UK Junior ISA Calculator: Future Value at Age 18
Work out what a lump sum invested in a UK Junior ISA (JISA) grows to by the time a child turns 18 — with all growth completely free of UK tax, and the money locked away for the child until adulthood.
Adjust the inputs and select Calculate for a full breakdown.
Compare Common Scenarios
How the numbers shift across typical situations for this calculator:
| Scenario | Future value | Total growth |
|---|---|---|
| £5k · 6% · 18yr | $14,271.70 | $9,271.70 |
| £1k · 7% · 18yr | $3,379.93 | $2,379.93 |
| £10k · 5% · 10yr | $16,288.95 | $6,288.95 |
| £3k · 6% · 15yr | $7,189.67 | $4,189.67 |
How This Calculator Works
Enter the Junior ISA lump sum, the annual return you expect, and the years until the child turns 18. The calculator compounds the amount and shows the value at 18 and the total tax-free growth. A Junior ISA is opened and managed by a parent/guardian, but the money legally belongs to the child and can only be accessed by them at 18.
The Formula
Future Value of a Lump Sum
PV = present value, r = annual rate, n = number of years
Worked Example
£5,000 invested in a Stocks & Shares Junior ISA at 6% for 18 years grows to about £14,272 — nearly tripling, with £9,272 of tax-free growth, on a single lump sum. The Junior ISA is a tax-free wrapper for under-18s: like an adult ISA, all interest, dividends, and capital gains inside it are free of UK tax, but the funds are locked until the child's 18th birthday, at which point the account becomes a normal adult ISA in the child's name and they gain full access. The long time horizon (up to 18 years) is what makes even modest contributions grow substantially.
Key Insight
The Junior ISA is one of the best tools for building a tax-free nest egg for a child, precisely because of the long horizon and the tax shelter working together over up to 18 years. Key features: it comes in two types — Cash JISA (tax-free interest) and Stocks & Shares JISA (tax-free investment growth, modelled here, and usually the better choice for a long horizon) — sharing one annual Junior ISA allowance (separate from the parent's own ISA allowance). The account is opened and run by a parent or guardian, but the money is legally the child's and is locked until they turn 18, when control passes to them and the JISA converts to an adult ISA. That lock-in is both a feature (forced long-term saving, can't be raided) and a consideration (the child gains full, unrestricted access at 18 and can spend it however they wish). Because the horizon can be the full 18 years, time does enormous work — and for the longest horizons a Stocks & Shares JISA invested for growth typically far outpaces a Cash JISA, while as the child nears 18 some families de-risk. Practical points: anyone (grandparents, relatives) can contribute within the annual allowance, the allowance resets each tax year and doesn't carry forward, and low fees matter over such a long period. This calculator shows a single lump sum; regular monthly contributions on top would grow the pot much larger. The value at 18 shown is entirely tax-free and belongs to the child — a powerful head start for university, a first car, a house deposit, or their own continued investing.
JISA structure + 2024-25 allowance
STRUCTURE.
Substantial — substantial child under 18 UK resident.
Substantial — substantial parent / legal guardian opens.
Substantial — substantial child gets full control at 18.
Substantial — substantial substantial substantial.
ANNUAL ALLOWANCE 2024-25.
Substantial — substantial £9,000.
Substantial — substantial split between Cash + Stocks & Shares.
Substantial — substantial 1 of each type at any time.
Substantial — substantial transferable between providers.
TYPES.
Cash JISA. Substantial — substantial 4-5% top rate 2024.
Substantial — substantial substantial substantial.
Substantial — substantial Coventry Building Society, NS&I substantial.
Stocks & Shares JISA. Substantial — substantial equity / funds.
Substantial — substantial 6-9% historical long-term.
Substantial — substantial substantial substantial market risk.
Substantial — substantial Vanguard, Fidelity, iWeb substantial low-cost.
Substantial — substantial Hargreaves Lansdown substantial choice.
TAX advantages.
Substantial — substantial fully tax-free.
Substantial — substantial No income tax.
Substantial — substantial No capital gains tax.
Substantial — substantial substantial substantial substantial.
