Shannon Diversity Index Calculator
Calculate the Shannon-Wiener diversity index (H), Pielou's evenness and the effective number of species from a list of species abundances.
How it works
H = −Σ pᵢ · log(pᵢ) · Evenness J = H / log(S) · Effective species = e^H (natural log) where pᵢ = nᵢ / N
The Shannon (Shannon-Wiener) index measures the uncertainty in predicting the species of a randomly chosen individual — it rises with both the number of species and how evenly they are represented. It is maximised (H = log S) when every species is equally abundant, so dividing by that maximum gives Pielou's evenness, a 0–1 measure of how balanced the community is. Because H itself is hard to interpret, ecologists often report e^H, the effective number of equally-common species. The log base only rescales H; evenness and effective species are unaffected.
Worked example
For counts 40, 30, 20, 10 (N = 100), the proportions are 0.4, 0.3, 0.2, 0.1. H = −(0.4 ln0.4 + 0.3 ln0.3 + 0.2 ln0.2 + 0.1 ln0.1) ≈ 1.28 (natural log). H_max = ln 4 = 1.386, so evenness = 1.28/1.386 ≈ 0.925, and e^1.28 ≈ 3.6 effective species.
Sources: Diversity index — Wikipedia
Frequently asked questions
What is a typical range for the Shannon index?
Using natural logs, H usually falls between about 1.5 and 3.5 for real communities. It is 0 when a single species dominates completely and rises with more, more-even species; there is no fixed upper limit.
Does the choice of log base matter?
It only rescales H (natural log gives nats, log₂ gives bits, log₁₀ gives decits). Evenness and the effective number of species are independent of the base because the base cancels out.
What is Pielou's evenness?
It is H divided by its maximum possible value log(S), giving a 0–1 score. Values near 1 mean species are almost equally abundant; low values mean a few species dominate.
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This calculator is for educational purposes. Double-check important results before acting on them.