Junior ISA Calculator
How much could your child's JISA be worth at 18?
Model tax-free growth from your child's current age to 18. Compares against a taxable account to show the JISA tax advantage, and optionally projects onwards as an adult ISA.
Child's age
0–17 completed years
Additional months
Contributions
JISA limit: £750/mo (£9,000/yr)
Increase contributions by this % each year (e.g. in line with salary growth)
Growth assumptions
Global equity index funds have returned ~7–8 % annually after inflation historically
Continue after 18
Continue as adult ISA
At 18 the JISA automatically becomes an adult ISA
Projected JISA balance at 18
£86,647
Tax advantage over taxable account
£11,999
A taxable account (at 80% effective growth rate) would reach £74,648 — the JISA saves £11,999 in tax drag.
Start one year earlier
+£8,757
Starting contributions now for a child aged newborn vs waiting until age 1 would add £8,757 more by 18 — that's the compounding value of acting today.
Growth over time
| Label | Value |
|---|---|
| Balance at 18 | £86,647 |
| Total contributions | £43,200 |
| Tax-free growth | £43,447 |
Year-by-year breakdown
| Year | Age | Annual contrib. | Balance | Total growth |
|---|---|---|---|---|
| 1 | 1 | £2,400 | £2,493 | £93 |
| 2 | 2 | £2,400 | £5,166 | £366 |
| 3 | 3 | £2,400 | £8,033 | £833 |
| 4 | 4 | £2,400 | £11,106 | £1,506 |
| 5 | 5 | £2,400 | £14,402 | £2,402 |
| 6 | 6 | £2,400 | £17,936 | £3,536 |
| 7 | 7 | £2,400 | £21,726 | £4,926 |
| 8 | 8 | £2,400 | £25,789 | £6,589 |
| 9 | 9 | £2,400 | £30,147 | £8,547 |
| 10 | 10 | £2,400 | £34,819 | £10,819 |
| 11 | 11 | £2,400 | £39,829 | £13,429 |
| 12 | 12 | £2,400 | £45,201 | £16,401 |
| 13 | 13 | £2,400 | £50,962 | £19,762 |
| 14 | 14 | £2,400 | £57,139 | £23,539 |
| 15 | 15 | £2,400 | £63,762 | £27,762 |
| 16 | 16 | £2,400 | £70,865 | £32,465 |
| 17 | 17 | £2,400 | £78,480 | £37,680 |
| 18 | 18 | £2,400 | £86,647 | £43,447 |
Want a complete retirement projection?
For a complete retirement projection including tax, State Pension, and 10,000 Monte Carlo simulations — use our full retirement planner.
This calculator is for illustrative purposes only and does not constitute financial advice. Projected values assume a fixed annual growth rate applied consistently over the full investment period — actual returns will vary and are not guaranteed. The Junior ISA annual allowance (currently £9,000) and adult ISA allowance (currently £20,000) are set by HMRC and subject to change. Tax treatment depends on individual circumstances. Past performance of investments is not a reliable indicator of future results. The value of investments can fall as well as rise and your child could get back less than was paid in. Selvox accepts no liability for decisions made in reliance on these outputs.