One of the reasons it’s so difficult to assess the length of your electrolysis journey is due to the variability of hair density and equally important: the size of the area being treatment. This body surface area calculator allows you to choose one of two methods to estimate the size of the area you’re targeting for treatment. If you choose to use the custom measurement option, you will need a flexible measuring tape. You can click on the third tab below to review the instructions for each measurement method.
-Betty Electrolysis (Chi)
Body Surface Area Calculator
Estimate the surface area of any body region in cm²
Step 1 — Enter height & weight to calculate total body surface area
Total body surface area
17,281 cm²
Step 2 — Select one or more body regions
Selected region(s)
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Step 1 — Choose the shape of the region
Step 2 — Enter measurements
How to use this calculator
This tool estimates the surface area (in cm²) of any part of the body using two methods. Choose the one that best fits your situation.
Method 1 — Rule of nines
Best when you need a quick estimate based on standard body proportions. The body is divided into regions, each accounting for a known percentage of total body surface area (TBSA).
- Enter the person's height and weight using the sliders. The Mosteller formula calculates their TBSA in cm².
- Tap any body region(s) to select them. Their combined surface area will appear below.
- You can select multiple regions at once — tap again to deselect.
Child mode: For patients under 15, enable Child mode and enter their age. The head and leg percentages adjust automatically using Lund-Browder values, since children have proportionally larger heads and smaller legs than adults.
Method 2 — Custom measurement
Best for precise measurements of a specific body part, lesion, wound, or burn site. Use a flexible tape measure and ruler.
- Choose the shape that most closely matches the region.
- Enter the measurements as prompted.
- The area is calculated instantly.
Shape guide
- Rectangle: Flat, roughly rectangular patches — enter length and width.
- Ellipse / oval: Oval patches — enter the longest and shortest diameters.
- Triangle: Triangular regions — enter base and height.
- Cylinder (limb): Arms, legs, fingers. Wrap a flexible tape measure around the limb to get its circumference, then measure the length of the segment. Circumference × length gives lateral surface area.
- Sphere (head): Measure the circumference around the widest part of the head (just above the eyebrows). The calculator assumes a perfect sphere — actual surface area is typically about 10% larger.
Tips for irregular shapes
- Break irregular regions into two or more simple shapes and add the results together.
- For curved body parts, a flexible cloth tape measure gives the most accurate readings.
- For very small areas (e.g. small wounds), a transparent wound-measurement grid can help.
Formulas used
- TBSA: Mosteller formula — √(height cm × weight kg ÷ 3600) × 10,000
- Rectangle: Length × Width
- Ellipse: π × (length ÷ 2) × (width ÷ 2)
- Triangle: ½ × Base × Height
- Cylinder: Circumference × Length
- Sphere: 4π × r², where r = circumference ÷ (2π)
This calculator is for clinical and educational reference. Always apply professional judgment when using these values for treatment decisions.