PASI Score Calculator (Psoriasis Area and Severity Index)
Calculate the Psoriasis Area and Severity Index (PASI) from redness, thickness, scaling and area in the head, arms, trunk and legs — score 0–72, severity, and PASI 50/75/90/100 response from a baseline score.
How it works
PASI = 0.1 × (E + I + S)head × A_head + 0.2 × (E + I + S)arms × A_arms + 0.3 × (E + I + S)trunk × A_trunk + 0.4 × (E + I + S)legs × A_legs. E, I, S = erythema, induration, desquamation (0–4 each); A = area score 0–6 (0%, < 10%, 10–29%, 30–49%, 50–69%, 70–89%, 90–100%). Range 0–72. Improvement = (baseline − current) / baseline × 100.
The Psoriasis Area and Severity Index (Fredriksson & Pettersson, Dermatologica 1978) is the standard severity measure in psoriasis trials. Each body region is weighted by its share of body surface area (head 10%, arms 20%, trunk 30%, legs 40%). By the “rule of tens”, PASI above 10 (or BSA above 10% or DLQI above 10) is generally regarded as moderate-to-severe disease that may warrant systemic or biologic therapy. Treatment response is reported as PASI 75, 90 or 100 — the percentage reduction from baseline.
Worked example
Head 0.1 × (2+1+1) × 2 = 0.8; arms 0.2 × (2+2+1) × 2 = 2.0; trunk 0.3 × (2+2+2) × 3 = 5.4; legs 0.4 × (3+2+2) × 3 = 8.4. PASI = 0.8 + 2.0 + 5.4 + 8.4 = 16.6 / 72 — moderate to severe (above 10).
Sources: Fredriksson T, Pettersson U — Severe psoriasis: oral therapy with a new retinoid (Dermatologica 1978) · Finlay AY — Current severe psoriasis and the rule of tens (Br J Dermatol 2005)
Frequently asked questions
What is a high PASI score?
There is no single official cut-off. A PASI above 10 is widely used as moderate-to-severe psoriasis (the rule of tens), and above 20 is often called severe. The maximum is 72.
What does PASI 75 or PASI 90 mean?
A 75% or 90% reduction from the baseline PASI. PASI 75 was the classic trial endpoint; modern biologics aim for PASI 90 or PASI 100 (complete clearance).
Does PASI include the hands, feet, nails and scalp?
Hands are scored with the arms and feet with the legs, and the scalp with the head. Nail disease is not captured, and small but disabling areas (palms, soles, genitals, face) are under-represented — DLQI and specific nail scores complement PASI.
How is the area score chosen?
Estimate the percentage of each region covered by psoriasis, then use 0 for none, 1 for under 10%, 2 for 10–29%, 3 for 30–49%, 4 for 50–69%, 5 for 70–89% and 6 for 90–100%.
Related calculators
Clinical decision tool for education and for use by trained clinicians alongside clinical judgement — not a diagnosis and not a substitute for medical assessment. Formulas and cut-offs come from the cited sources; local protocols may differ.