How Do You Split an ISA Allowance Between Cash and Shares?

In the 2026-27 tax year, the overall ISA allowance is £20,000 across all of your ISAs, and gov.uk says you can save up to that amount in one account or split it across multiple accounts. The pages read set the total, not a required proportion for cash or for stocks and shares. From April 2027 a separate, lower limit on cash ISA payments is due to apply to savers under 65; it is made in law and not yet in force.

The site's rules data was last checked against gov.uk on 26 September 2026. Figures typed in this guide may not yet reflect a later change, and the descriptions of gov.uk pages are summaries that may also lag. The current figures are on the ISA rules at a glance page. General information only, not financial advice.

How does splitting the ISA allowance work?

The GOV.UK page on how ISAs work says that every tax year you can save up to £20,000 in one account or split the allowance across multiple accounts. It gives an example of saving in one cash ISA, in another cash ISA and in a stocks and shares ISA in the same tax year. The tax year runs from 6 April to 5 April. gov.uk says you must be 18 or over, and resident in the UK (or in one of the other groups it lists), to open an ISA (GOV.UK: Individual Savings Accounts).

So £20,000 is a combined limit across all ISA types held in a tax year, and the split between Cash ISAs and Stocks and Shares ISAs only has to stay within that total. For example, £10,000 could go into a Cash ISA and £10,000 into a Stocks and Shares ISA, or £6,000 into cash and £14,000 into stocks and shares. These are assumed amounts, not suggestions. The Lifetime ISA is the exception to paying into several accounts of one type: gov.uk says you can only pay into one Lifetime ISA in a tax year, and the Lifetime ISA's £4,000 limit counts towards the overall £20,000, not in addition to it (GOV.UK: Lifetime ISA).

What is changing for cash ISAs?

Regulations made on 10 September 2026 (SI 2026/1018, regulation 6, read on legislation.gov.uk on 6 October 2026) set a separate yearly limit on how much savers who are under 65 at the end of the tax year can pay into cash ISAs, lower than the overall allowance, to apply from April 2027. The GOV.UK policy paper says the overall ISA limit is retained and that the cash ISA limit for investors aged 65 or over is not reduced (GOV.UK: cash ISA limit reduction). So younger savers who wish to pay in the whole allowance would need to put the rest into an ISA that is not a cash ISA, such as a Stocks and Shares ISA. The ISA Contribution Split Planner checks each tax year against the current limits. See also the £12,000 cash ISA limit explained.

What do Cash ISAs and Stocks and Shares ISAs offer?

Cash ISAs

A Cash ISA works much like a standard savings account, with the interest sheltered from UK Income Tax. It is lower risk than investing and access depends on the account terms. FSCS deposit protection is up to £120,000 per eligible person, per bank, building society or credit union. The FSCS pages read for this guide do not name ISAs, so ask your provider or FSCS whether a given ISA is covered. Cash interest rates can lag behind inflation, which means the real purchasing power of cash can fall over long periods.

Some people consider Cash ISAs for the following, as an illustration and not a rule:

Short-term savings goals (some think of under three to five years as short term)
Emergency funds
Money where a fall in value, even temporary, would be a problem
Capital preservation near retirement

Stocks and Shares ISAs

A Stocks and Shares ISA can hold shares in companies, unit trusts and investment funds, corporate and government bonds, and long-term asset funds (GOV.UK: how ISAs work). The potential for higher long-term returns is the usual reason people consider them, but the value of investments can fall as well as rise, and you could get back less than you put in.

Some people consider Stocks and Shares ISAs for the following, as an illustration and not a rule:

Longer-term goals (five years or more)
Savers who can tolerate short-term falls in value
Money intended for retirement or a significant future cost

What do people weigh up when splitting an allowance?

The rules set no required split between cash and stocks and shares. People weigh several factors, and the balance is personal.

Time horizon. If money is needed in two years, some people hold more in cash because there may be less time to wait out a market fall. If a goal is five or more years away, some choose more in stocks and shares.

Risk tolerance. A Stocks and Shares ISA can fall in value, for example by 20% within a year. Whether the holder could stay invested through that, or would feel compelled to withdraw, is a personal matter.

Overall financial picture. An ISA split sits alongside other savings. Some people with a cash emergency fund outside their ISA hold less cash inside it, while some whose ISA is their only cushion keep more of it in cash.

A worked example: splitting the allowance across two ISA types

This is an assumed scenario to show the arithmetic. It is not a recommendation.

