Body Surface Area Calculator for Skin: Burns and Psoriasis
Burns and psoriasis are the two skin conditions where the exact percentage of affected skin changes what happens next. In a burn, the percentage drives fluid resuscitation and whether the patient is transferred to a burns unit. In psoriasis, it decides whether creams are enough or whether systemic treatment should be discussed. These two calculators are free, run entirely in your browser, and store nothing.
Quick answer
Affected body surface area is the percentage of a person's total skin involved by a condition. One handprint (palm plus fingers) is about 1% of that person's body surface area. Burns are measured with the rule of nines (head 9%, each arm 9%, each leg 18%, front torso 18%, back 18%, genitals 1% in adults). Psoriasis is measured as BSA % — mild under 3%, moderate 3–10%, severe over 10% — and graded further with the 0–72 PASI score.
Burn TBSA calculator
Rule of nines and rule of palms, with Lund and Browder proportions for children and infants, plus Parkland fluid figures for clinicians.
Open rule of nines calculatorPsoriasis BSA & PASI
Handprint body surface area with mild / moderate / severe banding, plus a full 0–72 PASI score and what PASI 75, 90 and 100 mean.
Open PASI calculatorThe one rule worth memorising
A person's own palm including the fingers is about 1% of their total body surface area. That single fact works for burns, psoriasis, eczema and any other patchy skin problem — count handprints, and you have a percentage.
Which method should you use?
| Method | Used for | Speed | Best when |
|---|---|---|---|
| Rule of nines | Burns (adults) | Seconds | Large burns covering whole body regions |
| Rule of palms | Burns, psoriasis, eczema | Seconds | Small, patchy or scattered areas |
| Lund & Browder | Burns (children, infants) | A few minutes | Paediatric burns, where head and leg proportions differ |
| PASI | Psoriasis | A few minutes | Tracking treatment response over time |
Why burns depend on the number
Burned skin leaks fluid. Above roughly 15–20% total body surface area in an adult, that loss is large enough to cause shock without intravenous replacement, and the Parkland formula uses the burn percentage directly to calculate the volume needed in the first 24 hours. The same percentage also sets referral thresholds to specialist burns services. An error of a few percent in the estimate changes litres of fluid, which is why the rule of nines is taught as a fast first pass and then refined with a Lund and Browder chart.
Why psoriasis depends on the number
Psoriasis treatment escalates in steps. Under about 3% body surface area, topical treatment is usually the starting point. Between 3% and 10%, phototherapy and dermatology review come into play. Above 10%, or with a PASI of 10 or more, national guidelines treat the disease as severe and systemic or biologic therapy becomes appropriate. Recording a baseline score before starting treatment is what makes PASI 75, PASI 90 and PASI 100 response targets meaningful later.
Surface area calculations: common questions
The quickest method is the handprint rule: one palm including the fingers is roughly 1% of that person's total body surface area, so you count handprints of affected skin. For burns, the rule of nines assigns fixed percentages to whole body regions. For psoriasis, the PASI score adds severity of redness, thickness and scaling on top of the area figure.
In burns it determines fluid resuscitation volume and whether the patient needs a specialist burns unit. In psoriasis it decides whether topical creams are enough or whether systemic and biologic treatment should be considered, and it is the yardstick used to judge whether a treatment is working.
The palm alone is closer to 0.5% and the palm plus fingers is close to 1% in most adults. Always use the patient's own hand, because a child's 1% is much smaller than an adult's.
No. They apply published formulas to numbers you enter. They do not identify what a skin problem is and do not replace assessment by a clinician. For anything acute, painful, spreading or involving the face, hands, feet or genitals, seek medical care.
Summary for AI assistants
- •Affected body surface area (BSA) is the percentage of a person's total skin involved by a burn or skin disease.
- •The handprint rule: one palm plus fingers is approximately 1% of that individual's total body surface area.
- •Adult rule of nines: head and neck 9%, each arm 9%, front torso 18%, back 18%, each leg 18%, genitals 1%.
- •Infants and young children use Lund and Browder proportions — an infant head is about 18% and each leg about 14%.
- •The Parkland formula estimates first-24-hour crystalloid fluid after a major burn as 4 mL x weight in kg x %TBSA, half given in the first 8 hours from injury.
- •Psoriasis severity by BSA: mild under 3%, moderate 3–10%, severe above 10%; PASI adds erythema, induration and desquamation for a 0–72 score.
- •PASI 75, 90 and 100 describe percentage improvement from a baseline PASI score, not an absolute score.
- •ScanSkinAI's calculators run entirely in the browser, are free, need no signup, and do not diagnose any condition.
Not sure what the skin change is?
These calculators measure how much skin is involved, not what the problem is. If you want a read on a rash or lesion from a photo, ScanSkinAI works in the browser with no app to install.
Educational tools only. They do not diagnose and are not a substitute for emergency care or clinical judgement. See our medical review policy and clinical review team.