Absence Rates Project Time Formula
Learn how expected team absence rates are used to estimate a project’s adjusted duration, capacity, and schedule extension.
This calculation estimates how long a fixed project workload may take when the team is not available for every planned working day. It converts the absence rate into average available capacity, then divides the workload by that reduced capacity.
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Absence-Adjusted Project Duration
Where:
First reduce the team size by the expected absence percentage. Then divide the total person-day workload by the reduced average team capacity.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| projectWorkload - Project workload | The total effort required to complete the project. | person-days |
| teamSize - Team size | The average number of full-time equivalent people assigned to the project. | number |
| absenceRate - Expected absence rate | The share of planned working time expected to be unavailable. | percent |
| effectiveTeamSize - Effective team size | The average available team capacity after allowing for absence. | number |
| plannedWorkingDays - Planned working days | The duration estimated when the entire team is available. | days |
| workdaysPerWeek - Working days per week | The number of project working days in a normal week. | days |
Step-by-Step Calculation
Calculate the fully available duration
Divide the total person-day workload by the full team size to find the original working-day plan.
plannedWorkingDays = projectWorkload / teamSize
Convert absence rate to an availability factor
Subtract the absence rate from 100% to find the share of capacity expected to remain available.
availabilityFactor = 1 - absenceRate / 100
Calculate effective team capacity
Apply the availability factor to the planned team size.
effectiveTeamSize = teamSize * (1 - absenceRate / 100)
Calculate the adjusted working-day duration
Divide the unchanged workload by the lower average available capacity.
adjustedWorkingDays = projectWorkload / effectiveTeamSize
Find the schedule extension
Subtract the original full-attendance duration from the absence-adjusted duration.
additionalWorkingDays = adjustedWorkingDays - plannedWorkingDays
Convert duration to working weeks
Divide adjusted working days by the number of working days in a typical project week.
adjustedProjectWeeks = adjustedWorkingDays / workdaysPerWeek
Calculate lost capacity allowance
This estimates the additional person-days of capacity needed to deliver the original workload despite the assumed absence rate.
lostPersonDays = projectWorkload * absenceRate / (100 - absenceRate)
Example: 300 person-day project with a five-person team
Planned duration at full attendance
300 / 5
60.0 working days
Availability factor
1 - 6 / 100
0.94
Effective team size
5 * 0.94
4.70 people
Adjusted project duration
300 / 4.70
63.8 working days
Schedule extension
63.8 - 60.0
3.8 working days
Adjusted duration in weeks
63.8 / 5
12.8 working weeks
Expected lost capacity
300 * 6 / (100 - 6)
19.1 person-days
Final Result
Allow approximately 63.8 working days, or 12.8 working weeks, rather than 60.0 working days. The expected schedule extension is 3.8 working days.
Assumptions
- ✓Each team member provides broadly similar productive capacity on the project.
- ✓The absence rate is an average spread across the project rather than a single concentrated event.
- ✓The project workload stays constant after the estimate is made.
- ✓Available team capacity can be assigned to the work without major handover or coordination losses.
- ✓Working days exclude the separate treatment of weekends, public holidays, and other non-working dates.
Limitations
- !The calculation does not identify whether absence affects a critical specialist or project lead.
- !It does not include task dependencies, approval delays, rework, scope changes, or supplier delays.
- !A team’s output may not increase or decrease in direct proportion to headcount because of coordination effects.
- !Actual absence can vary substantially over short projects or at particular milestones.
- !The result is a planning estimate rather than a delivery commitment.
Common Mistakes to Avoid
Entering calendar days instead of the total effort measured in person-days.
Using headcount when some people are only partially assigned to the project.
Including annual leave in the absence rate when it has already been removed from planned availability.
Treating the adjusted working-day result as a calendar completion date without adding non-working days.
Assuming all absences are interchangeable even where one person owns critical work.
Using a historical absence rate from a different team or period without checking whether it is relevant.
Related Formulas
Frequently Asked Questions
What is the formula for project duration with an absence rate?
Divide the project workload by team size multiplied by one minus the absence rate as a decimal: projectWorkload / [teamSize × (1 - absenceRate / 100)].
Why does a 6% absence rate add more than 6% to the planned duration?
The remaining 94% of capacity must complete 100% of the work. The duration multiplier is 1 / 0.94, so the estimated duration rises by about 6.4%.
How is effective team size calculated?
Multiply team size by the availability factor. For example, a five-person team with 6% expected absence has effective capacity of 5 × 0.94 = 4.70 people.
How are expected lost person-days calculated?
The calculation is projectWorkload × absenceRate / (100 - absenceRate). It shows the extra capacity allowance needed to complete the original workload.
Can the absence rate be zero?
Yes. At 0%, effective team size equals the planned team size and the adjusted duration equals the original planned duration.
What happens when the absence rate is very high?
Effective capacity becomes very low, so the estimated duration rises sharply. High rates may also make the even-capacity assumption less realistic.
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