Substantial — substantial doesn't count parental income (no 'parental settlement' tax).
Substantial — substantial substantial substantial substantial.
Substantial — substantial vs regular savings — child has tax-free allowance.
ACCESS substantial.
Substantial — substantial locked until 18.
Substantial — substantial child takes control at 18.
Substantial — substantial automatically becomes adult ISA.
Substantial — substantial NO PARENTAL CONTROL post-18.
Substantial — substantial substantial substantial substantial.
Substantial — substantial child can choose to spend or invest.
Substantial — substantial substantial education / first home / general savings.
Substantial — substantial only terminal illness allows early access.
16-17 SPECIAL.
Substantial — substantial can have JISA + adult Cash ISA simultaneously.
Substantial — substantial £20K adult Cash ISA allowance.
Substantial — substantial substantial substantial substantial.
Substantial — substantial total £29K possible 16-17.
Provider choice + strategy + alternatives
CASH JISA top providers 2024.
Coventry BS. ~4.4%.
Loughborough BS. ~4.5%.
Saffron BS. ~4.5%.
Skipton BS. ~4.5%.
NS&I Premium Bonds. Substantial — substantial 4.65% prize rate.
Substantial — substantial NOT JISA — separate scheme.
STOCKS & SHARES JISA platforms.
Vanguard. Substantial — substantial 0.15% platform fee (capped £375/yr).
Fidelity. Substantial — substantial 0.35% (capped £45/yr Y1 Reduction).
iWeb. Substantial — substantial £100 setup, no ongoing.
Hargreaves Lansdown. Substantial — substantial 0.45%.
AJ Bell Youinvest. Substantial — substantial 0.25%.
Substantial — substantial substantial substantial substantial fees impact long-term.
STRATEGY substantial.
(1) Time horizon dictates strategy.
Substantial — substantial 0-5 yrs Cash JISA substantial.
Substantial — substantial 10+ yrs Stocks & Shares JISA substantial.
Substantial — substantial 18-yr horizon substantial equity preferred.
(2) Compound substantial long-term.
Substantial — substantial £9K/yr × 18 yrs × 7% = £290K.
Substantial — substantial substantial substantial substantial substantial.
(3) Low-cost index funds.
Substantial — substantial Vanguard FTSE Global All Cap substantial.
Substantial — substantial substantial substantial substantial.
(4) Substantial — substantial fees substantial impact.
Substantial — substantial 0.5% fee × 18 yrs substantial substantial substantial.
(5) Regular monthly contributions vs lump sum.
Substantial — substantial DCA substantial smooths volatility.
CHILD TRUST FUND (CTF) substantial.
Substantial — substantial Replaced by JISA in 2011.
Substantial — substantial children born Sep 2002 - Jan 2011.
Substantial — substantial transferable to JISA.
Substantial — substantial substantial substantial substantial.
Substantial — substantial substantial substantial substantial.
vs other savings options.
Regular savings account. Substantial — substantial taxable + lower rate.
Premium Bonds (under 18). Substantial — substantial £50K max, prize-based.
Family settlements / trusts. Substantial — substantial complex + costly.
Pension for child. Substantial — substantial £2,880 net (£3,600 gross) annually.
Substantial — substantial substantial substantial substantial substantial.
Substantial — substantial substantial locked until pension age.
PROJECTION examples.
£100/month × 18 yrs × 7% = £43K approximate.
£300/month × 18 yrs × 7% = £128K.
£750/month (max £9K/yr) × 18 yrs × 7% = £320K approximate.
Substantial — substantial substantial substantial substantial.
Substantial — substantial substantial significant adult-life capital.
PRACTICAL TIPS.
(1) Start early — substantial 18-yr compounding.
(2) Substantial automate monthly direct debit.
(3) Substantial transfer Cash JISA between providers for best rates.
(4) Substantial educate child as 18 approaches.
(5) Substantial consider Stocks & Shares for long horizon.
