The percentage of body surface area (BSA) affected is the simplest measure of the extent of an inflammatory skin disease and is used across psoriasis, atopic dermatitis and other dermatoses. The rule of nines was introduced by Wallace (1951) for estimating the area of burns: the head and neck represent 9% of total BSA, each upper limb 9%, the anterior and posterior trunk 18% each, each lower limb 18%, and the genitals 1%. The same regional weights are used for the extent component of the SCORAD index. In children under 2 years the head is proportionally larger and the legs smaller, and the SCORAD evaluation form assigns 17% to the head and neck and 12% to each lower limb, giving a total of 96%.

The palm method estimates BSA by counting the number of patient palms, including the fingers, needed to cover the affected skin, with each palm taken as approximately 1% of total BSA. The meta-analysis by Rhodes et al. (2013) found that the hand including the fingers represents about 0.8% of BSA in adults, so the method slightly overestimates extent; the palm alone without the fingers is closer to 0.5%. The approximation remains the recommended bedside method because of its speed and reproducibility, and it is the basis of the area component in several composite scores.

The National Psoriasis Foundation consensus (Pariser et al. 2007) classifies psoriasis as mild when less than 3% of BSA is affected, moderate at 3% to 10%, and severe above 10%. Finlay (2005) proposed the rule of tens, under which current severe psoriasis is defined by a BSA above 10%, a PASI above 10, or a DLQI above 10. The International Psoriasis Council recategorisation (Strober et al. 2020) moved away from BSA thresholds alone and defines candidates for systemic therapy as patients with a BSA above 10%, involvement of special areas (scalp, face, palms, soles, nails or genitals), or failure of topical therapy.

BSA is a common eligibility criterion for clinical trials and for public and private reimbursement of biologic therapy in Canada, where a BSA of 10% or more, alone or together with a PASI or DLQI threshold, is typically required. The product of the static Physician Global Assessment and BSA (PGA x BSA, range 0 to 400) was described by Walsh et al. (2013) as a simple static measure of psoriasis severity that correlates well with the PASI while remaining quick to record, and it has since been used as a treatment-response measure. The same construction is validated for atopic dermatitis as the IGA x BSA composite.

BSA captures extent only. It does not reflect the intensity of erythema, induration or scaling, the involvement of high-impact sites, or the burden of symptoms, and it is insensitive to change once the disease is confined to a small area. It should be read alongside a global assessment or a composite index and, where quality of life matters to the decision, a patient-reported measure.