Is this calculator accurate?
It follows the method HMRC publishes for employers: income tax is built up cumulatively from your tax code, National Insurance is worked out afresh on each pay day, and student loan is taken from pay above the plan threshold. Results should match a payslip to within a few pence, but a real payslip can differ if your tax code, pension scheme or pay dates are not what you entered.
PAYE tools for employers