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. The calculator applies HMRC's £20,000 adult allowance, 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.

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Balance after 20 years

£173,596

5% annual real return

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

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 % real over the most recent 30-year window per FTSE Russell, while the MSCI World Net Return in GBP has averaged % 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.