A payroll calculation has the same shape everywhere even though the rates differ: start from gross pay for the period, apply the deductions that are required, apply any voluntary deductions, and arrive at net pay. Separately the employer owes contributions on top, which do not appear in the employee's figure at all.
The structure of the calculation
Gross pay for the period, including any overtime, bonus or allowance. Then statutory deductions, which are calculated on defined bases and not always on the same base as each other. Then voluntary deductions such as pension contributions above any statutory minimum, or a salary sacrifice arrangement that changes the gross figure before tax is calculated at all.
Employer costs sit outside the payslip
Employer contributions, levies and pension contributions are a real cost of employment and do not reduce the employee's net pay. Budgeting from gross salary alone understates the cost of a hire by a meaningful margin, which is why employment cost rather than salary is the number to plan with.
Where errors actually enter
Mid period changes, almost always: a starter, a leaver, a rate change part way through, or an absence that affects pay. The arithmetic for a full period is reliable; the arithmetic for a partial one depends on a method that has to be consistent and documented. Check those cases rather than the ordinary ones.
Questions people ask about payroll example
Why does net pay change when salary did not?
Usually a threshold, a code change, or a deduction starting or stopping. The payslip should show which, and a product that cannot explain it is a poor one.
Are bonuses taxed differently?
They are generally taxed as pay, and the timing can make a period look unusual. The annual position normally evens out.
What is the real cost of an employee?
Gross pay plus employer contributions and any benefits. Plan from that figure rather than from salary.