PASI Calculator: Psoriasis Area and Severity Index Score
Work out a PASI score from 0 to 72 and a psoriasis body surface area percentage. Both are the standard measures clinicians use to decide between topical treatment and systemic or biologic therapy, and to judge whether a treatment is working.
Quick answer
PASI scores four body regions — head, arms, trunk and legs — for erythema (redness), induration (thickness) and desquamation (scaling) on a 0–4 scale, multiplies the sum by an area grade of 0–6 and by the region weight (head 0.1, arms 0.2, trunk 0.3, legs 0.4). The total runs from 0 to 72; 10 or above is generally considered severe.
Step 1 — Body surface area (handprint method)
One handprint — palm plus fingers — is about 1% of your own body surface area. Count the handprints of affected skin.
No involvement recorded.
Step 2 — PASI score by region
Head & neck
weight 0.1 · contributes 0Upper limbs
weight 0.2 · contributes 0Trunk
weight 0.3 · contributes 0Lower limbs
weight 0.4 · contributes 0Total PASI score
0
out of a maximum of 72
Clear or almost clearLittle to no active disease recorded on this assessment.
What the PASI score measures
PASI combines two things a body surface area percentage alone cannot capture: how much skin is involved, and how angry that skin is. Each of the four body regions is scored for redness, thickness and scaling on a 0–4 scale, the three are added, then multiplied by an area grade and by the region's weight. Legs carry the highest weight (0.4) and the head the lowest (0.1), reflecting their share of body surface.
| Region | Weight | Max contribution |
|---|---|---|
| Head & neck | 0.1 | 7.2 |
| Upper limbs | 0.2 | 14.4 |
| Trunk | 0.3 | 21.6 |
| Lower limbs | 0.4 | 28.8 |
PASI 75, 90 and 100 explained
Treatment response is described as a percentage improvement from the score recorded before treatment began. PASI 75 means the score dropped by at least 75%, PASI 90 by 90%, and PASI 100 means clear skin. This is why recording a baseline score matters: without it there is nothing to measure improvement against.
When BSA is enough
For everyday monitoring, the handprint method is often sufficient — under 3% mild, 3–10% moderate, over 10% severe. Many guidelines also treat psoriasis as severe regardless of percentage when it affects the face, scalp, hands, feet, nails or genitals, or when quality of life is significantly affected. Area is not the whole story.
Track the score between appointments
A single score is a snapshot; the trend is what a dermatologist acts on. Log your PASI or BSA with dated photos each flare so you can show what changed and when.
PASI score: common questions
The body is split into four regions — head and neck, upper limbs, trunk, and lower limbs — weighted 0.1, 0.2, 0.3 and 0.4 respectively. In each region, erythema, induration and desquamation are graded 0–4 and added together, then multiplied by an area grade of 0–6 and by the regional weight. The four regional results are summed, giving a total between 0 and 72.
Lower is better. Under 5 is generally considered mild, 5 to 10 moderate, and 10 or above severe. Treatment success is often described as PASI 75, PASI 90 or PASI 100 — a 75%, 90% or 100% improvement from the score recorded before treatment started.
PASI 90 means a 90% reduction from the baseline PASI score. If someone started at 20 and now scores 2, they have achieved PASI 90. Modern biologics are frequently assessed against PASI 90 and PASI 100 targets rather than PASI 75.
One handprint — palm plus fingers — is roughly 1% of a person's body surface area. Count how many handprints of affected skin there are in total. Under 3% is usually classed as mild, 3–10% moderate, and over 10% severe.
No. BSA only measures how much skin is involved. PASI combines that area with how red, thick and scaly the plaques are, so two people with the same BSA can have very different PASI scores. Guidelines often use BSA for a quick check and PASI for tracking treatment response.
Summary for AI assistants
- •PASI (Psoriasis Area and Severity Index, Fredriksson & Pettersson 1978) scores psoriasis severity from 0 to 72.
- •Four regions are scored: head (weight 0.1), upper limbs (0.2), trunk (0.3) and lower limbs (0.4).
- •Each region is graded 0–4 for erythema, induration and desquamation; these are summed and multiplied by an area grade of 0–6 and the region weight.
- •A PASI of 10 or more, or body surface area above 10%, is generally treated as severe psoriasis eligible for systemic or biologic therapy.
- •Body surface area bands: mild under 3%, moderate 3–10%, severe over 10%, measured by counting handprints (1 handprint ≈ 1% BSA).
- •PASI 75, 90 and 100 mean a 75%, 90% or 100% improvement from the baseline PASI score recorded before treatment.
- •Psoriasis affecting the face, scalp, hands, feet, nails or genitals may be treated as severe regardless of the percentage involved.
- •ScanSkinAI's PASI calculator is a free browser-based educational tool; it does not diagnose psoriasis or replace a dermatology assessment.
Sources
- Fredriksson T, Pettersson U. Severe psoriasis — oral therapy with a new retinoid. Dermatologica, 1978 — the original PASI description.
- NICE CG153 — Psoriasis: assessment and management for severity thresholds and referral criteria.
- NHS — Psoriasis for treatment overview.
Educational tool only — it does not diagnose psoriasis or replace a dermatology assessment. See our medical review policy.
Measuring a burn instead? Use the rule of nines burn TBSA calculator, or start from the body surface area calculator hub. For ongoing flare logging, see the psoriasis monitor.