About the ISA Calculator
This ISA calculator projects how much a UK Individual Savings Account could be worth after a number of years. Choose a cash ISA, a stocks and shares ISA or a Lifetime ISA, enter your starting balance, how much you pay in each month, the expected interest or growth rate and any platform and fund charges. The calculator shows the final value, how much of it is tax-free growth, what charges cost you and what the pot is worth in today's money.
It suits anyone deciding how much to put into an ISA this tax year, comparing a cash ISA rate with investing, or checking whether the Lifetime ISA's 25% government bonus beats a pension or regular ISA for a first home. Contributions are capped automatically: £20,000 per tax year across all your ISAs in 2026/27, of which no more than £4,000 can go into a Lifetime ISA.
Growth inside an ISA is free of income tax, dividend tax and capital gains tax, so no tax is deducted in the projection. The projection assumes today's limits stay the same every year. From 6 April 2027 the cash ISA limit falls to £12,000 a year for people under 65 (the overall £20,000 allowance is unchanged), so if you are under 65 keep cash ISA payments to £1,000 a month from then on. The government has also consulted on a new First Time Buyer ISA to replace the Lifetime ISA for home buyers; it has no confirmed start date or limits yet, and existing LISA rules still apply, so this calculator models the current LISA. Returns are assumed constant; stocks and shares can fall as well as rise.
With the default inputs, the isa value is £91,033.92. Change any value above to recalculate instantly.
How to use the isa calculator
- 1Choose the ISA type: cash, stocks and shares or Lifetime ISA.
- 2Enter your current ISA balance and how much you pay in each month.
- 3Enter the cash ISA rate (AER) or a cautious expected investment return.
- 4For investment ISAs, enter total platform and fund charges.
- 5Set the number of years and inflation to see the pot in today's money.
Formula and method
The expected annual return g, minus yearly charges c, is converted to an equivalent monthly rate r. Each month the balance grows by r and your contribution D is added at month end; for a Lifetime ISA the 25% government bonus on each payment is added too. Contributions above the annual limit are ignored, because you cannot pay more than the allowance into ISAs in a tax year.
Cost of charges is the difference between the value with no charges and the value after charges, which shows how fees compound over time. Real value divides the final pot by cumulative inflation to express it in today's pounds. No tax is deducted because ISA interest, dividends and gains are tax-free.
- g
- Expected annual growth or interest rate
- c
- Yearly platform and fund charges (0 for cash ISAs)
- D
- Monthly contribution (capped at the allowance)
- B(m)
- ISA balance after month m
Worked examples
Stocks and shares ISA: £10k plus £500/month for 10 years at 5%
You pay in £60,000 on top of the £10,000 already invested. Growing at 5% less 0.45% charges, the ISA reaches about £91,034 — £21,034 of tax-free growth. The charges cost about £2,437 over the decade, and at 2.5% inflation the pot is worth about £71,116 in today's money.
Cash ISA: trying to pay £2,000 a month at 4% for 5 years
£2,000 a month would be £24,000 a year, so contributions are capped at the £20,000 allowance (£1,666.67 a month). Over five years you pay in £100,000 and a 4% AER adds about £10,298 of tax-free interest. This assumes the full £20,000 cash limit every year, which holds only for savers aged 65 or over after 6 April 2027. Under-65s will be limited to £12,000 a year in cash ISAs from then.
Lifetime ISA: £4,000 a year for 15 years at 5%
Monthly payments are capped at the £4,000 yearly LISA limit, so you pay in £60,000 and the government adds a 25% bonus of £15,000. At 5% growth less 0.5% charges the account reaches about £106,047.
Frequently asked questions
What is the ISA allowance for 2026/27?+
You can pay up to £20,000 into ISAs in the 2026/27 tax year (6 April 2026 to 5 April 2027). You can split it across cash, stocks and shares, innovative finance and Lifetime ISAs, but no more than £4,000 can go into a Lifetime ISA.
Is the cash ISA limit changing?+
Yes. The government has announced that from 6 April 2027 people under 65 will be able to put at most £12,000 a year into cash ISAs, with the rest of the £20,000 allowance available for stocks and shares. Savers aged 65 and over keep the full £20,000 cash limit.
Do I pay tax on ISA interest or gains?+
No. Interest, dividends and capital gains inside an ISA are free of UK income tax and capital gains tax, and you do not need to report them on a Self Assessment return. Withdrawals are also tax-free.
Cash ISA or stocks and shares ISA?+
A cash ISA protects your capital and suits money you need within about five years. A stocks and shares ISA can grow more over the long run but its value can fall. Many people hold both: cash for short-term goals and investments for 5+ years.
How does the Lifetime ISA bonus work?+
If you open a LISA between 18 and 39, the government adds 25% to what you pay in, up to £1,000 a year on £4,000 of contributions, until you turn 50. You can use it for a first home costing £450,000 or less or withdraw from 60; other withdrawals face a 25% charge.
Is the Lifetime ISA being replaced?+
The government has consulted on a new First Time Buyer ISA that would pay its bonus at withdrawal instead of charging for other withdrawals. The consultation closed on 18 August 2026, and the limits and launch date have not been set. Until the new account launches, you can still open and pay into a Lifetime ISA under the current rules. Existing LISA savers will be able to use both accounts for the same purchase.
What happens to unused ISA allowance?+
It is lost. The allowance resets every 6 April and cannot be carried forward, so it is worth using as much as you can afford each tax year, even if you move the money between ISA types later via a transfer.
Results are estimates for educational purposes and are not financial advice. Rates, fees and terms vary — confirm with your lender or a licensed advisor before making decisions.