Example result based on the prefilled values.
ISA Value
£86,542.40
Value in Today’s Money
£70,994.91
Net Return Assumption
7.00%
Contributed
£60,000.00
Growth
£26,542.40
Annual Allowance Excess
£0.00
Continue your plan
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.
Source: HMRC — Individual Savings Accounts — source checked for 2026/27.
When to use this calculator
- Before choosing between saving, investing, or increasing your monthly contribution.
- When you want to compare best-case, base-case, and cautious return assumptions.
- When you need a quick projection before making a longer-term portfolio decision.
- When you are deciding how many more years of contributions are needed to reach a specific target balance.
- When you want to see whether starting earlier versus contributing more each month produces a bigger outcome.
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.
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
Use this metric to compare scenarios side by side and understand how changes in the key inputs drive the final outcome. If the figure surprises you, isolate one variable at a time and rerun the calculation to identify which assumption is responsible.
Value in Today’s Money
Use this metric to compare scenarios side by side and understand how changes in the key inputs drive the final outcome. If the figure surprises you, isolate one variable at a time and rerun the calculation to identify which assumption is responsible.
Net Return Assumption
Use this to compare scenarios over different time horizons and judge whether the projected outcome clears your minimum required target. Remember that real-world returns fluctuate, so also check the result under a more conservative growth assumption.
Contributed
Use this metric to compare scenarios side by side and understand how changes in the key inputs drive the final outcome. If the figure surprises you, isolate one variable at a time and rerun the calculation to identify which assumption is responsible.
Growth
Use this metric to compare scenarios side by side and understand how changes in the key inputs drive the final outcome. If the figure surprises you, isolate one variable at a time and rerun the calculation to identify which assumption is responsible.
Annual Allowance Excess
Use this metric to compare scenarios side by side and understand how changes in the key inputs drive the final outcome. If the figure surprises you, isolate one variable at a time and rerun the calculation to identify which assumption is responsible.
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 to include 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 platform fees or fund charges, which can reduce the real compounded return significantly over ten or more years.
- !Comparing ISA and general investment account projections without adjusting for the tax treatment of interest, dividends, or capital gains.
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
- 9m
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.
- 6m
UK ISA Guide 2025/26: 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.
- 7m
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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