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Useful next calculations
Rates & sources2026/27
UK ISA subscription limits for 2026/27. ISA interest, income and capital gains are sheltered from UK tax, subject to the rules for the ISA type.
| Band / figure | Rate |
|---|---|
| Total ISA allowance | £20,000 |
| Lifetime ISA allowance | £4,000 (included in £20k) |
| LISA government bonus | 25% on contributions |
| Junior ISA allowance | £9,000 |
Source: HMRC — Individual Savings Accounts — source checked for 2026/27.
When to use this calculator
- Before choosing between saving, investing or changing a contribution.
- When you want to compare cautious, base and optimistic return assumptions.
- When you need a projection before making a longer-term decision.
- When you want to see whether starting earlier or contributing more changes the outcome more.
Example: building a tax-efficient ISA plan in UK
Use these sample inputs as a quick scenario test, then change one variable at a time to compare outcomes.
| Input | Value |
|---|---|
| Starting ISA balance | £10,000 |
| Monthly contribution | £300 |
| Expected annual growth | 5.5% |
| Time horizon | 10 years |
Check that planned subscriptions stay within the annual allowance, then compare cautious and expected returns. For investments, subtract platform and fund charges from the return assumption and remember that the account value can fall as well as rise.
How to read your results
ISA Value
The figure produced by this output for the inputs you entered. Change one variable at a time if you want to see what drives it.
Value in Today’s Money
The figure produced by this output for the inputs you entered. Change one variable at a time if you want to see what drives it.
Net Return Assumption
A projected outcome under the growth rate you entered. Real-world returns fluctuate, so also check a more conservative assumption.
Contributed
The figure produced by this output for the inputs you entered. Change one variable at a time if you want to see what drives it.
Growth
The figure produced by this output for the inputs you entered. Change one variable at a time if you want to see what drives it.
Annual Allowance Excess
The figure produced by this output for the inputs you entered. Change one variable at a time if you want to see what drives it.
Method & assumptionsAuthoritative sources
This calculator models ISA growth using the annual return, annual charge and inflation assumptions you enter, a starting balance and contributions made at the end of each month. The projection compounds the return after subtracting the annual charge, then also shows the projected value in today’s-money terms using the inflation assumption. These are user assumptions, not a return, charge or inflation forecast. For a Cash ISA, enter a rate comparable with the provider's AER and any account charge. For a Stocks and Shares ISA, test lower-return scenarios because investments can fall as well as rise.
The allowance check compares twelve months of regular contributions with the annual allowance you enter. It does not track subscriptions to other ISAs, flexible withdrawals, transfers, the Lifetime ISA bonus or product-specific restrictions. Existing balances and valid transfers do not use the current-year subscription allowance. For 2026/27 the overall adult ISA allowance is £20,000 and the Lifetime ISA limit is £4,000 within it.
Common mistakes
- !Assuming a constant return without checking a more conservative growth rate.
- !Forgetting ongoing contributions, fees or tax wrappers where relevant.
- !Focusing only on the final balance instead of the path required to reach it.
- !Ignoring the drag of charges over a long period.
What to do next
- Check how much of the current tax year’s ISA allowance has already been used across all ISA types.
- Run a cautious return and a fee-adjusted return instead of relying only on the expected case.
- Compare cash and investment risk, access rules, charges, and time horizon before choosing an ISA product.
Go deeper — 3 guides reference this calculator
UK Tax Year 2025/26: Complete Guide to Allowances, Bands & Thresholds
Full reference for the 2025/26 UK tax year — income tax bands, Personal Allowance, National Insurance, ISA allowance, pension limits, stamp duty changes.
UK ISA Guide 2026/27: Types, £20,000 Allowance, Transfers, LISA
UK Individual Savings Account guide — Cash, Stocks & Shares, Lifetime and Innovative Finance ISAs, the £20,000 allowance, transfer rules and the 25% LISA bonus.
UK Tax Year 2026/27: Confirmed Rates, Upcoming Changes, Planning Checklist
What’s confirmed for the 2026/27 UK tax year, what’s changing (MTD for Self Assessment, IHT on pensions), and the allowances to lock in before 5 April 2026.
Frequently asked
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