Calculation notes

Calculator methodology

This page explains how the calculator turns the values entered by a user into estimated annual, monthly, weekly and scenario results.

Starting point

The calculation starts with gross annual employment income. It then applies the selected tax year, tax code, UK tax region, pension method, salary sacrifice, bonus, student loan settings and any other taxable income fields included on the form.

Tax code handling

Common PAYE tax codes are interpreted to estimate the personal allowance or adjustment used in the calculation. The calculator supports standard allowance-style codes, selected special codes such as BR, D0 and D1, and K-code style adjustments. Actual payroll systems may apply cumulative or week-one/month-one treatment differently.

Income tax and National Insurance

Income tax is estimated from taxable income and the selected tax region. Employee National Insurance is estimated from employment income after relevant salary sacrifice assumptions. The calculator presents results as annualised estimates and breaks the results into normal monthly and bonus-month views where relevant.

Pension and salary sacrifice

Pension inputs can be modelled as net pay, relief at source or salary sacrifice depending on the selected method. Salary sacrifice reduces contractual taxable pay in the estimate, but some benefits, such as company cars, can create Benefit-in-Kind tax and need separate treatment.

Bonus and pay-rise scenarios

Bonus can be entered as a fixed annual sum or a percentage of salary. Because bonuses are commonly paid in one payroll month, the calculator separates normal monthly take-home from bonus-month take-home. Pay-rise scenarios compare the current salary with a target salary using the same assumptions unless the user changes them.

Limitations

The calculator does not replace payroll software or professional advice. It does not know every employer payroll adjustment, tax-code notice, benefit arrangement, court order, attachment of earnings, workplace policy or personal circumstance. Results should be treated as estimates.