Junior ISA calculator
How much could their Junior ISA be worth?
See the value at 18, when it becomes theirs — and what it could grow to if they keep it invested.
Your plan
Growth compounds monthly at your chosen rate — 6% is a deliberately conservative default for a diversified global index fund. Contributions stop at 18; from there it's their money and their choice. Education, not financial advice — investments can go down as well as up.
Theirs at 18
£12,283
after 8 years of contributions — tax-free, and theirs to keep
£25,189
if they leave it invested until 30
Contributions stop at 18 — the growing doesn't
Child's age
At 18
£12,283
At 25
£18,674
At 30
£25,189
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Ready to start building their future?
Squids-In helps families track their children's investments, set goals — and teaches the kids how it all works along the way.
Prefer the version with free government money?
A Junior SIPP gets a 25% top-up on every contribution and grows until retirement. Our Junior SIPP calculator shows what that could become — many families use both accounts together.
What the numbers are built on
01
Compound growth
A conservative 6% a year (typical for diversified index funds), compounded monthly — so growth earns growth.
02
Contributions stop at 18
A Junior ISA becomes theirs at 18. The projection keeps going, because the smartest move is often to leave it invested.
03
Tax-free all the way
No income tax or capital gains tax inside a Junior ISA — and it rolls into an adult ISA at 18, still tax-free.
Frequently asked questions
How much can I pay into a Junior ISA each year?
The Junior ISA allowance is £9,000 per tax year (2026/27). Parents open the account, but anyone — grandparents, family, friends — can pay in, and all growth is completely tax-free.
What happens to a Junior ISA when my child turns 18?
At 18 the Junior ISA automatically becomes an adult ISA in your child's name, and the money is theirs. They can withdraw it — or keep it invested, where it keeps growing tax-free. Our calculator shows both moments: the value at 18, and what it could become if they leave it alone.
Is 6% a realistic growth rate?
Yes, 6% is a conservative estimate for a diversified global index fund over 10+ years. Historical returns for global index funds average around 8-10% annually, but we use 6% to be careful.
What's the difference between a Junior ISA and a Junior SIPP?
Junior ISAs are tax-free savings accessible at age 18, with a £9,000/year limit. Junior SIPPs are children's pensions with a 25% government top-up but accessible only from age 57+, with a £2,880/year contribution limit (£3,600 with the top-up). Many families use both. Read our full comparison.
Cash Junior ISA or stocks & shares Junior ISA?
This calculator models a stocks & shares Junior ISA, because over a whole childhood investment growth has historically beaten cash interest by a wide margin. Cash Junior ISAs suit short horizons; most children have the longest horizon there is.
Learn more about Junior ISAs
What is a Junior ISA?
Complete guide to Junior ISAs — how they work, contribution limits, and who can open one.
Read more →Junior ISA vs Junior SIPP
The two accounts compared — access ages, limits, tax treatment, and how families combine them.
Read more →One-off gift calculator
Birthday money or a grandparent’s lump sum — see what a single gift could become.
Read more →