UK Savings Interest Calculator 2026/27: Tax, ISA & PSA

A UK savings interest calculator is most useful when it distinguishes between the account balance, the rate, the type of return and the tax treatment. A headline interest rate does not tell you how much cash you will keep. The result depends on whether the account is taxable, inside an ISA, a Premium Bond holding with variable prizes, a joint account, or a child’s account funded by a parent. It also depends on your other taxable income because the Personal Allowance, Starting Rate for Savings and Personal Savings Allowance interact.

Use the Savings Interest Calculator→ to enter multiple accounts, balances, rates and product types. It can estimate gross interest, a simplified tax amount, net interest and an illustrative comparison with a Cash ISA or Premium Bonds. It cannot confirm your tax band, read HMRC’s bank-interest report, decide beneficial ownership or recommend a savings product.

This guide explains how to enter the numbers for 2026/27, how to read the allowances, why the tax year matters, when HMRC may change your tax code or require Self Assessment, and how to compare taxable savings with tax-free wrappers without treating a variable prize fund as guaranteed interest.

Key takeaways

What the calculator is estimating

The basic gross-interest calculation is:

Gross annual interest = balance × annual interest rate

If £20,000 is held at an illustrative 4%, the simple annual gross interest is £800. That does not mean the result is £800 in every tax year: the balance may change, the rate may be variable, the account may pay monthly, and a fixed-term or promotional product may have a different period. The calculator uses the balance and rate you enter, so save the assumptions beside the result.

The tax estimate then asks whether the return is taxable and how much of the person’s available allowances remain. A Cash ISA return is modelled as tax-free within the ISA rules. Premium Bond prizes are treated as tax-free, but the entered or displayed return is an illustrative expectation rather than contractual interest. A taxable bank account consumes the relevant savings allowances only after the income-ordering and tax-band questions are considered.

Do not use the result as a bank statement. Use statements and certificates to confirm the interest actually paid or credited in the tax year.

Step 1: identify the tax year and payment period

The UK tax year runs from 6 April to 5 April. Interest credited on a particular date can fall into a different tax year from the date you opened the account. A fixed account opened in January may pay no interest until maturity; the taxable timing can depend on the product terms and when the interest is treated as arising.

Before entering a result, record:

• the tax year being modelled;
• the account opening and maturity dates;
• whether interest is paid monthly, annually or at maturity;
• the balance on which the rate applies;
• any introductory or bonus rate expiry;
• whether the account is personal, joint, a child’s account or an ISA; and
• whether tax has already been collected through PAYE or another route.

An annual calculator estimate is most reliable when the balance and rate are reasonably stable. If the money will be added gradually, model each deposit period or use a lower average balance rather than applying the end-of-year balance to the whole year.

Step 2: enter every account separately

The tool supports multiple account lines because one blended rate hides the tax and access trade-off. Add the balance and expected rate for each account:

If you have several taxable accounts, enter them separately even if their rates are similar. This makes it easier to identify which account is consuming the available allowance and which account has a rate that may change. It also reduces the risk of treating an ISA or Premium Bond prize as ordinary bank interest.

Step 3: understand the three layers of tax-free interest

GOV.UK identifies three possible allowances: the Personal Allowance, the Starting Rate for Savings and the Personal Savings Allowance. The amount available depends on other income and the tax band. They are not three universal amounts that everyone can simply add together.

Personal Allowance

If wages, pension or other non-savings income does not use the full Personal Allowance, HMRC may apply the unused portion to savings interest. If employment income already uses the allowance, the calculator should not give you the same allowance again. The correct result begins with the full income picture.

Starting Rate for Savings

GOV.UK says that when other taxable income excluding savings interest and dividends is below £17,570, a person may qualify for a starting rate for savings of up to £5,000. The amount is reduced by £1 for every £1 of other income above the Personal Allowance. This means the full £5,000 is available only in a low-other-income situation; it is not an extra allowance for a normal salary earner.

Personal Savings Allowance

For 2026/27, the GOV.UK table states:

The interest itself can affect which band applies. If adding the interest pushes taxable income across a threshold, the available allowance and marginal tax on the excess may change. This is why entering only the savings balance without other income can produce an overly favourable result.

Step 4: compare gross interest with net interest

Suppose a taxpayer enters £30,000 in a taxable account at an illustrative 4% rate. The gross interest is £1,200. If the person is a basic-rate taxpayer and has no starting-rate or unused Personal Allowance complication, the 2026/27 PSA illustration may shelter £1,000, leaving £200 for the applicable savings tax calculation. If the person is higher rate, the normal PSA is lower; if additional rate, it is zero. These are illustrations, not a final liability.

