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UK ISA Calculator

Estimate how a Cash, Stocks & Shares or Lifetime ISA could grow with regular contributions and compound growth — including the government's 25% Lifetime ISA bonus.

Your ISA plan

£
£
%
20 years
Estimated balance after 20 years Growth share —
£—
You invested£—
Investment growth£—
Within the £20,000 ISA annual allowance for the 2026/27 tax year.
Contributions Growth (incl. LISA bonus)
Show yearly breakdown
YearContributionsGrowthBalance

ISA allowance and Lifetime ISA figures apply to the 2026/27 tax year (6 April 2026 – 5 April 2027). Sources: GOV.UK — Individual Savings Accounts · GOV.UK — Lifetime ISA. Rules and thresholds can change — check the latest official guidance before making financial decisions.

How it works

Choose the type of ISA you pay into, then set what you already have, your monthly contribution, an assumed annual return and how many years you plan to save. The calculator shows your estimated balance and splits it into three parts: what you invested, any Lifetime ISA government bonus, and the investment growth on top.

Growth is compounded monthly, and regular contributions are assumed to be paid at the end of each month. For a Lifetime ISA, the government bonus is modelled at 25% of each year's contributions up to £4,000 (so up to £1,000 a year), credited at the end of that year. The Lifetime ISA annual limit of £4,000 is part of your overall £20,000 ISA allowance.

Example calculation

You start a Stocks & Shares ISA with £1,000, pay in £250 a month, assume a 5% annual return and save for 20 years:

  • You invest: £1,000 + £250 × 240 = £61,000
  • Estimated balance: ≈ £105,471
  • Investment growth: ≈ £44,471 (about 42% of the final balance)

Example assumes constant 5% monthly-compounded return with no fees. Real returns vary and are not guaranteed.

Methodology & formula

With monthly compounding and end-of-month contributions, the future value is:

FV = P(1 + r/12)12t + M × [ (1 + r/12)12t − 1 ] / (r/12)
  • P = initial amount already in the ISA
  • M = monthly contribution
  • r = assumed annual return (as a decimal)
  • t = number of years

For a Lifetime ISA, each year's capped contribution (max £4,000) earns a 25% bonus (max £1,000), added to the balance at the end of that year before the next year compounds. ISA interest and growth are tax-free, so no tax is deducted.

Assumptions: constant return, no fees, no withdrawals, monthly compounding, contributions at month-end. The calculator does not model market volatility.

UK-specific information

  • 2026/27 allowance: you can pay up to £20,000 into ISAs in the 2026/27 tax year, split across Cash, Stocks & Shares, Innovative Finance and Lifetime ISAs. Unused allowance does not carry over. GOV.UK · 2026/27
  • Lifetime ISA: 25% bonus on up to £4,000 a year (max £1,000), openable at 18–39, for a first home (up to £450,000) or later life (60+). Early withdrawals pay a 25% charge. GOV.UK · 2026/27
  • Junior ISA: a separate £9,000 a year allowance for children under 18. GOV.UK/HMRC · 2026/27
  • Coming change: from 6 April 2027 the Cash ISA annual limit becomes £12,000 for investors under 65 (those 65 and over keep £20,000). Autumn Budget 2025
  • Everything inside an ISA is free of income tax, capital gains tax and dividend tax.

Frequently asked questions

Can I have more than one ISA?
Yes — you can pay into one Cash ISA, one Stocks & Shares ISA, one Innovative Finance ISA and one Lifetime ISA in the same tax year, as long as total contributions stay within £20,000 (2026/27).
Does the £20,000 allowance include the LISA bonus?
No. The £20,000 limit applies to what you pay in. The 25% government bonus is added on top and does not count against your allowance.
Can I withdraw money from an ISA?
Cash ISAs may be flexible (withdraw and replace in the same tax year) — check your provider. A Lifetime ISA is different: withdrawing for anything other than a first home, 60+ or terminal illness triggers a 25% charge.
What happens to the allowance I don't use?
It does not carry over to the next tax year. The allowance resets every 6 April.
Are Stocks & Shares ISA returns guaranteed?
No. Investments can fall as well as rise. The calculator assumes a constant return to illustrate compounding — it is not a forecast.
Is a Lifetime ISA right for me?
It can be if you are saving for a first home or retirement and are aged 18–39. Because of the 25% early-withdrawal charge, only use it for money you are confident you won't need before 60 for other purposes.

Related calculators

UK Savings Calculator · Home · Methodology

Sources

  • GOV.UK — Individual Savings Accounts (ISAs): gov.uk/individual-savings-accounts (accessed 2026, 2026/27 tax year)
  • GOV.UK — Lifetime ISA: gov.uk/lifetimeisa (accessed 2026)
  • GOV.UK — Tax-free savings newsletter 19 (Autumn Budget 2025): gov.uk
  • GOV.UK — ISA (Amendment) Regulations 2026 consultation (Cash ISA £12,000 under-65s from April 2027): gov.uk

Casvia provides educational calculators and general information. It does not provide personal financial, tax or investment advice. Rules and thresholds can change — always check the latest official guidance.