ISA Calculator
See what your ISA could be worth, with every penny of the growth free of tax, and how much of your allowance you are actually using. Cash ISA limits change in April 2027.
| Total paid in | £64,000 |
|---|---|
| Growth | £36,264 |
| Tax | None |
| ISA value | £100,264 |
You can pay £20,000 into a cash ISA this tax year, but from 6 April 2027 that falls to £12,000 a year, which is £1,000 a month. The overall £20,000 ISA allowance is unchanged: the other £8,000 can still be paid in, but only into a stocks and shares ISA.
- What you paid in
- Growth, all tax free
After 15 years the ISA reaches £100,264. Of that, £36,264 (36.2%) is growth you never paid in, and none of it is taxed.
| Year | ISA value | Paid in | Growth |
|---|---|---|---|
| 0 | £10,000 | £10,000 | £0 |
| 1 | £14,181 | £13,600 | £581 |
| 2 | £18,574 | £17,200 | £1,374 |
| 3 | £23,184 | £20,800 | £2,384 |
| 4 | £28,025 | £24,400 | £3,625 |
| 5 | £33,108 | £28,000 | £5,108 |
| 6 | £38,449 | £31,600 | £6,849 |
| 7 | £44,053 | £35,200 | £8,853 |
| 8 | £49,937 | £38,800 | £11,137 |
| 9 | £56,116 | £42,400 | £13,716 |
| 10 | £62,611 | £46,000 | £16,611 |
| 11 | £69,423 | £49,600 | £19,823 |
| 12 | £76,576 | £53,200 | £23,376 |
| 13 | £84,086 | £56,800 | £27,286 |
| 14 | £91,984 | £60,400 | £31,584 |
| 15 | £100,264 | £64,000 | £36,264 |
Information
An ISA is a wrapper, not a product. Money held inside one grows free of Income Tax and Capital Gains Tax, there is nothing to declare and nothing to put on a tax return. That shelter is unconditional: it does not matter what you earn, what rate you pay, or how big the pot grows. It is why this page never asks for your income, and why the savings calculator has to.
The price of the shelter is a limit. You may pay in at most £20,000 per tax year across all of your ISAs combined, which is about £1,666 a month. That allowance has been frozen since 2017/18, and it does not carry forward: whatever you have not used by 5 April is gone on 6 April. What is already inside the wrapper is untouched by any of this. It stays sheltered and keeps growing, however large it gets.
The cash ISA limit is being cut in April 2027. This is the change most savers have not heard about. From 6 April 2027 you may pay at most £12,000 a year into a cash ISA if you are under 65, down from £20,000. The overall ISA allowance is unchanged, so the other £8,000 can still be paid in, but only into a stocks and shares ISA. Savers aged 65 and over keep the full £20,000 for cash. If you are paying more than £1,000 a month into a cash ISA today, this will bite you, and the calculator above shows you exactly when and by how much.
The limit applies to what you pay in, never to what you hold. There is no cap at all on what an ISA may be worth. A pot built up over decades can run well into seven figures and every penny of it stays sheltered. The £20,000 limits the flow, not the stock, and this trips up more people than any other ISA rule.
What's simplified. One flat return is held for the whole term, whereas a cash ISA rate moves with the base rate and a stocks and shares ISA rises and falls with the market, sometimes for years at a time. Platform and fund fees are not modelled, and on a stocks and shares ISA they compound against you. The contribution is held flat, and the figures are in future pounds, not adjusted for inflation. Open the full planner to model a variable return, fees, a rising contribution, and your ISA in today's money.
FAQ
- Do I pay any tax on an ISA?
No. Interest, dividends and capital gains inside an ISA are all free of tax, there is nothing to declare and nothing to put on a tax return. It does not matter what you earn or what rate you pay: the shelter is unconditional. That is the whole point of the wrapper, and it is why this calculator does not ask for your income, while the savings calculator has to.
- Is the cash ISA limit really being cut to £12,000?
Yes, from 6 April 2027. The overall ISA allowance stays at £20,000 a year, but from that date at most £12,000 of it may go into a cash ISA if you are under 65. The other £8,000 can still be paid in, but only into a stocks and shares ISA. Savers aged 65 and over keep the full £20,000 for cash. If you are paying more than £1,000 a month into a cash ISA today, this is the change that will bite you, and the calculator shows you exactly when.
- How much can I pay into an ISA?
£20,000 per tax year across all of your ISAs combined, which works out at about £1,666 a month. It has been frozen at that since 2017/18. The allowance does not carry forward: whatever you have not used by 5 April is gone on 6 April, which is why providers see a scramble of contributions in the last week of the tax year. From 6 April 2027 the cash sub-limit above applies on top.
- Does the limit apply to what I have saved, or what I pay in?
Only to what you pay in. There is no cap at all on what an ISA may be worth: a pot built up over decades can run into seven figures and every penny of it stays sheltered. The £20,000 limits the flow, not the stock. This trips a lot of people up, so the calculator will happily model an ISA worth far more than the allowance, and is right to.
- Cash ISA or stocks and shares ISA?
This calculator will model either, and the arithmetic is the same; what differs is what the rate means. On a cash ISA it is an interest rate you are offered, and you will get it. On a stocks and shares ISA it is a return you are assuming, and real markets rise and fall, sometimes for years. Treat the stocks and shares figure as an average over a long horizon, not a promise, and do not use a long-run equity number for money you need soon. The two also differ in how much you may pay in, once the cash cut lands.
- What about a Lifetime ISA?
Not modelled here, because it is a genuinely different product: a 25% government bonus on up to £4,000 a year, an 18 to 39 opening window, and a 25% charge on anything you withdraw before 60 unless it is for a first home. That charge is worth understanding before you open one, because it takes back more than the bonus gave you: put in £4,000, receive £1,000, withdraw early and the charge is levied on the whole £5,000, leaving you £3,750. You lose £250 of your own money. It deserves its own calculator rather than a footnote here.
Sources
Disclaimer
Not financial advice. The figures above are a projection from the inputs and assumptions you provided. What you actually end up with depends on the rate your cash ISA pays, which can be cut at any time, or on what markets actually do, which nobody knows; on what you actually pay in; and on future rules. Investment returns are not guaranteed and you can get back less than you put in. Check your provider's own figures and consult a qualified adviser before making a decision based on these numbers.
