Predator-Prey Ratio Calculator
Find the predator-to-prey ratio, the number of prey supporting each predator and the predator count per 100 prey from two population counts.
How it works
Prey per predator = Prey ÷ Predators · Predators per 100 prey = (Predators ÷ Prey) × 100 · Predator share = Predators ÷ (Predators + Prey)
The predator-prey ratio is a quick index of how much prey biomass supports each predator in a food web. Because energy is lost at each trophic transfer (only roughly a tenth passes up a level), healthy systems tend to have far more prey than predators — a small predator share of the total. Watching this ratio over time reveals the classic Lotka-Volterra cycle, where predator numbers lag behind prey booms and crashes. A ratio that swings toward too few prey per predator warns of overpredation or prey collapse.
Worked example
A reserve holds 12 wolves and 480 deer. Prey per predator = 480 ÷ 12 = 40 deer each. That is 1 predator per 40 prey, or (12 ÷ 480) × 100 = 2.5 predators per 100 prey, with predators making up 12 ÷ 492 = 2.4% of the two populations.
Sources: Lotka–Volterra equations — Wikipedia
Frequently asked questions
Why are there usually far more prey than predators?
Only about 10% of the energy at one trophic level is available to the next, so a given prey base can support only a small predator population. This is why predator-prey ratios are typically well below 1.
What does a rising prey-per-predator figure mean?
It means prey are abundant relative to predators, which often precedes a rise in predator numbers as food is plentiful — the upswing of the classic predator-prey cycle.
Can I use this for parasites and hosts?
The same ratio logic applies to any consumer-resource pair, including parasites and hosts, though parasite loads per host are often expressed as an intensity rather than a whole-population ratio.
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This calculator is for educational purposes. Double-check important results before acting on them.