Population Size Calculator
Enter how many animals you caught and marked the first time, how many you caught the second time, and how many of those already had a mark. The calculator estimates the population and gives a 95% range.
- Mark and recapture
- 95% range
- Chapman's method
Leave enough time between catches for marked animals to mix back in, but not so long that many are born or die.
You can count a population without catching every animal. Mark a sample, release them and catch again later. If 10 of your second catch of 100 are marked, the 100 marked animals are about a tenth of the population, so there are about 1,000.
Population estimate chart
Chapman's estimator. Search for a survey.
| Marked | Second catch | Recaptured | Estimate | 95% range |
|---|---|---|---|---|
| 50 | 50 | 5 | 433 | 149 to 716 |
| 100 | 100 | 10 | 926 | 459 to 1,394 |
| 100 | 100 | 20 | 485 | 324 to 646 |
| 100 | 100 | 50 | 199 | 172 to 226 |
| 200 | 150 | 30 | 978 | 700 to 1,256 |
The range narrows as recaptures increase. Aim for at least 7, ideally 20 or more.
Recaptures and estimate
100 marked, 100 caught. Slide the recaptures.
The dots are the second catch. Coloured dots are animals that already carried a mark.
How mark and recapture works
Mark, release, recatch. Step through it.
First catch
Count and mark.
Use marks that do not harm the animal or change its chance of being caught, such as a small paint spot or tag.
Lincoln-Petersen, Chapman or Schnabel
Methods. Compare them.
M × C ÷ R.
Corrected version.
Many catches.
The Schnabel method uses several catches in a row and gives a tighter estimate for the same effort.
Before you survey
Field checks. Tick each item.
Tick each item as you go.
If marked animals avoid traps after the first catch, the estimate will be too high.
What you get
Population estimate. Each tile explains one result.
Chapman.
Low to high.
Lincoln-Petersen.
Of second catch.
Low to good.
Small recapture numbers give wide ranges. Doubling the effort in the second catch helps more than anything else.
Frequently Asked Questions
No question matches that search.
How does mark and recapture work?
Mark a sample, release it, recatch, and use the share of marked animals to estimate the total.
What is the Lincoln-Petersen formula?
Population = marked × second catch ÷ recaptured.
Why use Chapman's estimator?
It is less biased when recaptures are few, and works even with none.
How many recaptures do I need?
At least 7 for a rough figure, 20 or more for a good one.
What if no marked animals are recaptured?
The population is probably large. Catch more animals next time.
Does mark and recapture work for fish?
Yes, it is widely used for fish in ponds and lakes.
What assumptions does it make?
No births, deaths or moves, no lost marks and equal catchability.
Count what you cannot see
Two catches and a careful count give a sound estimate of the whole population. More recaptures give a tighter answer.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.