What this planner works out
This planner subtracts what you have already paid in from your annual ISA allowance and divides the remainder by the months left in the tax year, giving the monthly amount needed to use it fully.
The allowance is editable rather than fixed, because it is set by the government and has changed several times. Confirm the current figure on GOV.UK before relying on the result.
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Before you plan
- Confirm the current annual allowance, and remember it covers all your ISAs combined rather than each one separately.
- Count contributions from 6 April, when the tax year starts, not from January.
- Include everything paid into any ISA this tax year — cash, stocks and shares, innovative finance and Lifetime ISA contributions all count against the same allowance.
- Count the months remaining to 5 April, when the allowance resets and any unused portion is lost.
Unlike pension annual allowance, unused ISA allowance cannot be carried forward. Whatever is unused at midnight on 5 April is gone permanently. That is the single most important feature of the allowance and the reason a monthly figure is worth calculating.
How this calculator works
One subtraction and one division:
Remaining allowance = annual allowance − already contributed
Monthly amount to use it = remaining allowance ÷ months remainingThe remaining figure never goes below zero. Note that withdrawing from a flexible ISA and replacing the money in the same tax year does not use additional allowance — but this only applies to accounts specifically offering flexibility, and many do not.
Worked example: eight months left
Using the default figures — a £20,000 annual allowance, £5,000 already contributed and eight months remaining:
- Remaining allowance: £20,000 − £5,000 = £15,000
- Monthly amount to use it fully: £15,000 ÷ 8 = £1,875
£1,875 a month is out of reach for most households, and that is worth saying plainly: the full allowance is far larger than most people can use. The planner is more useful in reverse — if you can manage £400 a month, you will use £8,200 of the allowance, and the remaining £11,800 is not a failure. Very few people are constrained by the ISA allowance rather than by their income.
Common mistakes
- Treating the allowance as per account. It is a single limit across all your ISAs.
- Counting from January. The ISA year runs 6 April to 5 April.
- Forgetting the Lifetime ISA counts. Its own £4,000 limit sits inside the overall allowance.
- Assuming a withdrawal frees up allowance. Only flexible ISAs work that way, and not all are flexible.
- Leaving a large contribution to the final days. Transfers and applications can take longer than expected.
Frequently asked questions
What types of ISA are there?
Cash ISAs, stocks and shares ISAs, innovative finance ISAs and Lifetime ISAs, plus Junior ISAs for under-18s which have their own separate allowance. Rules on how many of each type you can pay into in a tax year have been relaxed in recent years, so check the current position on GOV.UK rather than relying on older guidance.
Is a cash ISA worth it?
It depends on whether you would otherwise pay tax on the interest. The Personal Savings Allowance lets basic rate taxpayers earn a certain amount of interest tax-free outside an ISA, so for smaller balances the shelter may add nothing. For higher rate taxpayers, larger balances, or anyone expecting rates to stay high, the ISA advantage is more meaningful and grows over time.
What is a Lifetime ISA?
An ISA for those aged 18 to 39 that pays a 25% government bonus on contributions up to £4,000 a year, intended for a first home or retirement. The bonus is substantial, but withdrawing for any other purpose before 60 carries a charge that can return less than you put in. It suits a specific situation well and is poor value outside it.
Should I transfer an old ISA?
Possibly, if the rate is poor — old cash ISAs frequently drift onto very low rates. Always use the provider's transfer process rather than withdrawing and re-depositing, because a withdrawal loses the tax-free status and the money counts against this year's allowance when you put it back.
Is what I enter stored?
No. Contribution figures are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — current ISA allowances, types and transfer rules
- MoneyHelper — guidance on choosing between ISA types
Sources are checked at publication and can change — how I choose and check references.