If you submit your tax return on-line, then your income tax (and, for the self-employed, your class 4 National Insurance bill) will be calculated automatically. This happens just before you submit the return on-line to HMRC. It is a good idea to check that the calculation is reasonable before you submit the tax return.  If the result of the calculation -  a bill or a refund – is unexpected, then there could be a mistake on your return. You should check all the entries in detail.

Possible errors might include, for example:

• Omitting the figure for tax paid from your P60 (employment income) or from your CIS income (for a self-employed Construction Industry Subcontractor)
• Putting an incorrect amount in the box for ‘Under-paid tax included in your PAYE code’
• Entering information for business turnover, expenses, profits or losses in the wrong box
• Including 12 month’s income rather than the amount received in the tax year if this is less – for example if you started to receive a pension part-way through the tax year
• Failing to claim for pension contributions paid (amounts paid to an employer’s pension scheme will normally have been deducted already from your PAYE income, and so do not need adjusting)
• Not making a claim for an allowance you are entitled to, such as the Blind Person’s Allowance or additional personal allowances available to those aged 65 and over - see  http://taxaid.org.uk/info/income-tax/your-personal-allowance. (You do not need to make a special claim for the basic personal allowance given to those aged under 65)
• Leaving out a claim for employee expenses or fixed rate deductions as an employee. See http://taxaid.org.uk/info/employee-expenses

If you submit your tax return on paper (due by 31 October), then HMRC should calculate the tax due and send you a calculation in time for the 31 January payment deadline.  You should check this calculation in the same way as for an on-line calculation.

There is more about how tax bills are worked out in the income tax section, where there is a worked example – http://taxaid.org.uk/info/income-tax/a-worked-example-based-on-2010-11