NIM08500 - Earnings Periods: Change of earnings period with the same employer: Change to shorter interval
Regulation 18 of the Social Security (Contributions) Regulations 2001 (SSCR 2001) (SI 2001 No 1004)
If an employee’s regular earnings period changes to a shorter interval, for example, monthly to weekly paid, or quarterly to monthly paid, work out the NICs on payments made after the change using the earnings period for the new interval.
Example
A monthly paid employee changes to weekly paid on the first day of the month having received a monthly salary the day before. They receive their first weekly pay on the 5th. Although this payment is in the same tax month as their monthly salary, work out the NICs for this payment separately using a weekly earnings period.