Function
The reference period is the statutory mechanism that enables operational flexibility in working hours while safeguarding workers against excessive weekly workloads. Under the Working Time Regulations 1998 (WTR 1998), adult workers cannot be required to work more than an average of 48 hours per week unless they have signed a voluntary, individual opt-out agreement under Regulation 5.
Because the 48-hour limit is applied as an average rather than an absolute cap in any single week, a business can schedule peaks in working hours (for instance, 55 or 60 hours in a busy week) without breaching employment law, provided that hours in subsequent weeks are sufficiently lower to bring the overall average across the reference period back down to 48 hours or fewer.
Calculation
Under Regulation 4 of the WTR 1998, average weekly working hours are calculated across a standard reference period of 17 consecutive weeks. For certain sectors (such as offshore work or where special exemptions apply), or where statutory modifications exist, standard reference periods may extend to 26 weeks.
The statutory formula for calculating average weekly hours is:
Average Weekly Hours = (A + B) / C
Where:
- $A$ is the total number of hours worked during the reference period.
- $B$ is the total number of hours worked immediately after the reference period to compensate for any statutory absences (such as statutory annual leave under Regulations 13 and 13A, sick leave, or maternity/paternity/shared parental leave).
- $C$ is the number of weeks in the reference period (normally 17).
Reference periods are typically administered in one of two ways:
- Successive (fixed) reference periods: Standard, consecutive 17-week blocks. While straightforward to track administratively, they require caution to ensure workers are not subjected to consecutive spikes across the boundary between two blocks.
- Rolling reference periods: A dynamic, rolling 17-week window that recalculates each week or day. This offers comprehensive oversight and eliminates threshold risks associated with transition dates between fixed periods.
Extension via Workforce Agreement
Under Regulation 23 of the Working Time Regulations 1998, employers and workers can agree to extend the standard 17-week reference period up to a maximum of 52 weeks (12 months) for objective, technical, or organisational reasons.
To be legally valid, this extension must be agreed through:
- A collective bargaining agreement with a recognised independent trade union; or
- A formally executed workforce agreement with elected employee representatives (where no trade union is recognised).
In the absence of a compliant collective or workforce agreement, the statutory 17-week baseline remains legally binding.
Monitoring & Compliance
Under Regulation 9 of the Working Time Regulations 1998 (reinforced by ECJ ruling C-55/18 CCOO), employers are under a positive legal duty to maintain adequate records showing whether the 48-hour average limit is being observed for all workers who have not opted out.
Because the reference period spans four months or longer, impending statutory breaches frequently go unnoticed when tracked retrospectively. A modern, automated time-tracking solution provides real-time visibility into rolling hour averages, highlighting potential breaches weeks in advance and allowing HR managers and team leaders to adjust shifts and allocate rest before non-compliance occurs.
- Updated on