Sarah, 34, earns £42,000 a year and has £6,000 in an easy-access account as an emergency fund. In this example she pays in the full £20,000 this tax year. Her two goals are a kitchen renovation in two years (about £7,000) and long-term retirement savings alongside her workplace pension.

Cash ISA: £7,000, matching the renovation cost and timeline.
Stocks and Shares ISA: £13,000, for the longer-term goal.

That is a 35/65 split between cash and stocks and shares. It reflects the two goals in the example and is not a rule. The ISA Calculator can show how each pot might grow under different rate and return assumptions, and its results are illustrations, not forecasts.

Can the split change in later tax years?

Yes. gov.uk gives the allowance for each tax year, so each new tax year the whole allowance can be allocated again, differently if circumstances have changed. If Sarah's renovation were finished, she could in the same example put the whole allowance into a Stocks and Shares ISA the next year. The guide read does not describe any carry forward of unused allowance, so this guide does not assume one.

Some ISAs are flexible. gov.uk says that in a flexible ISA you can take out cash and put it back in the same tax year without reducing your current year's allowance (GOV.UK: withdrawing your money). Existing balances can be moved between ISA types through the transfer process, and gov.uk says that if you withdraw the money without using it, you will not be able to reinvest that part of your tax-free allowance again (GOV.UK: transferring your ISA).

For a multi-year view, the Advanced ISA Planner models different contribution splits across tax years and ISA types.

What does splitting an allowance not do?

It does not guarantee returns. Money in a Stocks and Shares ISA can fall in value, and past performance is not a reliable indicator of future results.
It does not protect cash from inflation. If the interest rate is below inflation, the real value of the savings can fall even while the balance grows.
It does not rebalance itself. Neither the provider nor HMRC adjusts a split if markets move and the proportions drift. Rebalancing, if wanted, is the holder's own task.
It does not replace advice in complex situations. Where there is a large sum, an unusual tax position or complex goals, a regulated financial adviser may be worth consulting. ISAPlan provides educational tools and calculators and does not give financial advice.
It does not allow more than one Lifetime ISA. Several Cash or Stocks and Shares ISAs can be paid into in one tax year, but only one Lifetime ISA, and the total across all ISAs must stay within the allowance.

What frameworks do people describe for a split?

Age-based rule of thumb: some planners describe holding a percentage in lower-risk assets roughly equal to age (for example 35% at age 35). It is a rough heuristic, not a substitute for planning.
Goal-based splitting: setting aside the amount needed for short-term goals in cash and investing the rest for the long term. Sarah's example above works this way.
Equal split: some savers begin with 50/50 and adjust over time. It will not suit every situation.

The Cash ISA Calculator and the Stocks and Shares ISA Calculator can compare outcomes side by side. For the differences between the two account types, see Cash ISA vs Stocks and Shares ISA.

What to check and questions to ask

Goals and timing: What is the money for, and when is it needed?
Risk tolerance: How would a significant short-term fall in a Stocks and Shares ISA feel? There is no right or wrong answer.
Existing savings: Is there an emergency fund outside the ISA?
Illustrations: The calculators use assumed rates and returns, so their results are not forecasts.
Providers: Cash ISA rates and Stocks and Shares platform costs can be compared separately, and charges affect long-term returns.
The allowance: Total payments across all ISAs must stay within £20,000, and the new cash ISA limit may apply in the tax year being planned.
Review: Goals, time horizon and risk tolerance can change from one tax year to the next.
Old ISAs: For a Cash ISA with a low rate, what do the transfer rules and provider terms involve?

The calculators on ISAPlan need no sign-up; the Advanced ISA Planner is a paid tool.

Frequently asked questions

Is there a required split between cash and stocks and shares in an ISA?

The pages read set the £20,000 total, not a required proportion. gov.uk says you can save in one account or split the allowance across multiple accounts. From April 2027 a lower limit on cash ISA payments is due to apply to savers under 65.

Can I change my ISA split from one tax year to the next?

Yes. The allowance is given for each tax year, so it can be allocated again each year. The guide read does not describe any carry forward of unused allowance.

How will the new cash ISA limit affect a split?

From April 2027, savers under 65 at the end of the tax year can pay a lower yearly amount into cash ISAs, while the overall allowance stays the same. The rest can go into an ISA that is not a cash ISA. The change is made in law and is not yet in force.

This guide does not constitute financial advice. Your personal situation, tax position, and savings goals may mean different strategies are appropriate. Consider speaking to a regulated financial adviser if you are uncertain.