Junior ISA calculator — methodology

The term is derived, not entered

Every other projection on this site asks how long for. This one cannot: a Junior ISA runs until the child turns 18 and then ceases to be one. So the term is a subtraction, and the age field’s bounds are the wrapper’s own legal domain rather than round numbers — from birth to one year below maturity, because at maturity there is no Junior ISA left to calculate.

term in years   = maturity age − the child's age today
years to control = max(0, control age − the child's age today)

The lower bound of the age field is zero, which is also what an empty field substitutes — clamp(0, min, max). That is a real and common age for a Junior ISA holder rather than a neutral placeholder, so the term it produces is printed in words in the headline directly beneath the fields. An empty age box cannot silently lengthen a projection when the number of years it assumed is on screen next to the answer.

The schedule, and the conventions it states

The balance is a period-by-period schedule rather than a closed-form formula, so every headline figure can be read back off a row the reader can see. Each period, in this order:

balance ← balance + contribution   (paid at the start of the period)
balance ← balance × (1 + periodic rate)
balance ← balance − platform charge

The annual rate is treated as an effective rate, so the figure entered is the figure a year compounds to. Dividing an effective rate by twelve and applying it twelve times compounds to more than the rate entered, which over a term that can reach 18 years is not a rounding difference. The periodic rate actually used is printed on the calculator page beside the schedule, so the convention is checkable rather than merely stated.

The two ages are read off the same schedule

Each year of the rollup is labelled with the age the child reaches at the end of it. So the row marked 16 closes on that birthday, and the closing balance beside it is the pot on the day the child becomes the registered contact. Labelling rows with the age at the start would put the marker a year early, which on this page is the wrong direction to be wrong in.

age at the end of year i = age today + i + 1
balance at control       = closing balance of the row where that age = control age
balance at maturity      = the schedule's final closing balance
paid in after control    = sum of contributions in rows above the control age

For a child already at or past the control age there is no such row, and the answer is what the account holds today — not zero and not a dash. That is the normal case for a 16- or 17-year-old rather than an error, and the page says so in words rather than printing a balance with no explanation.

What changes at each age. At 16 the child takes over managing the account: choosing the investments, moving the provider. The adult who opened it can no longer do either. At 18 the account matures and the money is the child’s to withdraw in full, on any day, for any purpose. No condition can be attached to it by anyone who paid in.

The allowance is the child's, and it does not carry forward

The allowance check is a subtraction, not a projection: one year of contributions at the chosen frequency, measured against the published Junior ISA limit.

contributions a year = contribution × periods a year
left of the limit    = max(0, junior ISA limit − contributions a year)
over the limit by    = max(0, contributions a year − junior ISA limit)

Both are reported and neither is clamped away. A reader over the limit has something to act on, and a page showing them a tidy “£0 left” would be hiding it.

“Left of the limit” is never printed as headroom. An unused Junior ISA allowance does not carry forward: on 6 April what was unused is gone and a fresh limit starts. A remaining figure with no deadline attached reads as money waiting somewhere, which is a different and false claim, so the clause travels with the number everywhere it appears — on the calculator page, in the worked example and in the CSV.

The limit belongs to the child. It is not part of the adult ISA allowance and never enters that subtraction, which is why an adult can subscribe the full adult limit to their own ISAs in the same tax year they fill a child’s Junior ISA. That relationship, from the other side, is what the ISA allowance methodology sets out.

Limits, read from the rules files this calculator runs on

The two ages above are the only figures on this calculator that are not read from a rules file. They are constants in the page’s own model, because neither is an April figure and neither is a rate, an allowance or a threshold in money — the three things the rules directory versions. If every statutory figure is to live there, the fields to add are a control age and a maturity age beside the Junior ISA limit, and this page should read them instead.

Sources

• gov.uk — Junior Individual Savings AccountsThe annual limit, the two account types and the single limit shared between them, that the money belongs to the child, and the two ages — the child manages the account from 16 and can withdraw from 18.
• gov.uk — Individual Savings Accounts (ISAs)The adult subscription limit, which this page cites only to show that a Junior ISA does not touch it.
• gov.uk — Interest on savings for childrenThe parental settlement rule that applies to a taxable account and not to a Junior ISA. Its threshold is not held in this site’s rules files, so the mechanic is described on the page and the figure is deliberately not quoted.
• gov.uk — Tax-free savings newsletter 19 (November 2025)The limits are frozen until April 2031. This is what lets a second tax year be stated at all — the pages above each describe only the current one.

The stamp above covers the limit. It does not cover the balances.

A fixed-rate projection is not a forecast. The balances on this calculator come from arithmetic on a growth rate the reader typed. No rate, threshold or allowance published by government goes into them, so there is nothing for a gov.uk check to be about, and borrowing the stamp’s language for them would attach the credibility of a real check to a place no such check exists or could exist.

Nothing grows at the same rate every year. Real returns arrive in an order, and the order changes the outcome even where the average is identical. Treat the schedule as what a constant rate would have produced — a good way to compare two contribution plans and a poor way to predict a balance eighteen years out.

Five things that change the answer and are not modelled

• Inflation. Every figure is nominal, in today’s pounds. Over a term of up to 18 years this is the largest single caveat on the page, and it is not a conservative one — a balance that looks like a house deposit today may not be one on the day it matures.
• Limits in future tax years. The allowance check applies one published limit to one year of contributions. This site holds no figures for tax years the government has not published, and inventing a trajectory for them in order to check eighteen years of subscriptions would be the opposite of what the rules directory is for.
• Child Trust Funds. A child with one cannot also hold a Junior ISA until the two are combined, and a transfer between them uses no allowance. Whether that applies is a fact about the child’s date of birth and their existing accounts.
• The parental settlement rule, quantitatively. Outside a Junior ISA, income produced by money a parent gave a child is taxed on the parent once it exceeds a set amount in a year — the whole of it, not the excess. It is the strongest reason the wrapper exists for a parent’s own money, and it has no application inside a Junior ISA. The threshold is not held in this site’s rules files, so the mechanic is described and the figure is not quoted; it is a candidate field for the same reason the two ages are.
• Whether any of this is a good idea. This page computes; it does not advise. Whether to use a Junior ISA at all, rather than saving in an adult name and gifting later, turns on facts about a family that no calculator has.