P800 Tax Refund Calculator 2026/27

HMRC sends a P800 letter when their end-of-year calculation shows you overpaid or underpaid tax through PAYE. Overpayments are automatically refunded (usually within 5 weeks); underpayments are collected via an adjustment to the following year's tax code.

Common reasons for overpaying: wrong tax code (e.g. BR used instead of 1257L), leaving a job mid-year without using the full Personal Allowance, or Marriage Allowance not being applied.