Plant quadrat calculator
Paste your tree data to get basal area per hectare, the diameter class distribution, and each species' relative density, dominance and frequency with its importance value (IVI). Join the stems of a multi-stemmed tree with "+".
Input
Results
Everything is calculated in your browser; your data is never uploaded.
Formulas
Basal area
Basal area (m²) = Σ π × DBH² / 40000 per hectare = basal area × 10000 / plot area
DBH is in centimetres. If you enter girth, it is converted first with DBH = girth / π. For a 100 m² plot, basal area per hectare equals Σ girth² / 400π.
Importance value (IVI)
IVI = RD + RDo + RF
- Relative density RD = stems of the species / all stems × 100
- Relative dominance RDo = basal area of the species / all basal area × 100
- Relative frequency RF = subplots where the species occurs / the sum of that count over all species × 100
Each part runs from 0 to 100, so IVI runs from 0 to 300. Some reports divide IVI by 3 to give 0–100; state which you use. Herbs are measured by cover, not IVI; use the diversity index calculator instead.
Diameter classes
Trees are grouped in 10 cm classes up to the plot's largest DBH, with everything from 80 cm up merged into one "≥80cm" class (a common veteran-tree threshold). Regeneration surveys of seedlings and saplings use three classes: 1–3, 3–10 and over 10 cm.
Frequently asked questions
How do I convert girth to DBH?
DBH = girth / π. Set the input to girth and the tool converts it; for a 100 m² plot, basal area per hectare equals Σ girth² / 400π.
How do I enter a multi-stemmed tree?
Join the stems' diameters with "+", for example 21.4+12.1. The tool merges them into one equivalent DBH, √Σd², so the basal area equals the sum of the stems' basal areas.
How is the importance value calculated without subplots?
Relative frequency needs subplots. Without them RF is left out and IVI is RD + RDo; for the full IVI (0–300), add a subplot column at the start of each line.