ISA Calculator (2026)
Project how a UK ISA (Individual Savings Account) grows tax-free. Enter your starting balance, monthly contribution, expected annual return and time horizon to see the future value of a Cash ISA or Stocks & Shares ISA. The annual ISA allowance is £20,000 (2025/26). This is a projection, not a guarantee.
Project your ISA growth
Projection only. Investment returns are not guaranteed and capital can fall.
How to Use This ISA Calculator
- Enter your starting balance — any amount already in the ISA.
- Add your monthly contribution — keep total annual deposits within the £20,000 allowance.
- Set an expected annual return — modest for a Cash ISA, higher (but not guaranteed) for a Stocks & Shares ISA.
- Choose your time horizon and read the projected value, total contributions, and tax-free growth.
How ISA Growth Is Calculated
The projection combines two compound-growth formulas — one for your starting lump sum and one for your regular monthly contributions:
FV = P(1+r)n + M × [((1+r)n − 1) ÷ r]
where P is the starting balance, M the monthly contribution, r the monthly return (annual ÷ 12), and n the number of months. Because ISAs are tax-free, none of the growth is lost to income tax or capital gains tax.
Worked Example
Starting with £5,000, adding £500/month for 15 years at 5%:
- Total contributed: £5,000 + (£500 × 180) = £95,000.
- Projected value: about £144,000.
- Tax-free growth: roughly £49,000 — none taxed.
ISA Types and the £20,000 Allowance
- Cash ISA — tax-free savings interest; lower risk, lower return.
- Stocks & Shares ISA — invests in funds/shares; higher potential return, capital can fall.
- Lifetime ISA (LISA) — for a first home or retirement; 25% government bonus, £4,000/year limit (counts toward the £20,000).
- Innovative Finance ISA — peer-to-peer lending; higher risk.
You can split your £20,000 annual allowance across ISA types in the same tax year. The allowance resets each 6 April and cannot be carried forward.
Common Mistakes
- Exceeding £20,000 in a tax year across all ISAs.
- Assuming Stocks & Shares returns are guaranteed — markets fall as well as rise.
- Forgetting to use the allowance before 6 April — it doesn't roll over.
- Ignoring fees — platform and fund charges reduce real returns on investment ISAs.
Frequently Asked Questions
How much can I put in an ISA each year?
£20,000 per tax year (2025/26) across all your ISAs combined. The allowance resets every 6 April and can't be carried forward.
Is ISA growth really tax-free?
Yes — interest, dividends, and capital gains inside an ISA are free of UK income tax and capital gains tax, and you don't declare them on a tax return.
Cash ISA or Stocks & Shares ISA — which grows more?
Over the long term a Stocks & Shares ISA has historically out-grown cash, but with more risk and short-term ups and downs. Cash ISAs offer stability and a known interest rate.
Can I withdraw money from an ISA?
Usually yes. With a "flexible" ISA you can replace withdrawals in the same tax year without using extra allowance; with non-flexible ISAs, replacing money uses your allowance again.
What return should I assume?
For a Cash ISA, use the current advertised rate (around 4–5% in 2026). For a Stocks & Shares ISA, a long-run 5–7% is a common planning assumption — but returns are not guaranteed.