How much of my body surface is affected?
Paint the involved skin on front and back figures — or count palms — and get the percentage of total body surface area, by Rule of Nines, age-adjusted Lund–Browder, or the palm method. Pick your condition and the tool reads the number against the severity bands dermatologists actually use. Everything stays on this device.
Paint the figures or type a percentage into the breakdown to begin.
Saving builds a history so you can see whether the affected area is actually shrinking — the trend matters more than any single number. This is an educational estimate, not a diagnosis: confirm important readings with your GP or dermatologist.
Saved only on this device — and browsers erase site data. A history clear, a private window, or Safari's automatic deletion after seven days without a visit will take this log with it. The fix is a free Dermington account: it backs this up automatically and puts it on every device you sign in to — details in how storage works.
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How the BSA calculator works
Body surface area is dermatology’s yardstick for extent — how much skin a condition covers, as a percentage of your whole body. This tool computes it three ways. The Rule of Nines (Wallace, Lancet, 1951) divides the adult body into blocks of 9%: head and neck 9, each arm 9, each leg 18, front and back of the trunk 18 each. The Lund–Browder chart (1944) refines those weights by age, because a child’s head is a far larger share of their surface than an adult’s. And the palm method uses your own palmar surface — palm plus fingers, roughly 1% of your body — to count up small or scattered patches. When you paint the figures, the tool measures the painted fraction of each anatomical region pixel by pixel, multiplies by that region’s published weight, and sums the result; you can also type a percentage straight into any region row.
The percentage means different things for different conditions, which is why the tool asks what you’re measuring. For psoriasis, the National Psoriasis Foundation bands are under 3% mild, 3–10% moderate, and over 10% severe — the range where dermatologists discuss phototherapy, systemics, or biologics. For eczema, extent is one input alongside intensity and symptoms, with 10% or more commonly used as a moderate-to-severe criterion in trials — pair this with the POEM severity score for the fuller picture. For vitiligo, the value is in tracking the same map month over month. Saved maps stay in your browser and can be printed as a one-page report for appointments.
Common questions
Is this the same BSA measurement my dermatologist uses?
Same methods, same numbers. Clinicians estimate body surface area with the Rule of Nines, the age-adjusted Lund–Browder chart, or the palm rule — your palm plus fingers is roughly 1% of your own body surface. This tool implements all three; the painting approach just replaces eyeballing with pixel-counted regions, which tends to be more consistent between sessions than a visual guess.
What BSA counts as severe psoriasis?
The National Psoriasis Foundation consensus bands are under 3% mild, 3–10% moderate, and over 10% severe — and 10% or more is the range where dermatologists typically discuss phototherapy, systemic drugs, or biologics. Worth knowing: psoriasis on high-impact sites like the face, hands, or genitals can be treated as severe even at a low BSA, so the percentage is one input, not the whole story.
How do I measure lots of small scattered patches?
Switch to the palm method. Counting how many of your own palmar surfaces (palm plus fingers) the scattered spots would add up to is exactly what the palm rule was designed for — guttate psoriasis, patchy eczema, and hives are the classic cases. Each palm is about 1% of your body surface; a toggle for the 0.8% estimate some studies prefer is included.
Why does entering an age change the regional percentages?
Because body proportions change dramatically with growth. A newborn’s head is around 19% of their body surface but an adult’s is about 7%, while legs grow from roughly 10% to 14% each. The Lund–Browder chart corrects for this, which is why it’s the standard for children — the tool interpolates smoothly between the chart’s age anchors as you move the slider.
Where does my painted body map go when I save it?
Into your own browser’s local storage on this device — nothing is uploaded to Dermington or anywhere else. That privacy comes with a catch: clearing browser data erases saved maps, so if you’re building a history worth keeping, download a backup from Your data. Saved maps also show the change between assessments, which is the trend a dermatologist actually wants to see.