TL;DR - Compound Interest UK
- Compound interest is interest calculated on both the original sum (principal) and the interest already earned - meaning your interest earns interest, accelerating growth over time
- For UK savings: a £10,000 deposit at 4.5% interest compounded annually becomes £15,530 after 10 years - compared to £14,500 with simple interest
- Compounding frequency matters - daily compounding produces slightly more than monthly, which produces more than annual - most UK savings accounts compound daily or monthly
- For debt: compound interest works against you - credit card debt at 25% APR compounding monthly means £1,000 becomes £1,280 after one year if no payments are made
- ISAs benefit from compound interest tax-free - interest, dividends, and capital gains within a stocks and shares ISA or cash ISA compound without income tax or capital gains tax
- The formula: A = P(1 + r/n)^(nt) where P=principal, r=annual rate, n=compounding periods per year, t=years
Last reviewed: June 2026 - Sources: Bank of England, HMRC, FCA
KEY FACTS - COMPOUND INTEREST UK 2026 | |
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Compound interest is the process of earning interest on both the original amount saved or borrowed (the principal) and on the interest that has already accumulated. It is sometimes described as "interest on interest" and is the fundamental mechanism behind long-term savings growth and the accelerating cost of unpaid debt.
The Compound Interest Formula
The standard formula for compound interest is:
Where:
- A = the final amount (principal + interest)
- P = the principal (starting amount)
- r = the annual interest rate as a decimal (e.g. 4.5% = 0.045)
- n = the number of times interest is compounded per year (daily=365, monthly=12, annually=1)
- t = the number of years
UK Savings Examples
| Starting Amount | Rate | After 5 years | After 10 years | After 20 years |
|---|---|---|---|---|
| £5,000 | 4.5% annual | £6,230 | £7,765 | £12,063 |
| £10,000 | 4.5% annual | £12,462 | £15,530 | £24,117 |
| £20,000 | 4.5% annual | £24,925 | £31,061 | £48,234 |
| £10,000 | 5.0% annual | £12,763 | £16,289 | £26,533 |
These figures use annual compounding. Accounts that compound daily or monthly will produce slightly higher returns. The figures do not account for tax on interest above the Personal Savings Allowance (£500 for basic rate taxpayers, £0 for additional rate taxpayers in 2026-27) or within an ISA wrapper (tax-free).
How Compounding Frequency Affects Returns
Most UK savings accounts compound interest either daily or monthly. The more frequently interest is compounded, the higher the effective annual return. The difference is small at lower rates but becomes more significant over longer periods.
For a £10,000 deposit at 4.5% over 10 years:
- Annual compounding: £15,530
- Monthly compounding: £15,594
- Daily compounding: £15,600
When comparing savings accounts, look at the AER (Annual Equivalent Rate) rather than the gross rate - the AER standardises for compounding frequency and shows the true annual return, making accounts directly comparable regardless of how frequently they compound.
Compound Interest on ISAs
Individual Savings Accounts (ISAs) allow compound interest to accumulate tax-free. The annual ISA allowance is £20,000 per person in 2026-27. Within a cash ISA or stocks and shares ISA, interest, dividends, and capital gains compound without income tax or capital gains tax liability - which significantly increases long-term returns compared to a taxable account at the same rate.
For a basic rate taxpayer with savings above £500 in interest per year, using an ISA wrapper means the full 4.5% compounds - whereas outside an ISA, effective compounding is reduced by 20% tax on interest above the allowance.
Compound Interest on Debt
Compound interest works against borrowers. On credit card debt, interest is typically compounded monthly. At 25% APR compounded monthly, £1,000 of unpaid debt becomes approximately £1,280 after 12 months with no payments - and £3,292 after 5 years.
This is why the FCA introduced persistent debt rules requiring lenders to contact customers who have been making only minimum payments for 18 months - minimum payments on high-rate cards may not reduce the principal meaningfully when interest compounds at high rates.
For mortgages, compound interest works differently - the monthly payment is calculated to repay both interest and principal over the mortgage term, so the outstanding balance reduces progressively. Overpaying a mortgage reduces the principal and therefore the interest charged in subsequent periods - the compounding effect works in the borrower's favour when overpaying.
Related Guides
Disclaimer: Kaeltripton.com is an independent editorial publisher. This guide is factual information only and does not constitute financial advice. Savings rates change frequently. Figures shown are illustrative calculations only.
What is compound interest?
Compound interest is interest calculated on both the original principal and the interest already accumulated. Unlike simple interest (calculated only on the principal), compound interest means your interest earns interest - accelerating growth over time for savings, and accelerating the cost of unpaid debt.
What is the compound interest formula?
A = P(1 + r/n)^(nt). P is the principal, r is the annual interest rate as a decimal, n is the number of compounding periods per year, and t is the number of years. A is the total amount including principal and interest.
How does compound interest work on UK savings accounts?
Most UK savings accounts compound daily or monthly. The AER (Annual Equivalent Rate) shows the standardised annual return accounting for compounding frequency - use AER to compare accounts. ISAs compound tax-free, making them more effective than taxable accounts for long-term saving above the Personal Savings Allowance.
How does compound interest affect debt?
On credit card debt compounding monthly at 25% APR, £1,000 unpaid grows to approximately £1,280 after one year. Making only minimum payments may not meaningfully reduce the principal at high rates. Paying more than the minimum each month reduces principal faster and limits the compounding effect.
Sources: Bank of England base rate June 2026 (bankofengland.co.uk); HMRC ISA and savings allowances 2026-27 (gov.uk); FCA Consumer Credit sourcebook CONC persistent debt rules; FCA AER definition (fca.org.uk).
