Use this calculator to find animal mortality as either a simple death percent for a fixed group or a time-at-risk rate for a changing group.
Advanced options
How to use our Animal Mortality Rate Calculator
- Choose What are you calculating? based on your real setup: use Fixed group if the same animals were followed for the whole period, or Changing group if animals entered or left.
- Enter Time period length (days), Animals at start (count), and Deaths during period (count).
- If you chose Changing group, pick How will you enter changes? and then fill either Average animals present during period (count) or the entries and exits fields: Animals added during period (count), Day animals were added, Animals removed alive during period (count), and Day animals were removed alive.
- Open Advanced options if you want to change Show rate per or Decimal places, then click Calculate.
- Sanity-check the results: make sure Method used matches your situation, and check whether Average animals used in the calculation and Total animal-time at risk look reasonable for your records.
Definitions
Fixed group: A group where the same animals are tracked for the whole period, so a simple death percent is usually appropriate.
Changing group: A population where animals entered or left during the period, so time at risk is a better denominator than just the starting count.
Death percent over the period: Deaths divided by the starting group size, shown as a percent.
Total animal-time at risk: The total amount of time animals were actually present and able to experience the event, shown here in animal-days.
Time-at-risk mortality rate: Deaths divided by total animal-time at risk. This lets you compare periods or groups with different numbers present over time.
Average animals used in the calculation: The average number present during the period, either entered directly or implied by your entries and exits.
Animal-year: One animal present for 365 days. This page converts animal-days to animal-years using 365 days per year.
Common mistakes and quick fixes
Mistake: Using Fixed group even though animals were added, sold, adopted out, or moved during the period.
Fix: Change What are you calculating? to Changing group so the calculator can use Total animal-time at risk and Time-at-risk mortality rate .
Mistake: Entering a Deaths during period (count) value larger than Animals at start (count) in Fixed group mode.
Fix: Lower Deaths during period (count) , or switch to Changing group if the population changed during the period.
Mistake: Typing a day outside the study window for Day animals were added or Day animals were removed alive .
Fix: Keep each day from 1 to the value in Time period length (days) .
Mistake: Mixing methods in Changing group mode by filling Average animals present during period (count) when you meant to use the flow method.
Fix: Set How will you enter changes? to the correct choice and only use the fields for that choice.
Mistake: Reading Death percent over the period as if it were the same thing as Time-at-risk mortality rate .
Fix: Use Death percent over the period for a fixed group, and use Time-at-risk mortality rate when group size changed over time.
Mistake: Choosing Show rate per animal-year for a very short Time period length (days) and then being surprised by a large rate.
Fix: Check Check this note and consider a day-based basis such as 100 animal-days or 1,000 animal-days for short periods.
Limitations & Key Assumptions / Boundary Conditions
- The fixed-group percent assumes the same group was followed through the full period without meaningful entries or live removals.
- The changing-group flow method assumes each added or removed subgroup changed on one stated day and then stayed in that new status for the rest of the period.
- This calculator uses whole-group counts and timing, not individual animal histories, so results are summaries rather than exact event-by-event measures.
- If deaths are high compared with the implied average number present, the records may be inconsistent or the period may be too short for easy interpretation.
- Annualized outputs can look large when Time period length (days) is short, even when the raw number of deaths is small.
- The calculator measures mortality within the period you enter. Results can change a lot if the period start, period end, or counting rules are changed.
Methodology
How the calculator works
The calculator uses one of two methods. In Fixed group mode, it treats the starting group as the denominator and reports a percent. In Changing group mode, it calculates animal-time at risk and reports a rate per your selected basis.
Formulas
mortality_percent = (deaths / start_animals) * 100
animal_time_days = avg_animals_present * period_length
animal_time_days = start_animals*period_length + animals_added*(period_length - day_added + 1) - animals_removed_alive*(period_length - day_removed + 1)
mortality_rate = deaths / animal_time_days
rate_display = mortality_rate * basis_factor
The basis factor is 100 for 100 animal-days, 1,000 for 1,000 animal-days, 365 for animal-year, and 36,500 for 100 animal-years. The animal-year conversion uses 365 days per year.
How to read the outputs
Mortality result is the main answer. In Fixed group mode it matches Death percent over the period. In Changing group mode it matches Time-at-risk mortality rate in the basis you selected. Higher values mean more deaths relative to the denominator used.
Average animals used in the calculation is shown to help you check whether the denominator looks believable for your records. For changing groups, it is calculated as animal-time divided by period length.
Worked mini-example
Example 1, Fixed group: if Animals at start (count) is 100 and Deaths during period (count) is 5, then the death percent is 5%.
(5 / 100) * 100 = 5
Example 2, Changing group with average count: if Average animals present during period (count) is 90, Time period length (days) is 30, and deaths are 3, then animal-time is 2,700 animal-days and the rate per 100 animal-days is about 0.11.
animal_time_days = 90 * 30 = 2700
mortality_rate = 3 / 2700 = 0.001111...
rate per 100 animal-days = 0.001111... * 100 = 0.1111...
Assumptions and limits
Results depend on the method matching the real situation. A fixed-group percent and a time-at-risk rate are not interchangeable. If animals entered or left, the changing-group method is usually more meaningful because the amount of time present matters. The flow method is still a simplified model because it treats each addition or removal as happening on one stated day rather than across many different days.