Body Surface Area Calculator

What Body Surface Area Means and Why It’s Still Used

Body surface area (BSA) is an estimate of the total external surface area of the human body expressed in square meters (m²). It is not the same as body weight, and it is not the same as body composition. Instead, BSA sits between height and weight as a “size scaling” metric. The idea is straightforward: many physiological processes, organ sizes, and drug distribution patterns do not scale perfectly with weight alone. A person’s height changes their overall body shape, and two people with the same weight can have very different body frames. BSA tries to capture a more proportional measure of overall body size by combining height and weight in a single value.

In modern practice, BSA is commonly used for two major purposes. First, it can be used to compute medication doses that are prescribed per square meter (for example, mg/m²), which is common in oncology and in some other specialized dosing contexts. Second, BSA can be used to index measurements, such as reporting a physiologic value “per 1.73 m²” in research and clinical reporting. Whether BSA is the best scaling method depends on the use case, but it remains widely used because it is simple, familiar, and embedded in many established protocols.

How This Body Surface Area Calculator Works

This calculator accepts height and weight in either metric (centimeters and kilograms) or imperial (feet/inches and pounds). If you enter imperial values, the calculator converts them internally to centimeters and kilograms because that is how most BSA equations are expressed. Once height and weight are standardized, it computes BSA using five common formulas: Mosteller, Du Bois & Du Bois, Haycock, Gehan & George, and Boyd. The output is always in square meters (m²).

The BSA Calculator tab highlights one “primary” formula (your choice) and then lists the results from all formulas so you can compare them. The Dose per m² tab converts a prescribed dose-per-area (mg/m², g/m², or mcg/m²) into a total dose using your BSA. The Target Weight tab inverts the equation to estimate what weight would produce a goal BSA at a fixed height for the selected formula. Finally, the Formula Comparison tab summarizes the spread across equations and shows percent differences against a baseline so you can quantify how much your choice of formula matters for your inputs.

The Most Used BSA Formulas and What Makes Them Different

BSA is not directly measured in routine care. It is estimated using formulas derived from measured surface area in small groups of individuals, then generalized to broader populations. Each formula uses a different mathematical relationship between height and weight, which can produce small but meaningful differences.

Mosteller is widely used because it is easy: it uses the square root of height times weight (with a constant) and tends to be close to other formulas for many adults. If you need a quick BSA estimate and no guideline specifies a particular method, Mosteller is often the default in calculators and clinical tools.

Du Bois & Du Bois is a classic equation derived from early 20th-century measurements. It uses exponents on height and weight and remains frequently cited in medical references. Some clinical environments still reference this equation explicitly, and some software uses it as a default.

Haycock is commonly referenced in pediatric contexts because it was designed to perform well across a range of sizes, including children. Like Du Bois, it uses height and weight exponents, but the exponent values and constant differ.

Gehan & George is another exponent-based formula often found in oncology references and medical calculators. It may produce slightly different values compared with Mosteller and Du Bois for some body sizes, which is why side-by-side comparison can be valuable.

Boyd includes a logarithmic adjustment to better model surface area across a wider range of body sizes. Because it incorporates log10(weight), it can behave differently at extremes. It is less “mental-math friendly” but remains part of many multi-formula calculator sets.

Why Formula Choice Can Matter for mg/m² Dosing

When a medication is prescribed per m², the total dose is the product of BSA and the dose-per-area. This means any difference in BSA becomes a proportional difference in the final dose. If two formulas differ by 3% in BSA, the total dose differs by about 3% before rounding. In many clinical scenarios, that difference may be small compared with other sources of variability, but it can matter more for narrow therapeutic index drugs or for protocols with strict dose limits.

That is why dosing policies often specify a formula, rounding rules, and sometimes dose caps. This calculator does not impose a clinical policy; instead, it helps you do the math transparently. If your prescription or hospital guideline specifies “Mosteller BSA,” choose Mosteller. If it specifies another formula, match it. If it does not specify one, use a consistent method and document it.

