment can build it.

Frequently Asked Questions