BSA: Essential for dosing of dermatological therapeutics and quantitative assessment.
Body Weight
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Body Height
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Enter height and weight to determine the body surface area using Mosteller and DuBois formulas.
Guidelines & Evidence
Verified
Last Review: 2026-07-17
When to Use
When to Use
BSA estimation quantifies the percentage of total body surface area affected by a dermatological condition. It is a core component of disease severity assessment in psoriasis, atopic dermatitis, and other inflammatory skin diseases — used to classify mild vs. moderate-severe disease, determine eligibility for systemic or biologic therapy, and monitor treatment response.
Primary Clinical Uses
Psoriasis severity classification (mild < 3%, moderate 3–10%, severe > 10%)
Atopic dermatitis extent scoring (component of SCORAD and IGA)
Burn surface area estimation (Rule of Nines for large contiguous areas)
Determining eligibility for biologic or systemic therapy in insurance/guideline contexts
Tracking treatment response at follow-up visits
When NOT to Use as a Sole Measure
BSA alone does not capture lesion severity (thickness, scaling, erythema) — always pair with PASI for psoriasis or EASI for eczema
BSA does not reflect quality-of-life impact — always pair with DLQI, especially for sensitive areas (face, hands, genitalia)
Small BSA involvement in high-impact locations (palms, soles, genitalia) may meet "severe" criteria even with < 3% BSA
How it Works
Estimation Methods
Palm Method (palmar surface + fingers): The patient's entire hand (palm plus all five fingers extended together) ≈ 1% total BSA. Use the PATIENT'S hand, not the examiner's.
Palm Alone (palmar surface only, excluding fingers): ≈ 0.5% total BSA. Useful for very small or scattered lesions.
Rule of Nines (adult): Head/neck = 9%, each arm = 9%, each leg = 18%, anterior trunk = 18%, posterior trunk = 18%, perineum = 1%. Best for large contiguous areas (burns, erythroderma).
Lund-Browder Chart: Corrects Rule of Nines for age (children have proportionally larger heads and smaller legs). Preferred for pediatric burns.
BSA Severity Thresholds — Psoriasis (AAD/NPF Guidelines)
| < 3% BSA | Mild — topical therapy appropriate first line |
| 3–10% BSA | Moderate — phototherapy or systemic oral agents considered |
| > 10% BSA | Severe — biologic therapy strongly considered |
Important Caveats on the Palm Method
Studies show the patient's entire handprint (palm + all digits) is approximately 0.76–0.87% of total BSA in adults — not exactly 1%. The 1% convention is a clinically convenient approximation and consistent with guideline use. Inter-rater variability is high; overestimation of affected BSA is the most common error, particularly in plaque psoriasis where plaques are numerous but small.
Estimating Non-Contiguous Lesions
For scattered or non-confluent lesions (typical in psoriasis), mentally consolidate all plaques into a single imaginary patch and estimate how many "patient hand-units" that patch would occupy. Do not attempt to trace individual plaques — the cognitive consolidation method is faster and comparably accurate.
Clinical Pearls
Clinical Pearls
BSA is the most widely used severity measure in clinical practice because it requires no special tools and takes < 2 minutes, but it is the least nuanced of the validated scoring systems.
For regulatory/biologic approval purposes, "moderate-to-severe" is generally defined as BSA > 10% OR PASI > 12 (or 10 in some guidelines) OR DLQI > 10 — meeting any one criterion typically qualifies.
BSA overestimates severity in erythrodermic psoriasis (near-total BSA) and underestimates burden in palmoplantar or genital psoriasis (< 3% BSA but severely disabling).
In atopic dermatitis, BSA is used within SCORAD (where BSA contributes 20 points of the 103-point scale via Lund-Browder mapping). EASI uses a four-region BSA multiplier.
For biologic treatment approval documentation, BSA should be recorded with a specific numeric value (e.g., "BSA 18%"), not a category label alone.
Comparator Instruments
PASI (Psoriasis Area Severity Index) — captures erythema, thickness, scaling AND area per body region; more sensitive to change; preferred for clinical trials
DLQI (Dermatology Life Quality Index) — captures patient-reported impact; essential complement when BSA is in "mild" range but quality of life is severely impaired
IGA (Investigator Global Assessment) — fast single-score severity assessment (0–4 scale); often used with BSA for biologic approval criteria
Next Steps
Psoriasis: Treatment Decision Framework by BSA
01
BSA < 3% (Mild): Initiate topical therapy — corticosteroids (medium-high potency), vitamin D analogues (calcipotriol), or combination products. Reassess at 4–6 weeks.
02
BSA 3–10% (Moderate): Trial topical therapy first for 3 months. If inadequate response or body-region impact (hands, face, nails): consider narrow-band UVB phototherapy or systemic oral agents (acitretin, methotrexate, cyclosporine, apremilast). Refer to dermatology if not already involved.
Atopic Dermatitis: BSA-Based Guidance
01
BSA < 10% (Mild): Optimized emollients + topical corticosteroids or calcineurin inhibitors for flares.
02
BSA 10–30% or face/hands/genitalia involvement (Moderate): Step up to topical JAK inhibitors (ruxolitinib), or initiate systemic therapy discussion (dupilumab, tralokinumab).
The Evidence
Psoriasis Severity Thresholds — Guideline Reference
Joint AAD-NPF guidelines of care for the management and treatment of psoriasis with biologics.
Menter A et al. • J Am Acad Dermatol.. 2019;80(4):1029–1072. Defines BSA thresholds (< 3% mild, 3–10% moderate, > 10% severe) as part of the AAD-NPF guideline framework for biologic therapy eligibility.
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Origins & History
Body surface area estimation in dermatology does not have a single founding paper — it evolved from burn medicine, where the Rule of Nines was introduced by Pulaski and Tennison in 1947 and formalized by Wallace and Muir in the 1950s. The palm-unit method (1 palm = 1% BSA) entered routine dermatological practice as a rapid clinical approximation and was incorporated into psoriasis and atopic dermatitis severity frameworks by the 1990s. The Lund-Browder chart (1944) remains the gold standard for pediatric burn BSA in burn surgery. BSA thresholds for psoriasis severity classification (< 3% mild, 3–10% moderate, > 10% severe) are embedded in AAD/NPF and BAD guidelines and are widely used as biologic therapy eligibility criteria.
Last Comprehensive Review: 2026-07-17