A fixed-rate ISA means you can lock in a certain savings rate for a certain amount of time – giving you the stability and reassurance of a set amount of interest. But, it’s harder to withdraw money than with an easy access savings account and, if you spot a better deal, you’ll likely have to pay a penalty to get out early.
Our fixed-rate ISA calculator estimates whether paying the get-out penalty on your existing fixed-rate ISA is worth it to open a new one at a better rate.
Please note that these are approximate calculations and you should seek exact figures from both ISA providers before making any decision. The calculations do not take into account any delay in transferring the funds, and assume that interest compounds monthly.
1. Enter the current balance in your fixed-rate ISA.
2. Enter the interest rate on your fixed-rate ISA.
3. Add how long is left on the fixed term (e.g 1 year 2 months or 0 years 7 months).
4. Add your specific penalty – either the days of interest (e.g. 90 days) OR the fee as a fixed number (e.g. £50). You can check the penalty with your account provider if you are unsure.
The calculator will tell you how much you could make or lose by switching, and therefore whether it’s worth paying the penalty.
It also breaks down how much you would earn in interest from the current ISA, the penalty you’d pay for transferring, and how much you’d earn in interest from the new ISA so you can see how you’d either make or lose money.
The calculator assumes that interest is compounded monthly and there is no delay in transferring funds. It’s an estimate, so you should always check with the provider directly.
With a fixed-rate ISA, the provider promises to pay the agreed rate of interest for a set period as long as you keep your money in the account. Common lengths for a fixed-rate ISA include 1 year, 2 years, 3 years and 5 years. If you choose to withdraw before the end of the fixed term, you will typically have to pay a penalty fee.
Like other types of cash ISA, you don’t pay any tax on the interest you earn, but there is a limit to the amount you can place into any ISAs in a tax year. For the 2025/2026 tax year, the ISA allowance is £20,000.
For the 2025/2026 tax year, the personal savings allowance is as follows:
Our personal savings allowance calculator shows whether you could benefit from moving some of your savings into an ISA. This will be the case if you earn more interest than your personal savings allowance and are therefore paying tax.
When interest rates are high, it can be easier to go over your personal savings allowance, so you might want to consider an ISA.
Matt Mckenna
UK Head of Communications
T: +44 20 8191 8806
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