Savings calculators
See how your money grows over time with compound interest, and plan your £20,000 ISA allowance across a Lifetime ISA, a cash ISA and a stocks and shares ISA in one place.
The ISA allowance is one shared limit
You can pay £20,000 into ISAs in the 2026/27 tax year. That's a single allowance covering every ISA you hold, so £15,000 into a stocks and shares ISA leaves £5,000 for a cash ISA. Within it, a Lifetime ISA takes a maximum of £4,000, and the government adds a 25% bonus on top of whatever you pay in, worth up to £1,000 a year.
The allowance resets on 6 April and doesn't carry over. Anything unused on 5 April is gone for good, which is why contributions spike in the first week of April and the last week of March.
When you'll pay tax on savings interest
Interest earned outside an ISA can still be tax free thanks to the Personal Savings Allowance (a slice of interest each year that isn't taxed). Basic rate taxpayers get £1,000, higher rate taxpayers get £500, and additional rate taxpayers get nothing. Anything above your allowance is taxed at your normal income tax rate.
There's a separate starting rate for savings worth up to £5,000, which matters if your other income is low. Interest inside an ISA sits outside all of this: it's tax free and never uses up the allowance. At current rates, a basic rate taxpayer needs roughly £20,000 in an easy access account before interest starts being taxed at all, so an ISA earns its keep mainly on larger balances or at higher rates of tax.
Why compounding rewards time more than rate
Compound interest is interest paid on interest you've already earned. The effect is slow at first and then steep, because each year's growth is calculated on a larger balance than the last.
Starting earlier usually beats finding a slightly better rate. £200 a month for 30 years at 5% ends up well ahead of £200 a month for 20 years at 7%, despite the second one paying more. When comparing accounts, use the AER rather than the gross rate, since AER already accounts for how often interest is paid.
Which calculator to use
To model growth over time, or compare two saving plans side by side, use the compound interest calculator. To split this year's £20,000 across account types and see the Lifetime ISA bonus, use the ISA allowance planner.
For the detail behind the numbers, read the guides on the ISA allowance, tax on savings interest, and overpaying your mortgage versus investing.
Frequently asked questions
How much can I put into an ISA in 2026/27?
The allowance is £20,000 across every ISA you hold. It is one shared limit, so £15,000 into a stocks and shares ISA leaves £5,000 for a cash ISA. Of that £20,000, a maximum of £4,000 can go into a Lifetime ISA.
Does an unused ISA allowance carry over to next year?
No. The allowance resets on 6 April and anything you did not use is gone. That is why the weeks before 5 April see a rush of contributions. If you have the money available and the allowance spare, using it before the deadline is worth more than the equivalent contribution a week later.
Do I pay tax on savings interest outside an ISA?
Often not, because of the Personal Savings Allowance. Basic rate taxpayers can earn £1,000 of interest tax free each year, higher rate taxpayers £500, and additional rate taxpayers get nothing. Interest above your allowance is taxed at your normal income tax rate. Interest inside an ISA is always tax free and never counts towards the allowance.
Is a Lifetime ISA worth it?
The government adds 25% to whatever you pay in, so £4,000 becomes £5,000. The catch is what you can do with it. You can withdraw penalty free to buy a first home worth up to £450,000, or from age 60. Any other withdrawal takes a 25% charge, which claws back the bonus and a little of your own money on top.
What does compounding frequency change?
Interest paid monthly starts earning interest sooner than interest paid annually, so the same headline rate produces slightly more. The effect is small over a year and grows over decades. Compare accounts on AER, which already accounts for the frequency, rather than on the gross rate.
Should I save into an ISA or overpay my mortgage?
Compare your mortgage rate against the return you would get after tax. Overpaying at 4.5% is the equivalent of a guaranteed tax free 4.5% return, which beats most cash savings. Against a stocks and shares ISA over 20 years the sums usually favour investing, though the return is not guaranteed and the mortgage saving is.