Tax overpayments and underpayments

1. Your tax calculation

If you’re employed or get a pension, HM Revenue and Customs (HMRC) checks whether you’re paying the right amount of tax throughout the year.

They may adjust your tax during the year if your circumstances change, for example if you get a pay rise or a company benefit.

If you haven’t paid the right amount at the end of the tax year, HMRC will post you a tax calculation. This can be a P800 or a Simple Assessment letter.

Your tax calculation will show you how to get a refund or pay tax you owe.

When you might get a P800

You might get a P800 if you:

  • finished one job, started a new one and were paid by both in the same month
  • started receiving a pension at work
  • received Employment and Support Allowance or Jobseeker’s Allowance

You won’t get a P800 if you’re registered for Self Assessment. Your bill will be adjusted automatically if you’ve underpaid or overpaid tax.

P800s are sent out after the tax year ends on 5 April. You’ll normally get yours by the end of November.

When you might get a Simple Assessment letter

You might get a Simple Assessment letter if you:

  • owe tax that can’t be automatically taken out of your income
  • owe HMRC more than £3,000
  • have to pay tax on the State Pension

You can pay your Simple Assessment bill online.

Checking your tax calculation

Your letter will show the income you should have paid tax on. This includes any income from pay, pensions, state benefits, savings interest and employee benefits.

Compare the figures with your records, for example your P60, bank statements or letters from the Department for Work and Pensions. If your state benefit was paid every 4 weeks, work out the total paid in a year by multiplying your regular payment by 13 (not 12).

You may be able to use the HMRC tax checker to work out how much tax you should have paid.

If you think your tax calculation is wrong

Contact HMRC if you think the amounts used in your letter are wrong, or HMRC didn’t act on information you gave them.