Updated on 6 April 2025

Self assessment tax calculations

A key part of self assessment is calculating your tax (and, if appropriate, self-employed National Insurance contributions and student loan repayments) due for the tax year. In most cases, HMRC perform this calculation for you based on the information you include on your tax return. 

brown paper with a tear in the centre, through the tear you can see a white background with the words 'SELF ASSESSMENT TAX' in black text
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When you submit a tax return, HMRC will usually calculate the amount you need to pay under self assessment based on the entries you include on the form. This calculation is sometimes called an SA302 tax calculation.

Filing online

If you file your tax return online using HMRC’s online self assessment system, it will calculate how much you owe. You can view the tax calculation online or print it out. If you use third-party software to file the return, this should also do the calculation for you and allow you to print out a copy of the calculation for your own records.

Filing on paper

HMRC will also usually calculate your self assessment tax bill for you if you file your tax return on paper. Again, the calculation will be based on the entries on the tax return, and they will send you the tax calculation by post once the form has been processed.

The table on our tax return deadlines page shows the dates by which you must file your self assessment tax return so that HMRC can send the tax calculation to you before the tax payment deadline.

If you don’t manage to file your paper tax return by the relevant deadline set out in the table, HMRC will still do the calculation for you but they cannot guarantee that they will be able to send it to you before the date that any tax payment is due.

In this case, you may consider making a payment by the due date based on an estimate of your tax bill. Otherwise, if you make the self assessment payment late, penalties and interest may become payable. These are in addition to any penalties that might apply for late filing.

Tax paid at source

You may already have had tax deducted from some income – for example, if you are employed, or are registered under the construction industry scheme (CIS).

You should include all taxable income on the tax return, whether or not it has already been taxed, making sure to include the tax that has been deducted at source in the correct box on the return too. The tax calculation will automatically take account of any tax paid at source (for example, through PAYE or CIS) that has been included on the tax return.

The tax calculation will then show the amount that is left to be paid through self assessment or to be refunded. Note that it might need to be adjusted for any payments on account already made, as we explain under the heading Payments on account below.

Payments on account

If you needed to make payments on account, any payments on account already made for a particular tax year should be compared to the final tax position for that year to see whether there is any further amount to pay (known as the balancing payment) or whether a refund is due.

You do not enter any tax paid as a payment on account anywhere on the tax return form itself.

Example – self assessment calculation with payments on account

Chloe, who lives in England, is employed part-time and also runs her own business. During the 2024/25 tax year she earned £15,000 from her job and paid tax through PAYE of £486. She also earned profits of £4,000 from her self-employment.

When Chloe completes her 2024/25 tax return, she includes all her employment income and tax deducted through PAYE and her self-employment profits. Her tax calculation will look as follows (no self-employed National Insurance contributions are due in this case):

Tax calculation
 

£
Employment income

15,000
Self-employment profits

4,000
Total income received

19,000
minus Personal Allowance

-12,570
Total income on which tax is due

6,430
Income tax due at 20%

1,286
minus tax deducted via PAYE

-486
Total income tax due by 31 January 2026

800

If Chloe had higher self-employed profits in the 2023/24 tax year, she might have been due to make payments on account for the 2024/25 tax year.

Let’s say she paid £1,200 in total via payments on account, as follows:

  • 31 January 2025: £600
  • 31 July 2025: £600

When Chloe submits her 2024/25 tax return online in November 2025, her total income tax due would still show as £800 as per the calculation above. But she is actually due a refund of £400 (the £800 she owes, minus the £1,200 she has paid) as her payments on account were more than her final tax liability. Chloe will need to deduct her payments on account from the figure in the online calculation to work out she is due a refund.

After the online tax return has been automatically processed, Chloe should be able to see that she is due a refund in her personal tax account. This should be repaid to her by HMRC shortly after the tax return is processed but if not, she can claim this back from HMRC.

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