How Body Surface Area is calculated

Most BSA formulas use the same idea: your surface area grows with both height and weight, but not in a strictly linear way. If you double someone’s weight, their surface area does not double (because surface is like “skin area,” not volume). That’s why most formulas use square roots or exponents.

The Mosteller formula is popular because it’s simple, quick to compute, and close to other common formulas for many adults:

• Mosteller (metric): BSA (m²) = √((height(cm) × weight(kg)) / 3600)

• Du Bois & Du Bois: BSA = 0.007184 × height(cm)^0.725 × weight(kg)^0.425
• Haycock: BSA = 0.024265 × height(cm)^0.3964 × weight(kg)^0.5378
• Gehan & George: BSA = 0.0235 × height(cm)^0.42246 × weight(kg)^0.51456
• Boyd: BSA = 0.0003207 × height(cm)^0.3 × weight(g)^(0.7285 − 0.0188×log10(weight(g)))

Notice how the “advanced” formulas use exponents (like 0.725 and 0.425) instead of a square root. Those exponents were chosen by fitting real-world body measurements to a curve—basically, “what combination of height and weight best predicts measured surface area in a population?”

• Different datasets: each formula was derived from different groups of people.
• Different goals: some aim for simplicity (Mosteller), others for better fit in small bodies (Haycock).
• Different era: older formulas (Du Bois) are historically popular and still referenced.

What this calculator does (step-by-step)

Here’s exactly what happens when you press “Calculate BSA”:

• If you choose Imperial, we convert your height to centimeters and your weight to kilograms.
• 1 inch = 2.54 cm, 1 lb = 0.45359237 kg.

• Mosteller uses a square root.
• The others use exponents (and Boyd uses grams + a log term).

• BSA in m² (the standard clinical unit)
• BSA in ft² (helpful if you like intuitive “area” units)
• The formula name + a short explanation you can copy/share.

Because different formulas can produce slightly different results, this tool lets you switch formulas and instantly compare. For most everyday uses, small differences won’t change the takeaway—but if you’re in a context where precision matters, follow the formula and guidance your protocol requires.

Body Surface Area examples

Examples make BSA feel less abstract. Below are quick “plug-in” examples using the Mosteller formula, with a note about what the number roughly means.

• Height: 175 cm
• Weight: 70 kg
• Mosteller: √((175 × 70) / 3600) = √(12250/3600) = √3.4028 ≈ 1.84 m²

A BSA around ~1.7–2.1 m² is common for many adults. If you’re using BSA for “per m²” references, you can think of 1.84 m² as your personal size multiplier.

• Height: 160 cm
• Weight: 50 kg
• Mosteller: √((160 × 50) / 3600) = √(8000/3600) = √2.2222 ≈ 1.49 m²

This is a good place to test alternate formulas like Haycock. The output won’t be wildly different, but some protocols prefer a specific formula for smaller bodies.

• Height: 190 cm
• Weight: 100 kg
• Mosteller: √((190 × 100) / 3600) = √(19000/3600) = √5.2778 ≈ 2.30 m²

Larger bodies often land around 2.2–2.6 m², depending on height and weight. At higher weights, formula differences can widen slightly—another reason clinical settings may specify a method.

Frequently Asked Questions

• What is a “normal” BSA?

  There isn’t a single “normal” BSA because it’s a body-size measure, not a health score. Many adults fall roughly between 1.6–2.2 m², but shorter/smaller bodies can be lower and taller/larger bodies can be higher. Context matters more than “normal.”

• Which BSA formula should I use?

  If you’re using BSA casually or for general understanding, Mosteller is a solid default. If you’re following a medical protocol, use the formula specified by that protocol (often Mosteller or Du Bois). For smaller bodies, some references prefer Haycock.

• Is BSA better than BMI?

  They answer different questions. BMI is a weight-for-height screening metric. BSA is an estimate of surface area, often used to scale certain physiological quantities. Neither is a perfect “health” measurement, and both ignore body composition.

• Why does my BSA change so little when I lose weight?

  Surface area grows slower than weight. Weight is related to volume, and surface is related to area. That’s why most formulas use exponents less than 1. A 10% weight change typically produces less than a 10% BSA change.

• Can I use BSA for medication dosing?

  BSA appears in some dosing guidance, but you should not dose medication based on a generic calculator alone. Dosing can depend on diagnosis, kidney/liver function, age, protocol, and safety limits. Use this tool for context, then confirm with a clinician.

• Does body fat or muscle affect BSA?

  Indirectly. Because BSA formulas only use height and weight, they can’t “see” whether weight is muscle or fat. Two people with the same height and weight will get the same BSA even if their composition is very different.

How to use your BSA number

To make this number useful, treat BSA as a scaling factor. If a reference says “X per m²,” you multiply X by your BSA. For example, if a guideline uses “50 units/m²,” and your BSA is 1.84 m², the scaled amount would be 50 × 1.84 = 92 units (before any clinical adjustments).

• Clinical indexing: compare values like cardiac output per m² (indexed values can be more comparable).
• Size comparison: understand how much “bigger” or “smaller” two bodies are in surface area terms.
• Tracking: if you’re changing weight over time, BSA gives a smoother trend than weight alone.

• If you’re at an extreme weight or height.
• If a clinical protocol uses a different scaling approach (like adjusted body weight).
• If you’re trying to infer health status from BSA alone (it’s not a health score).

Want more context? Pair this with tools like BMI, Lean Body Mass, and Waist-to-Height Ratio to get a fuller picture.