How the Dose per m² Tab Helps in Real Workflows

The Dose per m² tab is designed for a common task: converting a protocol written as “X mg/m²” into a total amount based on your BSA. You can use the BSA from the calculator tab or enter a BSA manually (useful if you already have a BSA value from a chart, EHR system, or clinical worksheet). Then you enter the dose-per-area and select the unit. The tool returns a raw total dose and an optional rounded dose.

Rounding is a real-world step because medication strengths, vial sizes, and preparation constraints can make exact dosing impractical. Some workflows round to the nearest 1 mg, 5 mg, 10 mg, or another increment. The calculator includes common rounding options so you can see the difference immediately. If you also know a concentration (for example, 10 mg/mL), the calculator estimates a volume, which helps with preparation math. This feature is optional and should be used only when it matches how the medication is actually supplied and prepared.

Target Weight: Inverting BSA for Planning and Scenario Analysis

Sometimes you know the BSA value you want to model rather than the weight. For example, you may want to understand how weight changes might affect BSA-based calculations at a fixed height. The Target Weight tab solves that problem by searching for the weight that produces a desired BSA using the selected equation. This is a mathematical inversion. It is useful for scenario planning, sensitivity checks, and education, but it is not a health recommendation or a clinical decision tool.

Because the equations are nonlinear, the calculator uses an iterative search within a weight range that you control. If you suspect the answer is outside the default range, widen it. The “precision” setting controls how close the computed result must be to the goal BSA. Higher precision can be useful for research or careful sensitivity work, while standard precision is often sufficient for practical estimates.

Common Measurement and Unit Pitfalls

The most common errors in BSA calculations are not mathematical—they are unit errors. Height must be in centimeters for most formulas as written in medical references. Weight must be in kilograms, and for Boyd the “weight in grams” version is common. This calculator standardizes units internally to avoid those pitfalls, but you still need to enter your values correctly.

• Check inches: If you enter 5 ft 10 in, keep inches under 12. If you enter 70 inches as “inches,” your height becomes unrealistically high.
• Check weight units: If you enter 154 thinking it is kilograms, your BSA will be drastically wrong. Switch to imperial if you are entering pounds.
• Use current measurements: For dosing, use the height and weight required by your protocol (for example, current measured weight or adjusted weight rules).

How to Interpret Differences Between Formulas

The Formula Comparison tab focuses on what you can act on: the spread between formulas for your height and weight. It reports min and max BSA values, the average, and the maximum difference and percent difference relative to a baseline of your choice (Mosteller, another formula, or the average). This makes it easy to answer practical questions like:

• “If I switch from Mosteller to Du Bois, how much would my per-m² dose change before rounding?”
• “Do the formulas diverge more at my body size than at typical adult sizes?”
• “Is the difference smaller than the rounding step used in my workflow?”

In many cases, formulas will cluster tightly, which is reassuring. When they do not, the right move is usually not to choose the formula with the “best-looking” number, but to follow the formula required by your clinical context or to use a consistent institutional standard.

Limitations of BSA and Safer Use

BSA does not directly measure lean mass, fat distribution, hydration status, or organ function. Two individuals with the same height and weight can differ in body composition and pharmacokinetics. That is one reason why modern dosing sometimes uses alternative scaling (such as weight-based dosing, adjusted body weight, or drug-specific models), and why many drugs require monitoring or lab-based titration even when dosing starts from a BSA estimate.

If you are using BSA for medication dosing, treat this tool as a calculator for the arithmetic, not a dosing authority. Always follow protocol rules, apply dose caps and adjustments when required, and verify calculations with a clinician or pharmacist—especially in high-stakes contexts like chemotherapy, pediatrics, renal impairment, or liver impairment.

Practical Tips for Getting the Most Useful BSA Result

• Use one formula consistently within a single workflow unless a guideline requires otherwise.
• Document the formula in any note, spreadsheet, or worksheet used for dosing.
• Compare before you commit if you are unsure which method your environment expects.
• Apply rounding rules last so you can see the raw math and the rounding impact separately.
• Recalculate when inputs change because BSA is sensitive to weight changes and can affect per-m² totals.