Annual Absence Rates Project Time Formula
Learn how annual absence rates are used to estimate lost project hours, remaining capacity, person-days, and FTE capacity loss.
This calculation starts with the team’s planned annual project time and applies an expected absence rate. It helps project planners set a realistic capacity allowance before scheduling work or estimating staffing needs.
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Available Annual Project Hours
Where:
Calculate all planned project hours for the team, then reduce that total by the percentage of time expected to be lost to absence.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| teamSize - Team size | Number of people expected to contribute to the project. | number |
| workingDaysPerEmployee - Working days per employee per year | Planned annual working days after holidays and other planned non-working days, before absences are deducted. | days |
| hoursPerDay - Project hours per working day | Average productive project hours planned for each person on a working day. | hours |
| absenceRate - Annual absence rate | Expected percentage of planned work time that will be unavailable because of absence. | percent |
Step-by-Step Calculation
Calculate planned annual project hours
This is the team’s total annual project capacity before allowing for absence.
plannedAnnualHours = teamSize * workingDaysPerEmployee * hoursPerDay
Convert the absence percentage to a decimal
A percentage must be divided by 100 before it is applied to project hours.
absenceDecimal = absenceRate / 100
Calculate project hours lost to absence
This estimates how many planned project hours are unavailable during the year.
absenceHours = plannedAnnualHours * absenceDecimal
Calculate available project hours
Subtract estimated lost hours from planned annual hours to find the remaining capacity.
availableProjectHours = plannedAnnualHours - absenceHours
Express lost time in person-days and FTE
Lost hours can be converted into person-days, while FTE loss shows the average team capacity represented by the absence allowance.
absenceDays = absenceHours / hoursPerDay; fteCapacityLost = teamSize * absenceDecimal
Example: 10-person team with 5% annual absence
Find planned annual hours
10 × 220 × 7.5
16,500 hours
Calculate lost absence hours
16,500 × 5 ÷ 100
825 hours
Calculate available project hours
16,500 − 825
15,675 hours
Convert lost hours to person-days
825 ÷ 7.5
110 person-days
Calculate FTE capacity lost
10 × 5 ÷ 100
0.50 FTE
Final Result
The team is estimated to lose 825 project hours, or 110 person-days, and retain 15,675 annual project hours.
Assumptions
- ✓The absence rate applies evenly across the team over the full year.
- ✓Working days are entered after planned annual leave, public holidays, and other known non-working days have been removed.
- ✓Each employee is assumed to contribute the same number of project hours per working day.
- ✓Project hours lost to absence are assumed not to be replaced by overtime, temporary cover, or reprioritised work.
Limitations
- !Actual absences may be clustered rather than spread evenly, which can create delivery delays beyond the annual hours total.
- !Individual schedules, roles, part-time arrangements, and productivity levels may differ across the team.
- !The estimate does not include turnover, recruitment gaps, training, meetings, or changes in project scope.
- !A capacity allowance does not predict the timing or cause of individual absences.
Common Mistakes to Avoid
Including annual leave in both the working-days input and the absence rate.
Using contracted daily hours instead of the productive project hours realistically available each day.
Entering 5 as 0.05 when the calculator expects an absence percentage of 5%.
Treating the FTE result as a specific vacant role rather than an annual average capacity equivalent.
Assuming remaining annual hours are automatically available at the exact times the project needs them.
Related Formulas
Frequently Asked Questions
How do you calculate annual project hours lost to absence?
Multiply planned annual project hours by the annual absence rate divided by 100. Planned annual hours equal team size multiplied by working days per employee and project hours per day.
How is available project time calculated after absence?
Subtract estimated absence hours from planned annual project hours. The same result can be calculated by multiplying planned hours by one minus the absence rate as a decimal.
What does 0.5 FTE capacity lost mean?
It means the annual absence allowance is equal to half of one full-time employee’s annual capacity on average. It does not mean one person will be absent for half the year.
Should holidays be included in the absence rate?
Usually, planned holidays and public holidays should be removed when entering working days. The absence rate can then represent unplanned or separately tracked absences.
Why are absence days shown as person-days?
Person-days convert lost hours into whole or partial workdays using the project hours entered per day. They describe total team time, not necessarily consecutive calendar days.
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