Body Surface Area Calculator

Introduction: What body surface area (BSA) is and why it matters

Body surface area (BSA) is an estimate of the total external surface of the human body. While it is not something most people measure day-to-day, it is widely used in healthcare because many biological processes scale more closely with surface area than with body weight alone. For example, clinicians may use BSA to help standardize drug dosing, compare cardiac output between people of different sizes (cardiac index), or interpret kidney function and other measurements that are reported “per square meter.”

BSA is especially common in oncology, where chemotherapy protocols often specify doses in milligrams per square meter (mg/m²). In that context, a small change in BSA can change the calculated dose. That is why it is important to use accurate height and weight measurements and to follow the dosing rules in the relevant protocol (including rounding rules, dose caps, and adjustments for organ function). This page provides a quick estimate and a copyable summary, but it does not replace clinical judgment.

How to use this BSA calculator

1. Select a measurement system: Metric (cm/kg) or Imperial (in/lb).
2. Enter height and weight as positive numbers. Decimals are allowed.
3. Choose a formula: Mosteller (simple and popular) or Du Bois (older, empirically derived).
4. Optional: enter a dosage in mg/m² to estimate a total dose in mg.
5. Click Calculate to see BSA in m² and ft², plus dose if provided.
6. Click Copy Results to copy a one-line summary for notes, charts, or assignments.

Tip: if you are comparing results over time (for example, during a treatment course), try to measure weight under similar conditions (same scale, similar clothing, similar time of day). Consistency reduces noise and makes trends easier to interpret.

Formulas used and unit assumptions

This calculator uses height in centimeters and weight in kilograms internally. If you choose imperial units, it converts your inputs before applying the selected equation:

• Inches → centimeters: multiply by 2.54
• Pounds → kilograms: multiply by 0.45359237

The output is shown in square meters (m²). For convenience, the calculator also displays square feet (ft²) using 1 m² = 10.7639104167 ft².

• Mosteller (1987): BSA equals the square root of height times weight divided by 3600. $\mathrm{BSA}=\sqrt{\frac{h\times w}{3600}}$ where h is height (cm) and w is weight (kg).
• Du Bois (1916): BSA equals 0.007184 times height to the power 0.725 times weight to the power 0.425. $\mathrm{BSA}=0.007184\times h^{0.725}\times w^{0.425}$ with the same unit assumptions (cm and kg).

Worked examples (step-by-step)

Example 1 (metric): Height 175 cm, weight 70 kg. Using Mosteller: $\sqrt{\frac{175\times 70}{3600}}\approx 1.84$ so BSA ≈ 1.84 m². Converting to ft² gives 1.84 × 10.7639 ≈ 19.81 ft².

Example 2 (imperial + dose): Height 68 in, weight 150 lb, dosage 150 mg/m². First convert units: 68 in × 2.54 = 172.72 cm; 150 lb × 0.45359237 = 68.04 kg. Then compute BSA (about 1.79–1.80 m² depending on formula). If BSA is 1.79 m², the estimated total dose is 1.79 × 150 = 268.5 mg.

Example 3 (dose comparison): Suppose two protocols use 75 mg/m² and 100 mg/m² for the same patient with BSA 1.60 m². The total doses would be 75 × 1.60 = 120 mg and 100 × 1.60 = 160 mg. This illustrates why BSA-based dosing is convenient: once BSA is known, you can quickly scale different mg/m² regimens.

How to interpret the result

The BSA value is typically reported to two decimal places in routine use, but clinical workflows vary. Some institutions round BSA, some round the final dose, and some apply dose banding or maximum dose limits. If you are using this tool for coursework, it can be helpful to state which formula you used and how you rounded.

If you are using the optional dosage field, the calculator multiplies the dosage (mg/m²) by the calculated BSA (m²) to estimate a total amount in mg. This is a straightforward multiplication, but it is not the whole story for real prescribing: clinicians may adjust doses for kidney function, liver function, blood counts, toxicity, age, or drug interactions.

Limitations, edge cases, and clinical caution

BSA is an estimate, not a direct measurement. Equations assume typical body proportions and may be less accurate for children, very small or very large adults, pregnancy, edema, amputations, or unusual body composition. In obesity, for example, some protocols recommend specific dosing approaches (actual body weight, adjusted body weight, or capped BSA) depending on the drug and indication.

The Du Bois equation was derived from a small historical sample, and the Mosteller equation was designed as a practical simplification. For many everyday uses, both are “close enough,” but for specialized populations (pediatrics, critical care, extremes of size) a guideline may recommend a different equation such as Haycock, Gehan and George, or Boyd. This calculator focuses on Mosteller and Du Bois because they are widely referenced and easy to compare.

If you are using BSA for medication dosing, always follow the applicable clinical protocol and confirm with a qualified clinician. This tool is intended for educational and informational use.

Practical notes for accurate inputs

Height and weight errors are the most common reason BSA estimates are off. If you are entering values manually, double-check the unit system first (cm/kg vs in/lb). A common mistake is entering centimeters while imperial is selected (or vice versa), which can produce a dramatically incorrect BSA. If the result looks implausible, re-check the units and the decimal point.

For adults, typical BSA values often fall roughly between about 1.4 m² and 2.3 m², though smaller or larger values can occur. Children have smaller BSAs, and very tall or heavy adults may have larger BSAs. Use these ranges only as a rough “sanity check,” not as a diagnostic rule.

FAQ (quick answers)

Which formula should I choose?

If you are unsure, Mosteller is commonly used because it is simple and produces results close to other methods for many body sizes. If your institution or assignment specifies Du Bois, choose Du Bois for consistency. When in doubt for clinical use, follow the protocol used by your healthcare organization.

Why does the calculator show both m² and ft²?

Square meters (m²) is the standard unit in most medical dosing references. Square feet (ft²) is included for convenience and for users who are more familiar with imperial area units.

Does “Copy Results” copy the full table?

The copy button copies a one-line summary (for example, “BSA 1.84 m² (19.81 ft²), dose 268.50 mg”). This is designed to paste cleanly into notes, spreadsheets, or messages. If clipboard access is blocked by your browser, you can still select and copy the text manually.

Is BSA the same as BMI?

No. BMI (body mass index) is weight divided by height squared and is mainly used as a screening metric related to body size categories. BSA estimates surface area and is often used for normalization and dosing. They answer different questions and are not interchangeable.

Summary

This BSA calculator estimates body surface area from height and weight using either the Mosteller or Du Bois formula. It supports metric and imperial inputs, displays results in m² and ft², and can optionally estimate a total dose from a dosage in mg/m². Use it to learn, to check calculations, or to prepare notes—while remembering that real-world dosing decisions should follow clinical guidelines.