Shannon Index Calculator
Compute the Shannon diversity index H′ = −∑ pi · ln(pi) from species counts, plus Pielou’s evenness J′, species richness, effective species count, and the Simpson index as a complement. Compare two communities side by side. Everything runs in your browser — no data leaves your device.
Enter one row per species: the species name (optional) and how many individuals you counted.
Per-species contribution (−p·ln p)
| Species | Count | pi | −pi·ln(pi) |
|---|
Rough interpretation guide
Rules of thumb for ln-based H′, not ecological thresholds. A perfectly even community reaches H′ = ln(S); a single-species sample always gives H′ = 0.
Community A
Community B
| Metric | Community A | Community B |
|---|
Worked example
Counts 50, 30, 20 (N = 100):
| Species | Count | pi | −pi·ln(pi) |
|---|---|---|---|
| Species 1 | 50 | 0.50 | 0.3466 |
| Species 2 | 30 | 0.30 | 0.3612 |
| Species 3 | 20 | 0.20 | 0.3219 |
| H′ (sum) | 1.0297 | ||
Evenness J′ = 1.0297 / ln(3) = 0.94; Simpson complement 1−D = 0.6200; effective species e1.0297 = 2.80.
Frequently asked questions
What is the Shannon diversity index?
The Shannon diversity index (H') is an entropy-based measure of species diversity: H' = -sum(p_i * ln(p_i)), where p_i is the proportion of individuals belonging to species i. It combines species richness (how many species) and evenness (how evenly individuals are spread). This calculator uses the natural logarithm, the standard in ecology. A perfectly even community of 4 species (10 individuals each) gives H' = 1.3863, which equals ln(4), the maximum possible for 4 species.
How do I calculate the Shannon index by hand?
Take each species' count, divide by the total to get p_i, compute -p_i * ln(p_i) for each, then add them up. Example with counts 50, 30, 20 (total 100): proportions 0.50, 0.30, 0.20 give terms 0.3466, 0.3612, 0.3219, and the sum is H' = 1.0297. The calculator above does this instantly, including the per-species contribution table.
What is a good Shannon index value?
There is no universal good or bad value: H' is a summary of your sample, not an absolute ecosystem health score, and it grows with sampling effort. As a rough guide for natural-log H': below 1.0 is low diversity (a few species dominate), 1.0-2.0 is moderate, 2.0-3.0 is high, and above 3.0 is very high and uncommon in small samples. Always report the species richness and evenness alongside H'.
What is evenness (J') in the Shannon index?
Pielou's evenness J' = H' / ln(S) measures how close the community is to perfectly even, on a 0-to-1 scale. J' = 1 means every species has the same number of individuals (for example counts 10, 10, 10, 10 give J' = 1.00); low J' means a few species dominate (counts 90, 5, 5 give H' = 0.3944 and J' = 0.36). For a single-species sample, evenness is undefined because ln(1) = 0, and H' itself is 0.
What is the difference between the Shannon index and the Simpson index?
They measure different things, so one is not a replacement for the other. The Shannon index (H') is an entropy measure sensitive to rare species; the Simpson dominance index (D = sum of p_i squared) is the probability that two randomly picked individuals belong to the same species, so it is driven by the most abundant species. This calculator reports the complement 1-D (higher = more diverse) alongside H': an even community of 4 species gives 1-D = 0.7500, while a skewed community of 90, 5, 5 gives 1-D = 0.1850, even though both have richness 3 or 4.
Should I use the natural log, log base 2, or log base 10 for the Shannon index?
The choice of logarithm only rescales the index; it does not change any ranking or comparison. The natural log (ln) is the standard in ecology and is this calculator's headline value. Log base 2 expresses H' in bits (a perfectly even 4-species community is exactly 2.0000 bits), and log base 10 in bans. The calculator shows all three so results are comparable with any published study.
How do I compare the diversity of two communities with the Shannon index?
Compute H' for each community from its own species counts, then compare the two values along with richness and evenness. Use the Compare tab above: for example, community A with counts 10, 10, 10, 10 gives H' = 1.3863, while community B with counts 37, 1, 1, 1 gives H' = 0.3488, so A is substantially more diverse despite both having 4 species. Differences in H' are meaningful only when sampling effort is comparable.
Does the Shannon index measure ecosystem health?
Not directly. The Shannon index summarizes the structure of one sample: how many species it contains and how evenly individuals are distributed. A high H' does not prove a healthy ecosystem, and a low H' does not prove a degraded one — some naturally harsh habitats are species-poor yet pristine. Use H' together with richness, evenness, the species list, and knowledge of the habitat, never as a standalone health score.
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