PASI Calculator (Psoriasis Area and Severity Index)
Use this tool to estimate PASI score from lesion severity and area involvement across four body regions.
What is the PASI score?
PASI stands for Psoriasis Area and Severity Index. It is one of the most widely used tools in dermatology to quantify psoriasis severity. The score combines two things: how severe plaques look (redness, thickness, and scaling) and how much of each body region is affected.
PASI ranges from 0 to 72. A score of 0 means no signs of psoriasis. Higher values reflect greater overall disease burden. In research and clinical care, PASI helps track treatment response over time and compare outcomes across therapies.
How this PASI calculator works
1) Severity signs in each region
For each region, rate three clinical features from 0 (none) to 4 (very severe):
- Erythema: redness of plaques
- Induration: thickness/infiltration
- Desquamation: scaling/flaking
2) Area involvement
Enter the percentage of skin involved in each region. The calculator converts this into the PASI area class (0-6), which is part of the standard formula used in clinical studies.
3) Regional weighting
Body regions are weighted differently:
- Head: 0.1
- Upper limbs: 0.2
- Trunk: 0.3
- Lower limbs: 0.4
Formula used: PASI = Σ [weight × (E + I + D) × area score].
Interpreting results
PASI interpretation can vary by study protocol, but a common practical framing is:
- Below 10: generally mild disease burden
- 10 to 20: moderate burden
- Above 20: severe burden
Remember: clinical decisions are not based on PASI alone. Symptoms (itch, pain), quality of life, special-site involvement (scalp, nails, genitals, palms/soles), and comorbidities all matter.
PASI response milestones (PASI 50/75/90/100)
In treatment follow-up, clinicians often look at percentage improvement from baseline:
- PASI 50: 50% reduction from baseline
- PASI 75: 75% reduction (traditional efficacy benchmark)
- PASI 90: near-clear skin, now common target with newer therapies
- PASI 100: complete skin clearance
Example: if baseline PASI was 24 and current PASI is 6, improvement is 75% (PASI 75).
Practical tips for consistent scoring
- Score under similar lighting each visit.
- Use the same rater when possible to reduce variability.
- Assess average plaque severity for the region, not only worst spots.
- Document photos to support trend tracking over time.
Limitations and medical disclaimer
PASI is a validated clinical metric, but no calculator can replace an in-person assessment by a qualified professional. This page is for education and tracking support only. If you are concerned about worsening psoriasis, joint pain, or side effects, please contact your dermatologist.