Junior ISA Calculator

Junior ISAs have an annual contribution limit of £9,000 per child per tax year, available from birth until age 18. Any UK resident child under 18 who does not have a Child Trust Fund can have a JISA. Parents, grandparents, and anyone else can contribute, but the total from all sources cannot exceed £9,000. The child takes ownership at 18 and can withdraw or continue holding the ISA as an adult ISA.

The compound growth calculation is particularly powerful for JISAs because of the long time horizon (up to 18 years). At 6% annual growth, £9,000 contributed every year for 18 years grows to approximately £311,000 — of which £149,000 is pure investment growth. Even modest contributions of £100/month over 18 years at 6% growth produce approximately £38,700, nearly double the £21,600 contributed.

Cash JISAs offer guaranteed returns at a fixed or variable interest rate, suitable for shorter time horizons. Stocks and Shares JISAs invest in funds or shares and are generally recommended for JISAs because the 18-year time horizon smooths out short-term volatility. The calculator shows both scenarios side by side, illustrating how even modest differences in average return (e.g., 4% cash vs 6% equities) compound to substantial differences over 18 years.

Junior ISA basics. Junior ISA (JISA) is a tax-free savings/investment account for under-18s. Annual allowance £9,000 (2026/27) — much higher than the LISA £4,000 limit. Can be Cash JISA, Stocks & Shares JISA, or both. Funds locked until age 18, then convert to adult ISA. Anyone can contribute (parents, grandparents, family, friends) up to the annual limit. Cannot be opened or controlled by under-18s except in very limited cases.

Why Stocks & Shares JISA usually beats Cash for newborns. 18-year time horizon makes equity growth nearly certain to beat cash. £100/month from birth to 18 in S&S JISA at 7% real return = £42,500. Same amount in Cash JISA at 4% AER = £30,500. £12,000 difference for ~£21,600 contributed. Use low-cost global tracker funds (Vanguard FTSE Global All Cap, iShares MSCI World). At age 18 it automatically becomes an adult ISA without losing tax benefits.

JISA vs LISA at age 18. At 18, the JISA auto-converts to an adult ISA. The child can then choose: (1) Keep as Cash or S&S ISA; (2) Transfer to a Lifetime ISA (LISA) before 40 to qualify for 25% government bonus for first home or retirement at 60. JISA → LISA transfer uses the LISA annual limit (£4,000/year) — so a £42,500 JISA pot transferred at £4k/year takes 11+ years. Plan ahead: open the child's LISA at 18 for housing/retirement goals.

Tax planning for parents and grandparents. Money gifted from parents into a JISA is not subject to the £100 'parental settlement' rule (which would otherwise tax income above £100 as the parent's). JISA growth is entirely tax-free. Grandparents can use the £3,000 annual gift exemption + £250/recipient small gifts to fund JISAs without IHT implications. JISA contributions count for 'regular gifts out of income' IHT exemption if structured properly.