Cash ISA calculator
See how a Cash ISA grows tax-free over time, how much of your £20,000 annual ISA allowance you're using, and roughly how much tax you're avoiding by keeping it in the wrapper.
Final balance (entirely tax-free)
£51,776
Where your balance comes from
Total contributed
£41,000
Tax-free interest earned
£10,776
You're contributing £3,600 a year — 18% of the £20,000 annual ISA allowance.
Because it's in an ISA, none of this interest is taxable. Outside an ISA, based on £30,000 of other income, you'd owe an estimated £192 in tax just on your final year's interest alone — see the Savings Tax calculator for the full picture.
Balance over time
Move across the chart to inspect any point.
Assumes a fixed interest rate and consistent monthly contributions, compounding monthly, and that contributions stay within the annual ISA allowance in each tax year. The tax comparison is illustrative — it applies the current Personal Savings Allowance rules to your final year's interest only, not a full multi-year simulation. Not financial advice.
Why a Cash ISA is different from an ordinary savings account
Interest earned in an ordinary savings account counts as taxable income, shielded only by your Personal Savings Allowance (£1,000 for basic-rate taxpayers, £500 for higher-rate, nothing for additional-rate). A Cash ISA sits outside that system entirely — every penny of interest is tax-free, no matter how large the balance grows or what rate of tax you otherwise pay.
The trade-off is the £20,000 annual allowance for paying new money in (existing ISA balances keep growing tax-free with no cap) — and that allowance is shared across every ISA you hold, not £20,000 per account.
Frequently asked questions
Related calculators
Savings tax
See exactly how much tax you'd pay on the same interest outside an ISA.
Lifetime ISA
Saving for a first home or retirement? The Lifetime ISA adds a 25% government bonus.
Junior ISA
Saving for a child instead? The Junior ISA has its own £9,000 allowance.