Junior ISA Calculator United Kingdom

A Junior ISA is a tax-free savings and investment account for a child under 18, and regular contributions have a long time to grow before the money is unlocked. Enter a monthly contribution, an assumed annual return and the years until the child turns 18, and this calculator projects the tax-free pot at 18, showing how much you paid in and how much is growth.

How the projection works

The calculator treats your monthly contribution as a regular payment that compounds every month until 18. Each payment earns the assumed return for every remaining month, so the earliest payments grow the most and the last one barely has time to move. The longer the time to 18, the more compounding does, which is why starting early matters more than the exact rate. At a zero return the result is simply the number of months times the monthly amount.

The Junior ISA allowance

Every child has a Junior ISA allowance of £9,000 per tax year, covering everything paid into cash and stocks and shares Junior ISAs and shared by everyone who contributes, so a grandparent’s payments reduce the room left for a parent that year. The calculator converts your monthly figure to a yearly one and flags when it would break the allowance, which happens above £750 a month.

Contributions versus tax-free growth

The calculator splits the final pot into the money you put in and the growth on top. Over 18 years at a healthy assumed return, the growth can approach or exceed the contributions, and because it sits inside the Junior ISA, none of it is taxed. A modest monthly habit kept up through childhood can end up worth far more than the sum of the deposits.

A worked example

You pay £200 a month for 18 years and assume a 5 per cent annual return. Contributions total £43,200, and with monthly compounding the pot reaches about £69,840 by the child’s 18th birthday, about £26,640 of tax-free growth. £200 a month is £2,400 a year, well within the £9,000 allowance.

If the same £200 a month runs for only nine years, contributions halve to £21,600 but the pot reaches only about £27,200, with growth of under £6,000, because the early years that had longest to compound never happened. £100 a month from birth reaches about £34,900, more than £200 a month from age nine despite the smaller payments.

Who can pay in and how it is opened

Someone with parental responsibility opens the Junior ISA, but once it exists anyone can pay in, including grandparents, aunts, uncles and family friends, which makes it a natural home for birthday and Christmas money. Everyone shares the £9,000 cap, so coordinate to avoid breaching it. A child can hold one cash Junior ISA and one stocks and shares Junior ISA at a time, and money can be moved between providers or types without using any allowance. A Child Trust Fund can also be transferred into a Junior ISA, which usually offers more investment choice and lower charges.

Cash versus stocks and shares

A cash Junior ISA works like a savings account: the balance cannot fall, but the return follows interest rates and is often beaten by inflation over eighteen years. A stocks and shares Junior ISA invests in funds or shares, so its value rises and falls, but it has historically delivered higher long-run returns that better protect buying power. Because the money cannot be touched until 18, short-term market falls have time to recover, which is one reason many families favour the invested option while a child is young and consider moving towards cash in the final years.

Common mistakes to avoid

The most common is starting late, since the earliest years of compounding do the most work. Another is leaving a long-term pot entirely in cash, where inflation can erode its real value. A third is losing track of the shared £9,000 allowance when several relatives contribute. Also remember the money belongs to the child outright at 18, with no conditions; if you want more say over how it is used, a Junior ISA may not suit every goal.

Assumptions and limitations

This is an illustration, not a forecast. It assumes the same contribution every month, one steady return with no fees and no change to the Junior ISA allowance, which is stored as a parameter and updated when the government changes it. Real returns vary, cash and stocks and shares Junior ISAs behave very differently, and investments can fall as well as rise. Use the calculator to compare scenarios, then check the current rules in the official guidance or speak to a regulated adviser.

About This Junior ISA Calculator

Use this Junior ISA calculator to get an instant, accurate estimate. The figures follow official GOV.UK guidance and every rate and threshold is kept up to date for 2026. Adjust the inputs above to match your situation, or browse all our calculators to plan your next step.