This calculator computes body surface area (BSA) in m² using the Mosteller and Du Bois formulas. BSA is the two-dimensional measure of a person’s outer body, and in nephrology it does one specific job: it normalises an absolute clearance to the 1.73m² reference that eGFR uses, so values from differently-sized people can be compared on one scale.
What Is Body Surface Area?
Body surface area estimates the total surface of the skin from two inputs almost always on hand: height and weight. Unlike weight alone, it scales with both height and mass, which is why physiological rates that depend on overall body size — heat exchange, blood volume, glomerular filtration, and a few drug doses — are often expressed per unit of BSA rather than per kilogram. A typical adult BSA falls between about 1.5 and 2.0 m². The figure 1.73 m² is the historical average for a young adult and is the denominator baked into every modern eGFR equation.
The Mosteller and Du Bois Formulas
Two formulas dominate clinical practice. The Mosteller equation is the simpler and is the default in most tools:
Mosteller BSA (m²) = √( (height in cm × weight in kg) ÷ 3600 )
The older Du Bois and Du Bois equation uses fixed exponents derived from their 1916 study:
Du Bois BSA (m²) = 0.007184 × height0.725 × weight0.425 (height in cm, weight in kg)
Across the normal adult range the two agree within a few percent. Mosteller is preferred for transparency and ease of hand-calculation; Du Bois can read slightly lower at the extremes of size.
Worked Example
Take an adult who is 180 cm tall and weighs 80 kg. Multiply height by weight (180 × 80 = 14,400), divide by 3600 (14,400 ÷ 3600 = 4), and take the square root: √4 = 2.0 m² by Mosteller. The Du Bois result for the same person is about 1.99 m² — close enough that either could be used. Because this person’s BSA is larger than the 1.73m² reference, an eGFR reported per 1.73m² would understate their true absolute filtration in mL/min, which is exactly the gap the normalisation step closes.
BSA Reference Bands
| Body surface area | Interpretation |
|---|---|
| < 1.5 m² | Smaller-than-average adult; eGFR may overstate absolute clearance |
| 1.5 – 2.0 m² | Typical adult range (1.73 m² is the eGFR reference) |
| > 2.0 m² | Larger-than-average adult; eGFR may understate absolute clearance |
These bands are orientation aids, not diagnostic thresholds — BSA is a body-size descriptor, not a measure of kidney health on its own.
Using BSA to Compare CrCl and eGFR
A Cockcroft–Gault creatinine clearance is an absolute rate in mL/min, while eGFR is indexed to 1.73m². To put them on the same footing, de-index the eGFR (or index the CrCl) using BSA:
Absolute clearance (mL/min) = eGFR × BSA ÷ 1.73, and conversely indexed eGFR = clearance × 1.73 ÷ BSA.
The GFR unit converter performs this normalisation for you. The correction is small for average-sized adults but becomes meaningful for very large or very small patients — see CrCl vs eGFR for when the distinction changes a dosing decision.
A Second Worked Example
Now take a smaller adult who is 155 cm tall and weighs 50 kg. Multiply height by weight (155 × 50 = 7,750), divide by 3600 (7,750 ÷ 3600 ≈ 2.15), and take the square root: √2.15 ≈ 1.47 m² by Mosteller. Because this BSA is below the 1.73m² reference, an eGFR reported per 1.73m² would overstate this person’s true clearance in mL/min — the opposite of the larger patient above. The two examples bracket the reference and show why the direction of the normalisation depends on whether a patient is bigger or smaller than 1.73m².
Why Surface Area Rather Than Weight
It is reasonable to ask why physiology cares about surface area at all rather than simply weight. Many of the body’s exchange processes — losing heat, filtering blood, distributing some drugs — scale more closely with the body’s two-dimensional surface and metabolic size than with raw mass. A tall, lean person and a short, heavy person can weigh the same yet have very different surface areas, and their physiological rates follow the surface area more faithfully than the weight. That is the reasoning behind indexing glomerular filtration to BSA: it produces a fairer comparison between bodies of different shapes than dividing by kilograms would. The trade-off is that BSA, being derived from height and weight, inherits any error in those two inputs.
BSA in Drug Dosing
Several cytotoxic and a handful of other agents are dosed in milligrams per square metre (mg/m²) so that exposure tracks body size rather than weight alone. Carboplatin is a special case: it is dosed by target exposure through the Calvert formula rather than by BSA directly. If you need a body-size-based dose, use the BSA value above; if you are dosing carboplatin, use the Calvert calculator.
Limitations and Edge Cases
- BSA formulas were validated on adults; pediatric and very obese patients can fall outside the populations studied, so the estimate is least reliable at the extremes.
- Height and weight must use the correct units — centimetres and kilograms for Mosteller and Du Bois. A height entered in inches or a weight in pounds will produce a wildly wrong BSA.
- BSA estimates surface area; it is not the same as fat-free mass, ideal body weight, or dosing weight. For weight-based dosing decisions, see ideal body weight and adjusted body weight.
- BSA does not adjust for amputation, fluid overload, or other changes in true body geometry.