Calculating the return on cash savings in the UK requires understanding both mathematical interest formulas and the tax rules set by HM Revenue and Customs. Between compound interest, the Personal Savings Allowance, and the annual Individual Savings Account (ISA) limit, the headline interest rate quoted by a bank rarely represents the exact net return that reaches your pocket.
Understanding compounding frequency, tax bands, and ISA allowances ensures you retain more of your money. You can project your returns using the savings interest calculator and examine multi-year accumulation on the compound interest calculator.
Simple interest versus compound interest
Savings accounts grow through simple or compound interest:
- Simple interest: Calculated exclusively on the initial deposit (principal). The formula is: Interest = Principal x Annual Interest Rate x Time in Years
- Compound interest: Calculated on the initial principal plus all accumulated interest from previous periods. The formula is: Total Amount = Principal x (1 + Rate / Frequency) ^ (Frequency x Time)
The table below demonstrates the difference on a £10,000 deposit earning 5% annual interest over five years:
| Growth Mechanism | Year 1 Balance | Year 3 Balance | Year 5 Balance | Total Interest Earned |
|---|---|---|---|---|
| Simple Interest | £10,500.00 | £11,500.00 | £12,500.00 | £2,500.00 |
| Annual Compounding | £10,500.00 | £11,576.25 | £12,762.82 | £2,762.82 |
| Monthly Compounding | £10,511.62 | £11,614.72 | £12,833.59 | £2,833.59 |
Monthly compounding produces £333.59 more interest than simple interest over five years.
Financial institutions display the Annual Equivalent Rate (AER) to show what the interest rate would be if compounded annually, enabling comparison across accounts.
The Personal Savings Allowance (PSA) in 2026/27
Under UK tax rules, most individuals can earn a specified amount of interest on standard savings accounts without paying tax. This threshold is the Personal Savings Allowance.
Your allowance depends on your Income Tax band for the 2026/27 tax year:
| Income Tax Band | Taxable Income Threshold | Personal Savings Allowance | Tax Rate on Excess Interest |
|---|---|---|---|
| Basic Rate | £12,571 to £50,270 | £1,000 | 20% |
| Higher Rate | £50,271 to £125,140 | £500 | 40% |
| Additional Rate | Over £125,140 | £0 | 45% |
Basic rate taxpayers can earn up to £1,000 in savings interest per tax year with zero tax. Higher rate taxpayers receive a £500 allowance. Additional rate taxpayers receive no allowance, meaning every pound of non-ISA savings interest is taxed at 45%.
Individuals with non-savings income under £17,570 may qualify for the 0% starting rate for savings, providing up to an extra £5,000 of tax-free savings interest.
Banks and building societies do not deduct tax at source. Instead, they report annual interest directly to HMRC. HMRC collects tax owed by adjusting your PAYE tax code in the following tax year, or through your Self Assessment tax return.
The ISA allowance in 2026/27
The Individual Savings Account (ISA) allowance for the 2026/27 tax year is £20,000 per person.
All interest earned inside a Cash ISA is completely free from UK Income Tax. Key points to note:
- ISA interest does not count toward your Personal Savings Allowance.
- ISA interest does not count as taxable income when determining if you cross into higher tax brackets.
- Transfers between ISA providers do not consume your annual allowance, provided you use the official ISA transfer service.
Account comparison: cash balances before paying tax
When savings interest rates sit around 5%, standard savings accounts can breach your tax-free allowance with modest balances:
| Tax Band | Personal Savings Allowance | Maximum Balance at 5% Interest Before Tax | Net Return on £30,000 at 5% |
|---|---|---|---|
| Basic Rate (20%) | £1,000 | £20,000 | £1,400 (Effective 4.67%) |
| Higher Rate (40%) | £500 | £10,000 | £1,100 (Effective 3.67%) |
| Additional Rate (45%) | £0 | £0 | £825 (Effective 2.75%) |
A higher rate taxpayer holding £30,000 in a standard 5% savings account earns £1,500 gross interest. After the £500 Personal Savings Allowance is applied, the remaining £1,000 is taxed at 40%, creating a £400 tax bill. Placing that money inside a Cash ISA preserves the full £1,500 return.
Easy access versus fixed-rate bonds
When selecting an account structure, balance liquidity against tax timing:
- Easy access accounts: Allow withdrawals at any point. Interest rates are variable, fluctuating with Bank of England base rate adjustments. Interest is usually paid monthly or annually.
- Fixed-rate bonds: Lock your money away for a term of one to five years in exchange for a guaranteed interest rate. Withdrawals are generally prohibited during the term.
- The maturity trap: If a multi-year fixed-rate bond pays all accumulated interest on the final maturity date, that entire interest sum is treated as income in that single tax year. This sudden influx can breach your Personal Savings Allowance and push you into a higher income tax band. If possible, choose an option to pay interest annually to spread the taxable income across multiple tax years.
Frequently asked questions
When does HMRC collect tax on savings interest?
UK banks and building societies send interest records to HM Revenue and Customs after the end of the tax year on 5 April. For employees and pensioners taxed through PAYE, HMRC adjusts your tax code for the following tax year to collect the tax due. If you file a Self Assessment tax return, you declare your total savings interest on your return.
Does interest from a cash ISA count toward the Personal Savings Allowance?
No. Interest earned within an ISA is entirely exempt from UK tax and does not count toward your Personal Savings Allowance. You can earn unlimited interest inside an ISA without using any of your £1,000 or £500 allowance.
What happens if fixed-rate bond interest is paid at maturity?
HM Revenue and Customs assesses tax on savings interest in the tax year you can access the money. If a three-year fixed bond pays all accumulated interest at maturity, the full sum counts toward that specific tax year. This can cause you to exceed your allowance and pay higher rate tax on the interest.
Who is eligible for the 0% starting rate for savings?
The 0% starting rate for savings allows up to £5,000 of interest at zero tax. Eligibility depends on other earnings. If your non-savings income (such as wages or pensions) is below £17,570 in 2026/27, you can use the allowance. Every pound of wages above the £12,570 personal allowance reduces the £5,000 savings allowance by £1.