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ISA & savings rate calculator
Money sitting in a low-rate account costs you twice: a worse rate, and tax on the interest once you pass your allowance. Enter your savings to see what a better rate and a tax-free ISA are worth to you in a year.
Your tax band sets your Personal Savings Allowance: £1,000 of interest tax-free at the basic rate, £500 at the higher rate, nothing at the additional rate. Inside a cash ISA all the interest is tax-free, whatever you earn.
Switching could leave you
£600
better off a year on £20,000 of savings
- Extra interest from 4.5% vs 1.5%£600
- Tax saved by using an ISA£0
At 4.5% your £20,000 earns £900, which sits within your £1,000 Personal Savings Allowance, so there is no tax to pay yet. Grow your balance or your rate and an ISA starts to shield interest from tax.
Figures are for the 2026/27 tax year and assume the rate holds for a full year. A cash ISA caps deposits at £20,000 a year.
Common questions
- How much can I put in an ISA each year?
- You can pay up to £20,000 across your ISAs in the 2026/27 tax year. That total is shared between cash ISAs, stocks and shares ISAs, innovative finance ISAs and lifetime ISAs (the lifetime ISA has its own £4,000 cap inside the £20,000). The allowance resets every 6 April and you cannot carry an unused part into the next year.
- What is the Personal Savings Allowance?
- It is the amount of savings interest you can earn outside an ISA before any tax is due. Basic-rate taxpayers get £1,000 a year tax-free, higher-rate taxpayers get £500, and additional-rate taxpayers get nothing. Interest above your allowance is taxed at your usual income-tax rate (20%, 40% or 45%). Inside a cash ISA, none of the interest counts towards this, so it is always tax-free.
- Is a cash ISA worth it if I am under my allowance?
- If your interest stays comfortably under your Personal Savings Allowance, an easy-access or fixed-rate account often pays a higher headline rate than an equivalent cash ISA, and the interest is tax-free anyway. The ISA earns its keep once your balance or rate grows enough to breach the allowance, or if you expect to move into a higher tax band. The advantage also compounds: an ISA shelters that money for good, year after year.
- Easy-access or fixed-rate: which should I pick?
- Easy-access lets you withdraw any time and the rate can change. A fixed-rate (fixed-term) account locks your money away for a set period, often one to five years, usually for a higher rate, with a penalty for early access. Keep money you may need soon in easy-access; only fix cash you are confident you can leave alone for the full term.
- Does the headline figure include both benefits?
- Yes. The big number combines the extra interest a better rate earns against your current rate, plus the tax you avoid by holding that better-rate cash inside an ISA rather than a taxed account. Both are shown separately underneath so you can see where the gain comes from.
Not sure whether to chase a better rate, clear debt, or top up your pension first? The Money Health Check helps you decide where this cash works hardest.