Browser-only calculation 2026/27 tax year HMRC rules applied Live SSR result

UK ISA Calculator - 2026/27

Choose your annual contribution, time horizon, ISA type, and expected return. According to gov.uk and HMRC ISA Manager Guidance Notes (2026/27 tax year), the calculator applies HMRC's £20,000 adult allowance and the Lifetime ISA 25% government bonus where applicable, and compounds annually at your chosen rate. Results are computed server-side so they index and render before JavaScript.

Compiled from official public sources by the publisher.

Balance after 20 years

£173,596

5% annual real return, Near long-run equity market average

Total contributed

£100,000

£5,000 a year for 20 years

Government bonus

£0

No bonus on this ISA type

ISA advantage

£11,542

Versus a taxable General Investment Account at the same return

How the £173,596 balance after 20 years breaks down -- the same figures as the final row of the table below.

Year-by-year projection

Year Contributed Bonus Growth Balance
1 £5,000 - £250 £5,250
2 £10,000 - £763 £10,763
3 £15,000 - £1,551 £16,551
4 £20,000 - £2,628 £22,628
5 £25,000 - £4,010 £29,010
6 £30,000 - £5,710 £35,710
7 £35,000 - £7,746 £42,746
8 £40,000 - £10,133 £50,133
9 £45,000 - £12,889 £57,889
10 £50,000 - £16,034 £66,034
11 £55,000 - £19,586 £74,586
12 £60,000 - £23,565 £83,565
13 £65,000 - £27,993 £92,993
14 £70,000 - £32,893 £102,893
15 £75,000 - £38,287 £113,287
16 £80,000 - £44,202 £124,202
17 £85,000 - £50,662 £135,662
18 £90,000 - £57,695 £147,695
19 £95,000 - £65,330 £160,330
20 £100,000 - £73,596 £173,596

How the calculator works

Each year, your contribution is added to the previous balance, the government bonus is added if you selected the Lifetime ISA (25% on contributions up to £4,000, capped at £1,000 per year), and the combined total grows at your chosen real return rate. Real return means inflation-adjusted, the FTSE All-Share Total Return Index has averaged roughly 5% real over the most recent 30-year window per FTSE Russell, while the MSCI World Net Return in GBP has averaged 6.4% real over the same horizon.

The "ISA advantage" figure compares the projected ISA balance against the same contributions in a taxable General Investment Account, where dividend income above £500 and capital gains above £3,000 attract HMRC tax, typically a 1-1.5% annual return drag for a UK higher-rate taxpayer holding equity funds. Over 20+ year horizons the wrapper effect is material: a £10,000 annual contribution at 5% real for 30 years compounds to about £698,000 in an ISA versus around £580,000 outside.

The calculator does not model in-year subscription timing (whether you contribute lump-sum on 6 April or drip £833 per month), provider fees (~0.25% on platform-fee S&S ISAs), or sequence-of-returns risk (the variance of equity returns around the mean). For longer horizons the platform-fee headwind and the sequencing variance largely wash out, but for projections under 5 years the variance dwarfs the mean, see the methodology page for the full assumption set.

Download the HMRC ISA wrapper statistics extract cited on this page: hmrc-isa-wrapper-statistics.csv (Open Government Licence v3.0).

Every allowance, limit, and rate on PlainISA is rendered from the current gov.uk ISA rules and the HMRC ISA Managers Guidance Notes as recorded in our database, and no number is typed in by an editor. Projections are computed server-side in this page's committed TypeScript (annual contribution, LISA bonus cap, compound growth, taxable drag estimate); rebuild reproduces identical arithmetic from the same inputs. Read our editorial standards and corrections policy, the methodology behind these numbers, or report a data error. Our database was last rebuilt on 15 August 2026.

Primary statutory sources: gov.uk’s Individual Savings Accounts guidance and HMRC’s ISA Managers Guidance Notes. Historical context is checked against HMRC’s annual ISA statistics.

PlainISA publishes ISA reference data and projection tools for information only. It is not financial advice and not a personal recommendation, projections are illustrations rather than forecasts, and tax rules and allowances change, so check the current position on gov.uk or with an FCA-authorised adviser before you act. Appropriate use.