For most people, you simply add up your working time over the last 17 weeks and then divide by 17. In the jargon of the Working Time Regulations 1998 (WTR), this is a '17-week rolling reference period'.
If you were sick or on leave during the last 17 weeks, you start the reference period a week earlier, and make up the missing days from your last week (i.e. the 18th week from when the reference period now starts). If this still doesn't give 17 weeks, then you keep starting the reference period a week earlier until you have 17 weeks of working days.
If there is an emergency, employers can use a 26-week period. Also some groups have a 26-week reference period instead.
Workers who use a 26-week reference period include:
- security guards, caretakers and similar jobholders;
- workers whose jobs which involve travelling long distances;
- workers whose jobs ‘require continuity of service or production’, such as in hospitals, the media, prisons, docks, airports, post and telecoms, civil protection, agriculture, and industries where work cannot be interrupted, such as the utilities;
- workers in jobs where there are seasonal rushes, such as tourism and agriculture; and
- shift workers in the process of changing shift.
If you have not yet worked 17 weeks (or 26 weeks if you are in one of the groups above), your reference period starts on the first day of work. In other words if you have had your job for five weeks, you add up your working time since you started and divide by five.
Bear in mind that the reference period can be extended to 52 weeks by a relevant agreement with a trade union or representatives of the workforce, made under the rules set out in the WTR. Also, the offshore industry has a standard 52-week reference period.