UK payslip guide
National Insurance explained
National Insurance is a separate payroll deduction from Income Tax. Most employees see it deducted from each pay period.
In short
Employee National Insurance is generally calculated from your weekly or monthly pay, which is why a payslip can differ from a simple annual calculation.
It is calculated per pay period
Unlike cumulative Income Tax, employee Class 1 National Insurance usually uses the earnings in that individual pay period. A bonus, overtime or commission can therefore change one deduction without affecting every month.
Why it changes during the year
The rate can change once pay passes a threshold. Salary sacrifice can also reduce the earnings used for National Insurance, depending on the arrangement.
Employer contributions are separate
Your employer normally pays its own National Insurance contribution as well. It is not deducted from your pay, but it can be relevant when comparing an offer or contractor rate.
See HMRC's employee National Insurance guidance, or use the UK payslip calculator for a pay-period estimate.