The same account can produce a different net amount for a different person. A pensioner may have unused Personal Allowance or starting-rate entitlement. A higher earner may have a £500 PSA or no PSA. A Scottish taxpayer’s savings income uses UK-wide savings rules, but their non-savings income can use Scottish bands and affect the wider calculation. The calculator should therefore be used as a scenario tool, not as a universal tax-rate table.

Step 5: Cash ISA and taxable savings are different decisions

GOV.UK states that ISA savings are tax-free and that the 2026/27 maximum ISA subscription is £20,000. A Cash ISA can therefore shelter interest that would otherwise consume the PSA, subject to the subscription limit, provider terms and the account’s access and rate.

The comparison should not stop at “ISA equals tax-free”. Check:

• the gross rate and whether it is fixed or variable;
• access restrictions and notice periods;
• withdrawal or transfer terms;
• the annual ISA subscription room available;
• whether a transfer uses the official ISA transfer process;
• whether the taxable account has a bonus rate that will expire; and
• whether the money is an emergency reserve, house deposit or long-term investment.

A lower ISA rate can still produce more net interest than a higher taxable rate for someone whose PSA is already used. Conversely, an accessible taxable account may be appropriate when the ISA product has restrictions or a materially lower rate. Use the calculator to show the tax difference, then compare the actual product terms.

Step 6: treat Premium Bonds as a variable outcome

NS&I describes Premium Bonds as entries into a monthly prize draw with tax-free prizes. The prize fund rate is not a guaranteed interest rate paid to each holder. A calculator may use an expected or illustrative return to make the comparison visible, but the actual result can be above or below that assumption, including no prize for a particular holding or period.

Keep three lines separate:

1. the amount invested;
2. the expected or assumed prize return; and
3. the actual prize outcome.

Premium Bonds can offer tax-free prize treatment and accessibility, but that does not mean they are automatically better than a deposit account. Compare certainty, access, deposit protection or the NS&I structure, expected return, volatility of outcomes and the household’s need for predictable interest. Do not enter a Premium Bond expected return as if it were a fixed account rate.

Step 7: joint accounts and children’s savings

Interest on a joint account is generally considered between the account holders according to beneficial ownership. The calculator can show a split scenario, but it cannot decide whether a different ownership declaration is valid. Keep the statement, ownership basis and any transfer records.

Children usually have their own allowances, but GOV.UK has a specific parental-gift rule. If a child receives more than £100 of interest in a tax year from money given by a parent, the parent may have to pay tax on all the interest above their own allowances. The £100 limit does not apply to money from grandparents, relatives or friends, or to money in a Junior ISA or Child Trust Fund under the conditions described by GOV.UK.

Do not solve a child-savings tax question by simply moving the account into another person’s name. The source of the money, beneficial ownership, the product and the family relationship matter. When a child’s account has a large balance or a parental gift is near the limit, retain the evidence and check HMRC’s current guidance.

Step 8: understand how HMRC may collect tax

GOV.UK says that the tax collection route depends on the amount of interest and the individual’s circumstances. If you already complete Self Assessment, report the interest there. If you are employed or receive a pension, HMRC may adjust your tax code using information from the bank or building society, or issue a Simple Assessment letter if the code cannot be changed.

If savings interest is more than £10,000 in a tax year, GOV.UK says you must tell HMRC through a Self Assessment tax return and may need to register if you do not already file. That is an administrative reporting threshold, not a statement that the first £10,000 is tax-free. The allowances still determine whether tax is due.

Check HMRC’s calculation against your bank statements. HMRC may use an estimate based on a previous tax year, while your current account rate, balance or fixed-term maturity may be different. Correcting an estimate early can prevent an inaccurate tax code from carrying forward.

Step 9: make a savings decision, not just a tax calculation

The best place for cash also depends on purpose. An emergency fund may need instant access. A house deposit may have a known date and low tolerance for volatility. Long-term money may need a broader comparison with pensions or investments, but those products are not equivalent to cash savings. A savings-interest calculator can show tax drag; it cannot choose the right risk level or time horizon.

Common questions

Run the Savings Interest Calculator→ with each account separated, then compare the result with the Savings Interest Tax guide, the ISA Allowance guide and the official sources below.

Official sources

• GOV.UK — Tax on savings interest: how much is tax free
• GOV.UK — Tax on savings interest: how you pay tax
• GOV.UK — Individual Savings Accounts
• GOV.UK — Interest on savings for children
• NS&I — Premium Bonds
• GOV.UK — Income Tax rates and allowances

This guide is educational. It does not provide tax advice, confirm product suitability, determine beneficial ownership or replace HMRC’s calculation, a bank statement or professional advice.