UK Junior ISA benchmarks (2024-25)
Reference JISA structure + projections.
| Item | Detail |
|---|---|
| Annual allowance 2024-25 | £9,000 |
| Cash JISA top rate | 4-4.5% |
| Stocks & Shares historical | 6-9% |
| Tax treatment | Fully tax-free |
| Locked until | Age 18 |
| 16-17 adult Cash ISA additional | £20K |
| Terminal illness early access | Yes |
| £100/mo × 18 yr × 7% | ~£43K |
| £300/mo × 18 yr × 7% | ~£128K |
| £750/mo (max) × 18 yr × 7% | ~£320K |
| Vanguard platform fee | 0.15% (capped) |
| Hargreaves Lansdown fee | 0.45% |
JISA fully tax-free + doesn't count parental income (no 'parental settlement' tax). Child takes full control at 18 — no parental control post-18. 16-17 can have JISA + adult Cash ISA (£29K total). Long horizon 18-yr substantial equity preferred. Low-cost index funds substantial. HMRC + FCA + Moneyfacts research.
Frequently Asked Questions
How is Junior ISA growth calculated?
The lump sum is multiplied by (1 + annual return) raised to the number of years until age 18. £5,000 at 6% for 18 years is £5,000 × 1.06¹⁸ ≈ £14,272 — all of it tax-free within the JISA.
When can the child access a Junior ISA?
At 18. The account is opened and managed by a parent or guardian, but the money legally belongs to the child and is locked until their 18th birthday, when control passes to them and the JISA becomes a normal adult ISA. They then have full, unrestricted access to spend or invest it as they choose.
What are the types of Junior ISA?
Two: a Cash Junior ISA (tax-free interest, like a savings account) and a Stocks & Shares Junior ISA (tax-free investment growth, modelled here). They share one annual Junior ISA allowance. For long horizons, a Stocks & Shares JISA invested for growth typically outpaces a Cash JISA, with some families de-risking as 18 approaches.
Who can pay into a Junior ISA?
Anyone — parents, grandparents, and other relatives or friends can contribute, up to the annual Junior ISA allowance in total. The allowance is separate from a parent's own adult ISA allowance, resets each tax year, and doesn't carry forward, so it's use-it-or-lose-it each year.
Is the growth really tax-free?
Yes — all interest, dividends, and capital gains earned inside a Junior ISA are free of UK tax, just like an adult ISA. Combined with the up-to-18-year horizon, this tax-free compounding is what makes the JISA so effective for building a child's nest egg from even modest contributions.
When is this calculator unreliable?
Less reliable when Cash vs Stocks & Shares JISA different return characteristics (Cash 4-5% nominal vs equity 6-9% with volatility), when JISA allowance changes (was £9K since 2020, prior £4,368), when Child Trust Fund (CTF) transfer to JISA pre-2011 has different rules, when JISA locked until 18 (no early access except terminal illness — child takes control at 18 with no parental restriction), when market volatility for Stocks & Shares JISA substantial, or when provider fees differ (Vanguard 0.15% capped vs Hargreaves Lansdown 0.45%).
References & Authoritative Sources
- HM Revenue & Customs (HMRC) — Junior ISA Rules · consulted June 1, 2026 · UK federal tax authority
- Financial Conduct Authority (FCA) — ISA Regulations · consulted June 1, 2026 · UK regulator
- Moneyfacts / MoneySavingExpert — Junior ISA Comparisons · consulted June 1, 2026 · Consumer financial research
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Methodology & Review
UK Junior ISA (JISA) growth = contributions × (1 + return)^years. Calculator returns 18-year maturity. JISA 2024-25 allowance £9,000/year. Cash JISA 4-5% top rates; Stocks & Shares JISA equity growth 6-9% historical avg. Substantial tax-free. Locked until 18 (child takes control). 16-17 can also have adult Cash ISA simultaneously. RELIABILITY: Reliable for documented contributions + return assumption. Less reliable when (a) Cash vs Stocks & Shares JISA different return characteristics; (b) JISA allowance changes (was £9K since 2020, prior £4,368); (c) Child Trust Fund (CTF) transfer to JISA pre-2011; (d) JISA locked until 18 (no early access except terminal illness); (e) market volatility for Stocks & Shares JISA; (f) provider fees substantial — Vanguard, Fidelity, Hargreaves Lansdown differ.
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