About the UK Junior ISA Calculator
This Junior ISA calculator projects how much a child’s Junior ISA (JISA) could be worth when they turn 18 and the money becomes theirs. Enter the child’s age, the current balance, how much family members pay in each month, and the expected interest or investment growth rate, and it shows the value at 18, how much of that is tax-free growth, what charges cost and what the pot is worth in today’s money.
Parents and grandparents use it to decide how much to put away each month, to compare a cash JISA with a stocks and shares JISA over a long horizon, or to see what starting at birth rather than at age 10 is worth. Anyone can pay in, but the total for each child is capped at £9,000 per tax year in 2026/27 across all their Junior ISAs; the calculator caps contributions at £750 a month to stay within it and assumes the limit stays the same in future years.
Rules to know: a child must be under 18 and living in the UK. The child can take control of the account at 16 but cannot withdraw until 18, when the JISA becomes an adult ISA. Growth and interest are free of income tax and capital gains tax, so no tax is deducted. The projection assumes a constant net return; investments can fall as well as rise.
With the default inputs, the value at 18 is £33,045.39. Change any value above to recalculate instantly.
How to use the uk junior isa calculator
- 1Enter the child’s age and any balance already in the Junior ISA.
- 2Enter how much the family will pay in each month in total.
- 3Choose a cash or stocks and shares JISA and enter the expected rate.
- 4For investments, enter the yearly platform and fund charges.
- 5Read the value at 18 and the value in today’s money.
Formula and method
The current balance B grows for n months until the child’s 18th birthday, and each monthly contribution C (capped at £750, so no more than £9,000 a year) is added at the end of the month and grows for the remaining months. The annual growth rate r minus yearly charges f is converted to an equivalent monthly rate i.
Cost of charges is the difference between the value at the gross rate and the value after charges. The value in today’s money divides the projected pot by (1 + inflation) for each year to 18. Because a Junior ISA is tax-free, no income or capital gains tax is deducted.
- FV
- Projected value at 18
- B
- Current balance
- C
- Monthly contribution (≤ £750)
- r, f
- Annual growth rate and yearly charges
- n
- Months until 18
Worked examples
£100 a month from birth in a stocks and shares JISA
£100 a month for 18 years is £21,600 paid in. At 5% growth less 0.45% charges the pot reaches about £33,045, of which £11,445 is tax-free growth. In today’s money, after 2.5% inflation, that is about £21,188. Charges cost roughly £1,480 over the period.
Maximum £9,000 a year into a cash JISA from age 5
Paying the full £750 a month for 13 years plus the existing £3,000 puts in £120,000. At 4% interest the cash JISA would hold about £157,361 at 18.
Starting at 10 and trying to pay £1,000 a month
Contributions are capped at £750 a month by the £9,000 allowance, so £72,000 goes in over eight years. At 6% less 0.45% charges the pot reaches about £89,859.
Frequently asked questions
How much can I put in a Junior ISA?+
The Junior ISA limit is £9,000 per child for the 2026/27 tax year. It covers all contributions from everyone, across both cash and stocks and shares Junior ISAs, and does not affect the parents’ own £20,000 ISA allowance.
When can my child access their Junior ISA?+
The child can take control of the account at 16, but cannot withdraw money until they turn 18. At 18 the Junior ISA becomes an adult ISA in their name, and they can withdraw it or keep it invested tax-free.
Is a cash or stocks and shares Junior ISA better?+
Over 10–18 years a stocks and shares JISA has historically been likely to grow more than cash, though its value can fall. A cash JISA is safer and suits short horizons or cautious savers. A child can hold one of each.
Who can pay into a Junior ISA?+
A parent or guardian with parental responsibility opens the Junior ISA (16 and 17-year-olds can open their own), but anyone — grandparents, relatives or friends — can pay into it, as long as total contributions stay within the £9,000 yearly limit.
What happens to a Child Trust Fund?+
Children who have a Child Trust Fund cannot also hold a Junior ISA unless the CTF is transferred into one. Transferring is often worthwhile because Junior ISAs usually have lower charges and more choice.
Results are estimates for educational purposes and are not financial advice. Rates, fees and terms vary — confirm with your lender or a licensed advisor before making decisions.