What this calculator works out
This calculator projects a Junior ISA forward from its current balance and monthly contributions, at an assumed annual growth rate, up to the point the child turns 18 and gains access.
Junior ISAs have a long horizon by design, which is what makes them effective — and what makes the growth assumption matter more than in almost any other calculation on this site.
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Before you rely on the projection
- Check the current Junior ISA allowance. It is separate from the adult allowance and set annually by the government.
- Choose a growth rate appropriate to the account. Cash Junior ISAs and stocks and shares Junior ISAs behave very differently over 18 years.
- Deduct charges from the growth rate on a stocks and shares account.
- Understand that the money becomes the child's absolutely at 18. That is a feature of the product, not a detail.
A Junior ISA opened at birth has the longest horizon most families will ever invest over. Over that period the growth assumption dominates the outcome, and the historical case for equities over cash is strongest precisely because there is time to ride out falls.
How this calculator works
The opening balance and the contribution stream are grown separately:
i = annual growth ÷ 12 ÷ 100, n = years × 12
Projected value = balance × (1 + i)n + monthly × [ ((1 + i)n − 1) ÷ i ]Growth inside a Junior ISA is free of UK income tax and capital gains tax, so no tax adjustment is applied. The projection is in future money and makes no allowance for inflation — a useful sum in eighteen years will need to be considerably larger than the same sum today.
Worked example: £100 a month for ten years
Using the default figures — a £1,000 current balance, £100 a month, 5% annual growth and ten years until the child turns 18:
- Projected value at 18: £17,175.24
- Total contributed: £1,000 + (£100 × 120) = £13,000
- Growth: about £4,175
Start the same contributions at birth rather than at age eight and the eighteen-year projection rises to roughly £36,000 — more than double, for eight more years of the same £100. Nothing else in this calculation has anything like that leverage, which is the argument for starting early rather than for contributing more.
Common mistakes
- Assuming you keep control at 18. The money becomes the child's absolutely and unconditionally.
- Choosing cash for an eighteen-year horizon by default. Over that period inflation is a larger risk than volatility.
- Forgetting charges on a stocks and shares account. Over eighteen years they compound substantially.
- Confusing the Junior ISA allowance with the adult one. They are separate limits.
- Reading the projection in today's money. It is a future figure and will buy less.
Frequently asked questions
Who controls the money?
A parent or guardian manages the account until the child is 16, at which point the child can take over management. At 18 the account converts to an adult ISA and the money is theirs outright, to use as they choose. There is no mechanism to restrict this, and it is worth being comfortable with that before committing large sums.
Cash or stocks and shares?
Over a horizon measured in many years, historical evidence favours stocks and shares — inflation is the more certain threat to cash over eighteen years than volatility is to equities. Cash makes more sense where the horizon is short, perhaps for a child already in their teens, or where the family is genuinely uncomfortable with the value falling.
Can grandparents contribute?
Yes. Anyone can pay into a Junior ISA, though only a parent or guardian can open one and manage it. All contributions from all sources count towards the same annual allowance, so it is worth coordinating to avoid exceeding it.
What if the child needs the money earlier?
They cannot have it. Junior ISA funds are locked until 18 except in cases of terminal illness or death. If you may need access before then, an ordinary savings account in the parent's name is more appropriate, though the interest may be taxable.
Is what I enter stored?
No. Balances and contributions are processed entirely in your browser and are never transmitted or retained.
Related tools
References
- GOV.UK — Junior ISA rules, allowances and what happens at 18
- MoneyHelper — guidance on saving and investing for children
Sources are checked at publication and can change — how I choose and check references.