Junior ISA Calculator — Invest for Your Child's Future
A Junior ISA (JISA) allows you to invest up to £9,000 per year tax-free for a child under 18. The account becomes theirs at 18. Starting from birth, even modest contributions compound dramatically over 18 years. At £100/month with 7% returns, a JISA grows to approximately £43,000 by age 18 — with just £21,600 contributed. Grandparents, family members, and friends can all contribute within the annual limit. If multiple family members contribute £50/month each, the numbers grow quickly. The most valuable aspect of a JISA isn't just the money — it's giving your child a head start on understanding investing and compound growth at the age when time is most on their side.
Illustrative estimate only — not a guarantee
~£43,072 after 18 years
£21,600 contributed + £21,472 interest
Based on a hypothetical constant return. Actual returns will vary.
By the CompoundWise Team · Updated April 2026
UK-based financial education · Not financial advice
Invest £100/month for 18 years at 7%
£21,472
earned in interest alone
Total value
£43,072
You put in
£21,600
To reach £43,072, most UK investors use a Stocks & Shares ISA
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Compare other platforms ↓Keeping this in a savings account? You'd have ~£11,522 less
Compared to investing at 7% vs a 4% cash savings account
Growth Over Time
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- Your money earns ~£3/day in interest — that's £21,472 earned while you sleep.
- Saving just £50 more per month would add £21,536 to your final balance — that's £10,800 invested for £21,536 extra.
- 5 more years would add £25,147 — nearly 58% more, showing how powerful time is.
- Starting 5 years earlier would add £17,739 to your final balance. Every year you wait costs real money.Start investing now →
- Consistency beats timing — investing £100/month for 18 years matters more than picking the perfect moment to start.
- At your current plan, you reach £25k in 13 years. That's a real milestone — and it compounds from there.
- 70% of your total wealth is built in the final 10 years. Patience is everything.
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Year-by-Year Junior ISA Growth From Birth to Age 18
Investing £100 per month at 7% returns from birth, your child JISA reaches approximately £1,242 by age one. By age five, the balance hits roughly £6,960 — already a meaningful sum. Age 10 is where compound interest starts pulling its weight: the JISA holds approximately £17,400, with over £5,400 from compound growth. By age 13, the balance crosses £25,000, and annual interest income exceeds £1,600. At age 15, the portfolio reaches roughly £31,700. The final three years add approximately £11,300, bringing the total to roughly £43,000 at age 18. Your total contributions are £21,600, meaning compound interest has generated over £21,400 — nearly doubling your money. The child receives access to the full amount on their 18th birthday.
JISA Rules, Allowances, and What Happens at Age 18
The Junior ISA annual allowance is £9,000 per child (separate from the adult ISA allowance). Both cash JISAs and stocks and shares JISAs are available, and you can hold one of each type simultaneously. Parents, grandparents, and any other person can contribute, provided total contributions stay within the £9,000 limit. The account is managed by the registered contact (usually a parent) until the child turns 16, at which point the child can manage it themselves. At 18, the JISA automatically converts to an adult stocks and shares ISA or cash ISA. The child can then withdraw funds, continue investing, or add further contributions within the adult ISA allowance. Importantly, JISA investments do not count toward the parent income tax threshold — a common concern when gifting money to children.
How to Set Up a Junior ISA and Choose the Right Investments
Open a stocks and shares JISA with an FCA-regulated platform that offers low fees on smaller portfolio sizes — Vanguard (0.15% annual fee, no dealing charges on funds) and Fidelity (no platform fee for JISAs) are popular choices. With an 18-year time horizon, a 100% equity allocation is appropriate — global equity index funds provide maximum growth potential over this long period. Set up a £100 monthly direct debit and enable automatic dividend reinvestment. If grandparents or other family members want to contribute, provide them with the JISA reference number to make direct transfers. Consider timing additional contributions at the start of the tax year (April) to maximise the time money has to compound. A standing order set for early April is a simple way to front-load annual contributions.
What If the Whole Family Contributed to the Junior ISA?
If parents contribute £100 per month and grandparents add an additional £50 per month (total £150 per month), the JISA at 7% returns reaches approximately £64,500 by age 18 — enough for three years of university costs outside London, or a substantial deposit on a first property. At the maximum allowance of £750 per month (£9,000 per year), the JISA could grow to approximately £323,000 by age 18 — a truly life-changing sum. Even modest additional contributions make a meaningful difference: adding just £25 per month from grandparents (total £125 per month) pushes the final balance from £43,000 to approximately £53,700 — an extra £10,700 from £5,400 in additional contributions, with £5,300 of that coming from compound growth on the extra money.
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