Stocks and Shares ISA Calculator (2026/27)
See how regular saving could grow inside a stocks and shares ISA, how much of the result is your money and how much is growth, and whether your payments fit the £20,000 ISA allowance for the 2026 to 2027 tax year. Results update instantly and the formula is shown with your numbers. No sign-up.
- Total you pay in£0
- Growth£0
- Fees paid£0
- Into ISAs this tax year£0
- 2026/27 allowance left£0
What you pay in vs what it could grow to
Grey = your contributions. Green = projected value. The gap between them is growth.
Quick answer
Paying £500 a month into a stocks and shares ISA for 20 years, at an assumed 6% return less a 0.2% fee, ends at about £225,624: £120,000 paid in and £105,624 of growth. That uses £6,000 of the £20,000 2026/27 ISA allowance. An illustration on an assumed constant return, not a forecast. Capital is at risk.
Different monthly amounts, same assumptions
Your own amount is highlighted. The largest row is the most you could pay in each month and stay within £20,000 this tax year (after any other ISA payments). ⚠ marks amounts over the allowance.
| Per month | You pay in | Growth | Final value |
|---|
What a stocks and shares ISA is
An ISA (Individual Savings Account) is a wrapper you hold savings or investments in. GOV.UK lists four types: cash ISAs, stocks and shares ISAs, innovative finance ISAs and Lifetime ISAs. A stocks and shares ISA holds investments such as funds or shares, so its value can go down as well as up.
GOV.UK describes ISA saving as tax-free. In plain terms, growth inside the wrapper is not reduced by tax along the way, which is why this calculator has no tax line. Outside an ISA, tax may apply depending on your circumstances — this page does not model that.
GOV.UK also sets age conditions: you must be 18 or over to open a stocks and shares ISA, and a Lifetime ISA can only be opened if you are under 40.
The £20,000 allowance for 2026/27
The tax year runs from 6 April 2026 to 5 April 2027. GOV.UK: “In the 2026 to 2027 tax year, the maximum you can save in ISAs is £20,000.”
- It is per person. Each adult has their own limit.
- It is shared across all your ISAs. Paying £8,000 into a cash ISA leaves £12,000 for everything else in the same tax year. Enter other ISA payments above and the calculator checks the total.
- Per month, £20,000 ÷ 12 = £1,666.67. The calculator warns you when your monthly amount times 12, plus other ISA payments, comes to more than £20,000.
The projection repeats your monthly amount every year. GOV.UK states the limit for 2026 to 2027 only; allowances for later years can change, so the check applies to this tax year.
Example: £500 a month for 20 years
Nothing invested to start, £500 a month, an assumed 6% a year before fees and 0.2% a year in fees, for 20 years.
- Projected value: £225,624.
- You pay in £120,000 — £6,000 a year, well inside the £20,000 allowance.
- Growth: £105,624, close to half the final value.
- Fees paid: £3,642. With no fee the same plan would reach £231,020.
| £500 a month for | You pay in | Growth | Projected value |
|---|---|---|---|
| 10 years | £60,000 | £21,056 | £81,056 |
| 20 years | £120,000 | £105,624 | £225,624 |
| 30 years | £180,000 | £303,467 | £483,467 |
From 20 to 30 years you pay in another £60,000, and the projected value rises by £257,843. Most of the later growth comes from money already invested, which is why the time a plan runs for matters as much as the amount.
What this calculator does not model
- Real-world returns. A constant return is assumed. Markets go down as well as up, sometimes for years, and you may get back less than you put in.
- Inflation. Figures are in future pounds. Enter a return net of inflation to think in today's money.
- ISA rules beyond the allowance check. Transfers, withdrawals and the specific rules of Lifetime ISAs are not modelled.
- Which investments to choose. The calculator does not suggest any.
The formula
- P — what is already in this ISA
- PMT — your monthly contribution, added at the end of each month
- i — (assumed return − annual fees) ÷ 12
- N — years × 12
- Allowance check — PMT × 12 + other ISA payments, compared with £20,000
See the formula with your own numbers
These update live from the calculator inputs above.
Frequently asked questions
How much can I put into an ISA in 2026/27?
GOV.UK says that in the 2026 to 2027 tax year the maximum you can save in ISAs is £20,000. That limit is per person and is shared across all your ISAs, not £20,000 in each one.
How much is £20,000 a year per month?
£20,000 divided by 12 is £1,666.67 a month. If you also pay into another ISA in the same tax year, the room left for this one is smaller — the calculator works that out when you enter your other ISA payments.
Is a stocks and shares ISA tax-free?
GOV.UK describes saving in an ISA as tax-free. That is why this calculator shows no tax line. The value of the investments themselves can still fall as well as rise — tax-free does not mean risk-free.
What return should I assume?
Nobody knows, which is why the rate is an input. Returns are not constant and can be negative. Try a cautious rate and an optimistic one and treat the gap between the two results as the honest range.
Does the projection assume the allowance stays at £20,000?
The projection simply repeats your monthly amount every year. The £20,000 check applies to the 2026 to 2027 tax year, the year GOV.UK states the limit for. Allowances for later years can change.
Method & sources
- Calculation: Monthly compounding at (assumed return − fees) ÷ 12, contributions at the end of each month. Constant return; no inflation, withdrawals or transfers.
- ISA allowance, types and age conditions: GOV.UK, Individual Savings Accounts — gov.uk/individual-savings-accounts (checked 18 October 2026).
- Tax year dates: GOV.UK, Income Tax rates and Personal Allowances — gov.uk/income-tax-rates.
- Verification: The engine is checked against the closed-form future-value formula in an automated test (
test/isa.test.js). - Reviewed: .
Educational illustration only, not personal financial advice. Capital at risk: investments can fall as well as rise and you may get back less than you put in. Tax rules can change and depend on your circumstances.
Run your own numbers
Enter what you plan to pay in each month, any other ISA payments this tax year, and the return you want to test.
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