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.
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 the key drivers affect the final outcome.
Contributed
Use this metric to compare scenarios side by side and understand how the key drivers affect the final outcome.
Growth
Use this metric to compare scenarios side by side and understand how the key drivers affect the final outcome.
Est. Tax Saved
Review this figure alongside your gross income so you can understand the true cost of deductions.
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.
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.
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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