Attrition rate measures departures as a share of average headcount over the period. It counts departures even when new hires replace those who leave. This rate covers the selected period; it is not annualised.
Average headcount is the sum of the headcounts for each calendar day, divided by the number of days in the period. An employee is counted through their last day under contract, inclusive; a new hire is counted from their arrival date. Weekends are included. The date used is the contract end date, even if the last day actually worked is different.
Two possible calculation methods
1. Previous approximate method (midpoint)
For comparison with a manual calculation, the midpoint method estimates average headcount by taking the average of headcount on the first and last days.
This method only considers those two dates: it ignores when arrivals and departures occur between them.
2. Reflect method: daily average
Reflect uses the daily average headcount over the period, available in the Average employee count measure.
Impact and example
Example: a new site opening in 2025
Period: 1 January–31 December 2025 inclusive, or 365 calendar days.
In this fictional example, a company has 200 employees on 1 January.
A new site opens on 1 March: 40 employees join the company that day.
20 fixed-term contracts end on 30 September, giving 20 departures during the year. All employees are included in the population being measured; there are no other changes.
Daily headcount
1 January–28 February: 200 employees for 59 days.
1 March–30 September inclusive: 240 employees for 214 days.
1 October–31 December: 220 employees for 92 days.
Arrivals increase headcount from 1 March. Employees whose contracts are ending are still counted on 30 September; headcount decreases on 1 October.
Results compared
Midpoint method: headcount goes from 200 employees on the first day to 220 on the last. Estimated average headcount is 210 employees and the attrition rate is 9.52%.
Reflect method: headcount is 240 employees for seven months. Daily average headcount is 228.49 employees and the attrition rate is 8.75%.
The difference is 0.77 percentage points. The midpoint method ignores the seven months at 240 employees: in this case, it underestimates average headcount and overestimates attrition.
The calculations use the unrounded average. Displayed to one decimal place in the application, the rates are 9.5% and 8.8%.
Check a difference from your own calculation
Compare the same period and population: use the same filters and employees included in headcount. The overall rate is not limited to resignations.
Check arrival and departure dates, then the daily average. Do not use closing headcount alone or the number of distinct employees present at any time during the period.
Use the same departure convention: attrition uses HR departures, rather than Finance departures recorded the next day. If the difference persists, send your Reflect contact the period, filters and calculation details.
If average headcount is zero, the rate cannot be calculated: a missing value does not mean 0% attrition. Turnover and retention use different formulas; their results are not interchangeable with attrition.
Further reading: Departures: HR vs Finance and Headcount: end of period vs over the period.