First, compare HMRC’s calculation with your tax return, payslips, tax code and supporting records, as an incorrect figure may result from missing or duplicated information. For Self Assessment, you can usually amend your return within 12 months of the filing deadline; otherwise, contact HMRC to explain the error and provide evidence. If the calculation forms part of a formal decision or penalty, you can usually appeal in writing within 30 days of the decision date, although the notice should confirm the procedure. PAYE issues may require HMRC to correct your tax code through your employer. Check the relevant GOV.UK guidance, and consider professional advice if the amount or dispute is complex.