ISA calculator — methodology

One limit, four account types

The four adult types gov.uk lists are the cash ISA, the stocks and shares ISA, the innovative finance ISA and the Lifetime ISA. There is one subscription limit across all four, and the split between them is free:

used      = cash + stocks and shares + innovative finance + lifetime
remaining = max(0, overall limit − used)
excess    = max(0, used − overall limit)

Remaining and excess are reported separately 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.

The Lifetime ISA has two limits on it at once

Its own annual limit, and its share of the overall one. gov.uk states the relationship directly: “The Lifetime ISA limit of £4,000 counts towards your annual ISA limit.” So a full Lifetime ISA subscription does not sit beside the overall allowance — it comes out of it:

bonus            = 25% × min(paid into a lifetime ISA, lifetime ISA limit)
left for others  = overall limit − lifetime ISA limit

The bonus is not a subscription. HMRC adds it, so it uses no allowance, and the account therefore ends the year holding more than its own annual limit. That is not a contradiction and it is the single figure readers most often expect this page to get wrong.

The rate and the cap are both published, and they reconcile — 25% of £4,000 is £1,000. The engine derives the cap from the rate and the limit rather than storing it, so a mistake in either one would stop the two published figures agreeing, which is what packages/tax-core/test/hmrcGoldenFixtures.test.ts asserts.

The Junior ISA is a different person's allowance

It is measured against its own limit and never enters the adult subtraction, because the account holder is the child. gov.uk: “Parents or guardians with parental responsibility can open a Junior ISA and manage the account, but the money belongs to the child.” The child takes control at 16 and can withdraw at 18.

Limits, read from the rules files this calculator runs on

Paying into more than one ISA of the same type

Allowed since 6 April 2024, with the Lifetime ISA as the exception. This rule changed recently enough that a page stating it from memory would be stating the old one, so it is quoted. gov.uk’s own worked example: “You could save £10,000 in one cash ISA, £3,000 in another cash ISA and £7,000 in a stocks and shares ISA in one tax year.” And the exception, from the same page: “You can only pay into one Lifetime ISA in a tax year.”

The overall limit binds regardless of how many accounts the subscriptions are spread over, which is why this calculator asks for a total per type rather than per account.

Sources

• gov.uk — Individual Savings Accounts (ISAs)The overall subscription limit and the four adult ISA types.
• gov.uk — How ISAs workThe rule on splitting the allowance across accounts, the worked example showing two cash ISAs in one tax year, and the restriction to one Lifetime ISA a year.
• gov.uk — Lifetime ISAThe £4,000 annual limit, the 25% bonus and its £1,000 cap, and the sentence that puts the Lifetime ISA limit inside the overall one.
• gov.uk — Junior Individual Savings AccountsThe Junior ISA limit, and that the money belongs to the child.
• gov.uk — Tax-free savings newsletter 19 (November 2025)The limits are frozen until April 2031. This is what lets an earlier tax year be stated at all — the three pages above each describe only the current year.

Four things that change the answer and are not modelled

• Subscriptions you have forgotten. Nothing here reads an account. Every figure is one you typed, and the most likely reason this page and HMRC disagree is a payment made in April that nobody remembers in February.
• Flexible ISAs. Money withdrawn from a flexible ISA and replaced in the same tax year does not use allowance again. Whether your account is flexible is a fact about your provider, and this calculator cannot know it — so it treats every payment as a subscription, which is the safe direction to be wrong in.
• Transfers. Moving an ISA from one provider to another, including money subscribed in an earlier year, uses no allowance. Only new money does. Enter transfers as nothing.
• Eligibility of any kind. Residence, age, the Lifetime ISA rules on opening before 40 and paying in until 50, and the withdrawal charge on taking money out of a Lifetime ISA other than for a first home or after 60, are all outside this arithmetic. A subscription this page reports as within the limits can still be a subscription you were not entitled to make.