This psoriasis area and severity index (PASI) score calculator measures the psoriasis severity based on lesion severity, appearance and skin area affected. There are instructions below the form on how the PASI calculator computes the score.


1Grade of affected skin area
2Erythema (redness)
3Induration (thickness)
4Desquamation (scaling)

Arms/Upper extremities

5Grade of affected skin area
6Erythema (redness)
7Induration (thickness)
8Desquamation (scaling)


9Grade of affected skin area
10Erythema (redness)
11Induration (thickness)
12Desquamation (scaling)

Legs/Lower extremities

13Grade of affected skin area
14Erythema (redness)
15Induration (thickness)
16Desquamation (scaling)

How does the psoriasis area and severity index (PASI) score calculator work?

This health tool is used to measure the severity of psoriatic lesions. In order to do so, it combines the severity of lesions with the spread area of the condition and the plaque appearance.

The PASI is based on the model released in 1978 by Fredericksson and Pettersson.

The four sections of the body, that are considered in the PASI score calculator, are as follows:

■ Head (10% of skin surface);

■ Arms/Upper extremities (20% of skin surface);

■ Trunk (30% of skin surface);

■ Legs/Lower extremities (40% of skin surface).

The percent of skin area affected by the disease is accounted in one of the following grades:

■ Grade 0 – 0% involvement;

■ Grade 1 – less than 10% involvement;

■ Grade 2 – 10 to 29% involvement;

■ Grade 3 – 30 to 49% involvement;

■ Grade 4 – 50 to 69% involvement;

■ Grade 5 – 70 to 89% involvement;

■ Grade 6 – 90 to 100% involvement.

Within each area, there are three clinical signs by which the severity of the lesions is evaluated:

■ Erythema (redness);

■ Induration (thickness);

■ Desquamation (scaling).

The clinical signs are awarded on a scale from 0 (none), 1 (slight), 2 (moderate), 3 (severe) to 4 (very severe).

In order to produce the PASI score, for each section of the body, the points awarded to the three severity signs are summed and then multiplied by the involvement grade and the weight of that section (i.e. 0.1 for head, 0.2 for arms, 0.3 for trunk and 0.4 for legs).

This can be written like:

PASI = [0.1 x (Eh + Ih + Dh) x Gh] + [0.2 x (Ea + Ia + Da) x Ga] + [0.3 x (Et + It + Dt) x Gt] + [0.4 x (El + Il + Dl) x Gl]

The score results range from 0 to 72, where the highest the score, the higher the severity of the disease.

For example, taking the case of a patient’s arms where the psoriasis involvement is grade 4 and erythema is evaluated at 3, induration is evaluated at 2 and desquamation is evaluated at 2, the PASI component for the arms will be: (3+2+2) x 4 x 0.2 = 5.6.

The PASI is used in clinical trials, often to define subject’s entry condition and afterwards to measure outcomes. In this case, PASI is presented as a percentage response rate. For example, a PASI 50 of 65 means that from the clinical trial, a number of 65 patients have achieved a 50% or more reduction in their PASI score.

Because the method can sometimes be difficult to put in practice, a simplified version of the psoriasis area and severity index has also been devised for clinical use.

One of the criticisms received by the model concerns the fact that the clinical severity of the disease cannot be explicitly described through a PASI result and that it needs to be used alongside other evaluation systems such as the Psoriasis Global Assessment (PGA).


1) Feldman SR, Krueger GG. (2005) Assessment: domains and instruments: Psoriasis assessment tools in clinical trials. Ann Rheum Dis; 64:ii65-ii68.

2) Fredriksson T, Pettersson U. (1978) Severe psoriasis--oral therapy with a new retinoid. Dermatologica; 157(4):238-44.

3) Louden BA, Pearce DJ, Lang W, Feldman SR. (2004) A Simplified Psoriasis Area Severity Index (SPASI) for rating psoriasis severity in clinic patients. Dermatol Online J; 10(2):7.

4) Faria JR, Aarão AR, Jimenez LMZ, Silva OH, Avelleira JCR. (2010) Inter-rater concordance study of the PASI (Psoriasis Area and Severity Index). Anais brasileiros de dermatologia; Vol.85(5), pp.625-9.

5) Robinson A, Kardos M, Kimball AB. (2012) Physician Global Assessment (PGA) and Psoriasis Area and Severity Index (PASI): why do both? A systematic analysis of randomized controlled trials of biologic agents for moderate to severe plaque psoriasis. J Am Acad Dermatol; 66(3):369-75.

08 Dec, 2016