One of the most popular pages on my website contains a discussion of the pros and cons of 12-hour shifts. Here's a link to that page if you haven't seen it:  www.shift-schedule-design.com/12-Hour_Shifts.

This article will address a different aspect of 12-hour shifts, namely how the longer shifts fit with different 24/7 workload distributions.

Constant Workloads
When the volume of work is steady throughout the day, the best shift length is either 8-hours or 12-hours. That's because they divide evenly into a 24-hour day. With 8-hour shifts, there would be three shifts with the same coverage on each shift. With 12-hour shifts, there would be two shifts with the same coverage.

Other shift lengths, e.g., 9-hour or 10-hour shifts, do not divide evenly into 24 hours. This means they will have overlapping shifts, with double coverage during the overlaps. 9-hour shifts will have 3 hours in which the shifts overlap. 10-hour shifts will have 6 hours in which the shifts overlap. Overlaps can be achieved with 8 or 12-hour shifts simply by including an unpaid meal break in each shift.

These other shift lengths (9s or 10s) are not an efficient choice for organizations with a constant workload distribution. In addition to the producing double coverage for part of the day, they also require a larger staff because the organization has essentially extended the hours of operation to 27 or 30 hours a day. 9-hour shifts require 12.5% more personnel and 10-hour shifts require 25% more personnel than 8-hour or 12-hour shifts.

Now that we've shown the best choice for a constant workload is 8 or 12-hour shifts, let's take a quick look at the staffing requirements. Here is a simple example. If you need 2-person coverage around-the-clock, you will need 336 hours of coverage per week (2 people * 24 hours/day * 7 days/week = 336 hours/week). If you divide this by 40 hours per week, you get 8.4 employees. If you divide this by 42 hours per week, you get 8 employees. 42 hours/week is commonly used in 24/7 schedules because it allows the use of 4 crews in the schedule.

The only exception to this is 8-hour fixed shifts. With these, each shift's staffing requirements must be calculated separately. In the example with 2-person coverage on each shift, this will require 3 people on each shift working an average of 37.3 hours a week (2 people * 8 hours/day * 7 days/week = 112 hours/week / 3 employees = 37.3 hours/week). The total staffing with fixed 8s would be 9 employees.

With the exception of 8-hour fixed shifts, 8s and 12s for steady coverage require the same number of employees working the same average hours per week. Since both shift lengths cost the same, this generally means that the choice between 8s and 12s can be left up to employees. When employees are asked if they want 8s or 12s, most will say 8s. However, when they are shown examples of schedules with both shift lengths, most will say 12s.

Variable Workloads
Many smaller establishments do not have constant workloads. They are often slower at night and on the weekends. For example, on Monday-Friday, they may need 4-person coverage from 0700 to 2300 and 2-person coverage from 2300 to 0700. On Saturday and Sunday, they only need 2-person coverage all day.

When the workload is lighter on weekends, this can be accommodated with either 8-hour or 12-hour shifts. When the workload is lighter at certain times of the day, this can be more difficult to match with 12-hour shifts. That's because most groups find their busy hours correspond more closely with one or two 8-hour shifts, i.e. for 8 or 16 hours a day. Let's look at examples of each of these.

Example #1. In this example, the organization needs 4-person coverage for 8 hours and 2-person coverage for 16 hours a day. This requires 11.2 employees working an average of 40 hours a week with 8-hour rotating shifts (12 people with 8-hour fixed shifts). If you use one 12-hour shift with 4-person coverage and one with 2-person coverage, you have 4-person coverage for 4 hours longer than necessary. It also would require 12 employees working an average of 42 hours a week.

The best way to accommodate 12-hour shifts in this scenario is to use 12-hour shifts for the 2-person coverage around-the-clock and an 8-hour overlay shift that matches the busy period. This would require 11 employees. You could either combine the two shift lengths into a single schedule or make a separate schedule for each shift length.

• 8 people on 12s --> 2-person coverage for 24/7
• 3 people on 8s --> 2-person coverage for 8/7
  

Example #2. In this example, the organization needs 4-person coverage for 16 hours and 2-person coverage for 8 hours a day. This requires 14 employees working an average of 40 hours a week with 8-hour rotating shifts. (15 people with 8-hour fixed shifts). If you use one 12-hour shift with 4-person coverage and one with 2-person coverage, you have 4-person coverage for 4 hours less than necessary. If this is unacceptable, you would have to have 4-person coverage on both shifts. This would require at least 16 employees working an average of 42 hours a week.

The best way to accommodate 12-hour shifts in this scenario is to use 12-hour shifts for the 2-person coverage around-the-clock and two 8-hour overlay shifts that match the busy period. This would require 14 employees. You could either combine the two shift lengths into a single schedule or make a separate schedule for each shift length.

• 8 people on 12s --> 2-person coverage for 24/7
• 6 people on 8s --> 2-person coverage for 16/7

Conclusions
There are lots of other possibilities when it comes to variable workloads. If you want to include 12-hour shifts in the schedule, you can follow the generic approach desribed in this article:

• If the workload varies by day of the week, use two 12-hour shifts with a modified work pattern. Popular 12-hour patterns such as Pittman, Panama, DuPont, 3-on-3-off, etc. have the same coverage throughout the week. Therefore these patterns must be modified to match the daily variations. This is best handled by someone who is experienced in schedule design. They can ensure that the schedule: (1) matches the daily coverage variations, (2) averages close to 40 hours a week, and (3) maximizes weekends off for employees.
   
• If the workload varies by hour of the day, you can use two 12-hour shifts for the base coverage. Then you will use one or more additional shifts that match the length of the busy period. I discussed 8-hour and 16-hour busy periods above. Your busy period might require something other than one or two 8-hour shifts. For example, if your busy period is longer than 18 hours, you might use an 8-hour and a 10-hour overlay shift. In some cases, you may want these shifts to overlap so that the coverage is increased further during the hours of overlap.