What this calculator does
A mortality rate is deaths divided by the population that could have died, expressed as a percentage. The part that decides whether the figure is right is the denominator: animals born during the period were also at risk, so they belong in it. Dividing by opening stock alone overstates the rate.
With 100 animals at the start, 20 born and 15 deaths, the at-risk population is 120 and the rate is 12.5%. Using the opening 100 instead would give 15%, which is not a small difference and runs in the same direction every time, so it makes a herd look worse than it is in exactly the situations where births are highest.
The formula
The at-risk population is the opening stock plus the births during the period. Deaths are divided by that total and expressed as a percentage. The closing stock is the at-risk population less the deaths, which assumes nothing was bought, sold or moved during the period.
| Term | Meaning |
|---|---|
| At-risk population | Opening stock plus births, the denominator of the rate. |
| Mortality rate | Deaths as a percentage of the at-risk population. |
| Survival rate | The complement of the mortality rate, the percentage that did not die. |
| Closing stock | At-risk population less deaths, assuming no purchases, sales or transfers. |
The inputs explained
| Field | What to enter |
|---|---|
| Opening stock (animals) | The number of animals at the start of the period. |
| Births during period (animals) | Animals born during the period. Include any that were born and then died within the period, since they were at risk. |
| Deaths during period (animals) | Deaths during the period, from any cause. |
When to use it
Reporting a herd or flock figure
Mortality is a standard reporting measure in livestock management, and using the at-risk denominator is what makes one period comparable with another when birth rates differ.
Monitoring for a problem
A rate rising across consecutive periods is a signal worth acting on well before the absolute numbers look alarming, particularly where the herd is also growing.
Comparing against a benchmark
Industry benchmarks are almost always quoted against the at-risk population, so a rate calculated on opening stock alone will not be comparable with them.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does the mortality rate change with the number of deaths?
A fixed opening stock and birth count with an increasing number of deaths.
| Deaths | Mortality rate | Survival rate | Estimated closing stock |
|---|---|---|---|
| 5 | 4.17% | 95.8% | 115 animals |
| 10 | 8.33% | 91.7% | 110 animals |
| 15 | 12.5% | 87.5% | 105 animals |
| 25 | 20.8% | 79.2% | 95 animals |
| 40 | 33.3% | 66.7% | 80 animals |
Questions
Why include births in the denominator?
Because those animals were present and could have died. A period with many births has more animals exposed to risk than the opening count suggests, and ignoring them inflates the rate. The correction matters most in exactly the herds where it is most likely to be overlooked.
What about animals bought or sold during the period?
This calculation does not handle them, and the closing stock figure assumes none. Where stock moves in or out, the rigorous approach is animal-days at risk, which weights each animal by how long it was actually present rather than counting it whole.
Should stillbirths be counted?
Convention varies, so the important thing is to be consistent and to say which you used. Counting them requires including them in the births as well as the deaths; excluding them requires leaving them out of both. Mixing the two produces a figure that is wrong in a way that is hard to spot later.
Is a mortality rate the same as a death rate?
They are generally used interchangeably in livestock work. In human epidemiology the terms are distinguished more carefully, with mortality rate usually measured against person-time rather than a headcount, which is the same idea as animal-days at risk.
For projecting a population under a capacity ceiling, see the logistic growth calculator. For an interacting predator and prey pair, see the Lotka-Volterra calculator.