What is a P800?
A P800 tax calculation is how HMRC tell you you've paid the wrong amount of tax. It's shows what tax you owe and what tax you've paid. Usually you one get one when you've paid too much or too little tax. If you haven't paid all the tax you owe, HMRC will take action to get it from you but if you've overpaid, you can get a tax refund.