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.

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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.