ISA Calculator
See how much your ISA could be worth with tax-free compound growth. Compare Cash ISA (fixed rate) vs Stocks & Shares ISA (market-based returns).
How ISA Calculator works
ISA annual allowance
The annual ISA allowance is £20,000 per person for 2024–25 and 2025–26. You can split this between Cash ISA, Stocks & Shares ISA, Innovative Finance ISA, and Lifetime ISA (LISA, capped at £4,000). All growth and withdrawals are tax-free.
Cash ISA vs Stocks & Shares ISA
Cash ISAs offer a guaranteed return but inflation can erode real value. Stocks & Shares ISAs offer higher long-term potential returns but with investment risk. The Vanguard FTSE All World index has returned approximately 8–9% annually over 20 years; 7% is a conservative real estimate.
Tax benefit of ISA
Without an ISA, interest income above the Personal Savings Allowance (£500 for higher-rate taxpayers, £1,000 for basic-rate) is taxed at your marginal rate. In an ISA, all interest and growth is tax-free regardless of amount. The benefit grows as your balance grows.
Frequently asked questions
What is the ISA allowance for 2025-26?
£20,000 per person. You can split this between different ISA types (Cash, Stocks & Shares, Lifetime). The Lifetime ISA (LISA) has a separate £4,000 sub-limit within the £20,000 allowance.
What rate should I assume for a Stocks & Shares ISA?
Historical global equity returns have averaged 8–10% annually (nominal). After inflation (2%), a real return of 5–7% is commonly used in planning. Past performance does not guarantee future returns. 7% is a conservative benchmark many planners use.
Can I have both a Cash ISA and a Stocks & Shares ISA?
Yes. You can hold multiple ISA types simultaneously and contribute to one of each type per tax year, as long as total contributions don't exceed £20,000 in the tax year.
What happens to my ISA if I withdraw money?
For most ISAs, withdrawals are permitted but the withdrawn allowance is lost for that tax year. Flexible ISAs allow you to replace withdrawals in the same tax year without using additional allowance. Check your provider's terms.