What this calculator does
Takes the child's current age, annual contribution, starting balance, and expected annual return. Returns the projected balance at age 18 when the JISA matures.
The formula
Standard future value with annual contributions: A = P × (1 + r)^t + C × [(1 + r)^t − 1] / r Where: P = current JISA balance C = annual contribution (max £9,000 in 2025/26) r = annual return rate (decimal) t = years until child turns 18 JISA annual allowance (2025/26): £9,000
Assumptions
- Contributions are made at the end of each year.
- Constant return rate throughout the period.
- No tax on growth or income inside the JISA wrapper.
- The JISA matures at 18 and converts to an adult ISA. The child controls the account from age 16 and can withdraw from age 18.
- The allowance is modelled at the current year's figure and is not assumed to increase.
Data sources
| Figure | Value used | Source | Last checked |
|---|---|---|---|
| JISA annual allowance | £9,000 | HMRC — Junior ISA allowances | May 2026 |
Limitations
- Does not model what happens to the fund after the JISA matures at 18 — the calculator assumes the child accesses the money at that point.
- Return rate is constant; actual investment returns vary year to year.
- Does not account for platform or fund fees.
- The allowance is modelled at the current year's figure and is not assumed to increase in future years.
Worked example
Inputs: child aged 3, £0 starting balance, £200/month (£2,400/year) contributions, 6% annual return, 15 years to 18.
A = £0 × (1.06)^15 + £2,400 × [(1.06)^15 − 1] / 0.06 = £0 + £2,400 × [2.3966 − 1] / 0.06 = £2,400 × 23.28 = £55,861 at age 18
Changelog
| Date | Change |
|---|---|
| May 2026 | Initial publication |