Calculate∑ What your result means
ISAs let your savings or investments grow completely free of UK Income Tax and Capital Gains Tax, up to the £20,000 annual contribution allowance.
Stocks & Shares ISA / Cash ISA Compound Interest Calculator
Project the tax-free growth of regular ISA contributions over time.
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Verified for 2026-27Methodology 2026-27.1 · reviewed 2026-08-08 · review by 2027-03-01
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Change monthly contribution to see the effect on your result.
Understand your result
Clear context for the values above.
Initial lump sum
Initial lump sum is included so you can interpret this estimate and make a better-informed comparison.
Monthly contribution
Monthly contribution is included so you can interpret this estimate and make a better-informed comparison.
Expected annual growth
Expected annual growth is included so you can interpret this estimate and make a better-informed comparison.
Annual platform and fund fees
Annual platform and fund fees is included so you can interpret this estimate and make a better-informed comparison.
How to use this calculator
- 1Enter your current initial lump sum, monthly contribution, expected annual growth, annual platform and fund fees, inflation for today’s-money view, time horizon.
- 2Review the headline projected isa value and the supporting figures beside it.
- 3Change one input at a time to see which assumption has the biggest effect.
- 4Confirm current UK rates and thresholds with an official source before acting.
Explore this calculator
What your result means →The headline projected isa value is based on the inputs currently shown above.Key terms explained →Review the assumptions, applicable rules, timing and real-world fees before making a decision.How to use this calculator →Enter your figures, review the live result, compare scenarios, then save or print your estimate.The calculation formula →The calculation follows the standard relationship between the displayed inputs. Results update whenever an input changes.Worked example →Using the current values produces £93,212.Assumptions and limitations →Review the assumptions, applicable rules, timing and real-world fees before making a decision.Costs or factors not included →Review the assumptions, applicable rules, timing and real-world fees before making a decision.Frequently asked questions →How accurate is this estimate? What should I check before relying on it? Can I save or share the result?
▦ How this calculator works
The calculator compounds your initial lump sum and monthly contributions monthly at your expected annual growth rate over the chosen time horizon, using the standard future-value-of-an-annuity formula.
◇ Assumptions and what's not included
This assumes constant gross growth, fees and inflation. The annual ISA allowance warning uses the contributions entered here and cannot see contributions to your other ISAs.
▤ What to do next
Keep your total ISA contributions across all providers within the annual allowance, and treat the growth rate as an assumption to stress-test with a more conservative scenario too.
UK-specific contextFigures use GBP and current published rates and thresholds.
Reviewed for clarityUpdated 3 August 2026.
ImportantFor planning and educational use — not tax, legal or financial advice.
Your privacy mattersInputs stay in your browser.
Frequently asked questions
Is a Cash ISA calculated the same way as a Stocks & Shares ISA?+
The compounding maths is the same, but a Cash ISA's rate is a savings rate set by the provider while a Stocks & Shares ISA's return is market-linked and not guaranteed.
What is the annual ISA allowance?+
It's currently £20,000 per tax year across all your ISAs combined — contributions above that don't get the tax-free treatment.
Do I pay tax when I withdraw from an ISA?+
No, withdrawals from an ISA are entirely tax-free, which is the core benefit compared with a general investment account.
Does the growth rate include fees?+
No, enter a growth rate net of fees if you want a more realistic projection, since platform and fund charges will reduce your actual return.
Official references
Rules and thresholds used by this calculator are labelled for 2026–27 or formula-labelled period. Review the primary guidance before making a filing, borrowing, benefit or investment decision.
Last source review: 3 August 2026. Calculator results are estimates and may exclude circumstances described in the methodology.