UK Junior ISA Calculator: Future Value at Age 18
Work out what a lump sum invested in a UK Junior ISA (JISA) grows to by the time a child turns 18 — with all growth completely free of UK tax, and the money locked away for the child until adulthood.
Adjust the inputs and select Calculate for a full breakdown.
Compare Common Scenarios
How the numbers shift across typical situations for this calculator:
| Scenario | Future value | Total growth |
|---|---|---|
| £5k · 6% · 18yr | $14,271.70 | $9,271.70 |
| £1k · 7% · 18yr | $3,379.93 | $2,379.93 |
| £10k · 5% · 10yr | $16,288.95 | $6,288.95 |
| £3k · 6% · 15yr | $7,189.67 | $4,189.67 |
How This Calculator Works
Enter the Junior ISA lump sum, the annual return you expect, and the years until the child turns 18. The calculator compounds the amount and shows the value at 18 and the total tax-free growth. A Junior ISA is opened and managed by a parent/guardian, but the money legally belongs to the child and can only be accessed by them at 18.
The Formula
Future Value of a Lump Sum
PV = present value, r = annual rate, n = number of years
Worked Example
£5,000 invested in a Stocks & Shares Junior ISA at 6% for 18 years grows to about £14,272 — nearly tripling, with £9,272 of tax-free growth, on a single lump sum. The Junior ISA is a tax-free wrapper for under-18s: like an adult ISA, all interest, dividends, and capital gains inside it are free of UK tax, but the funds are locked until the child's 18th birthday, at which point the account becomes a normal adult ISA in the child's name and they gain full access. The long time horizon (up to 18 years) is what makes even modest contributions grow substantially.
Key Insight
The Junior ISA is one of the best tools for building a tax-free nest egg for a child, precisely because of the long horizon and the tax shelter working together over up to 18 years. Key features: it comes in two types — Cash JISA (tax-free interest) and Stocks & Shares JISA (tax-free investment growth, modelled here, and usually the better choice for a long horizon) — sharing one annual Junior ISA allowance (separate from the parent's own ISA allowance). The account is opened and run by a parent or guardian, but the money is legally the child's and is locked until they turn 18, when control passes to them and the JISA converts to an adult ISA. That lock-in is both a feature (forced long-term saving, can't be raided) and a consideration (the child gains full, unrestricted access at 18 and can spend it however they wish). Because the horizon can be the full 18 years, time does enormous work — and for the longest horizons a Stocks & Shares JISA invested for growth typically far outpaces a Cash JISA, while as the child nears 18 some families de-risk. Practical points: anyone (grandparents, relatives) can contribute within the annual allowance, the allowance resets each tax year and doesn't carry forward, and low fees matter over such a long period. This calculator shows a single lump sum; regular monthly contributions on top would grow the pot much larger. The value at 18 shown is entirely tax-free and belongs to the child — a powerful head start for university, a first car, a house deposit, or their own continued investing.
Frequently Asked Questions
How is Junior ISA growth calculated?
The lump sum is multiplied by (1 + annual return) raised to the number of years until age 18. £5,000 at 6% for 18 years is £5,000 × 1.06¹⁸ ≈ £14,272 — all of it tax-free within the JISA.
When can the child access a Junior ISA?
At 18. The account is opened and managed by a parent or guardian, but the money legally belongs to the child and is locked until their 18th birthday, when control passes to them and the JISA becomes a normal adult ISA. They then have full, unrestricted access to spend or invest it as they choose.
What are the types of Junior ISA?
Two: a Cash Junior ISA (tax-free interest, like a savings account) and a Stocks & Shares Junior ISA (tax-free investment growth, modelled here). They share one annual Junior ISA allowance. For long horizons, a Stocks & Shares JISA invested for growth typically outpaces a Cash JISA, with some families de-risking as 18 approaches.
Who can pay into a Junior ISA?
Anyone — parents, grandparents, and other relatives or friends can contribute, up to the annual Junior ISA allowance in total. The allowance is separate from a parent's own adult ISA allowance, resets each tax year, and doesn't carry forward, so it's use-it-or-lose-it each year.
Is the growth really tax-free?
Yes — all interest, dividends, and capital gains earned inside a Junior ISA are free of UK tax, just like an adult ISA. Combined with the up-to-18-year horizon, this tax-free compounding is what makes the JISA so effective for building a child's nest egg from even modest contributions.
Related Calculators
Methodology & Review
Future value is the lump sum compounded at the annual return over the period. It assumes the amount is invested at once in a Junior ISA and left untouched at a constant return; growth is tax-free. It ignores fees and the annual Junior ISA allowance.
Written by Ugo Candido · Last updated May 22, 2026.