ISA Value
£86,542.40
Contributed
£60,000.00
Growth
£26,542.40
Est. Tax Saved
£5,308.48
Rates & sources2025/26
UK ISA allowance (2025/26). All gains + income within the wrapper are tax-free.
Source: HMRC — Individual Savings Accounts — figures refreshed at the start of each tax year.
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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
This example helps because it shows how a tax wrapper becomes more valuable as the balance and time horizon increase. It is rarely just about this year contribution; it is about the full compounding path.
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.
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.
Est. Tax Saved
Review this figure alongside your gross income so you can understand the true cost of deductions and plan around any thresholds before the tax year closes. If the figure looks higher than expected, check whether any pension or gift-aid contributions could reduce your taxable income.
Method & assumptionsAuthoritative sources
This calculator models the growth of ISA savings over time by applying a fixed annual return to your starting balance and regular contributions. For Cash ISAs, the return rate should reflect the AER offered by your provider. For Stocks and Shares ISAs, you might use an assumed annual growth rate net of fund charges — many UK planners use 5–7% as a long-term equity assumption before inflation.
The calculator does not simulate tax-free benefits explicitly, as all growth within an ISA is already tax-exempt. It assumes contributions are made at the start of each year and that the annual allowance is not exceeded. The model does not account for the Lifetime ISA 25% government bonus, which applies separately to LISAs. Charges vary significantly between ISA platforms, so deducting your annual platform fee from the return rate gives a more accurate net projection.
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
- Test a cautious, expected, and optimistic growth rate instead of relying on a single projection.
- Compare this result with related savings or retirement tools before committing more money.
- Use the linked guides to understand which assumptions matter most over longer periods.
- Consider running the same figures in an ISA and a general account scenario to see how the tax treatment changes the outcome over ten or more years.
- If the projected balance falls short of your target, use the tool to work backwards — increase the monthly contribution until the result meets your goal.
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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