BSA calculator: affected body surface area
Choose a method: rule of nines (share of involvement per region) or palm method (the patient's palm including fingers corresponds to about 1% of body surface area).
⚠︎ Learning tool for medical education – not for use in patients or for treatment decisions.
Classification (original publication)
| Score | Classification (original publication) |
|---|---|
| 0% | No involvement |
| ≤ 10% | Limited extent – on its own not a rule-of-tens criterion |
| > 10% | Extensive – rule of tens met – BSA > 10% indicates moderate-to-severe psoriasis |
Rule of nines and palm method
Rule of nines in adults: head/neck 9%, each arm 9%, front trunk 18%, back 18%, each leg 18%, genitals 1%. Enter what percentage of each region is affected; the calculator converts to total body surface area. For scattered lesions the palm method is often more accurate: the patient's palm including fingers corresponds to about 1% of body surface area.
Interpretation
In psoriasis, according to the rule of tens (Finlay 2005), a BSA above 10% – like a PASI or DLQI above 10 – indicates moderate-to-severe disease. In children the proportions differ (larger head); the rule of nines is then only an approximation.
FAQ
How large is a palm as a percentage of body surface area?▾
The patient's palm including fingers corresponds to about 1% of body surface area.
References
- Finlay AY. Current severe psoriasis and the rule of tens. Br J Dermatol 2005;152:861–7. · PubMed
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Open in browser Download on the App StoreIntended purpose: learning tool for medical education only. Not intended for use in patients or for diagnostic or treatment decisions; not a medical device. Calculations follow the cited original publications; no guarantee of completeness or correctness. Last updated: 09/2026 · Dr. Pascal Bafteh