Body Surface Area Calculator

Body Surface Area Calculator: Estimate Your BSA for Drug Dosing, Burns, and Clinical Use

TL;DR: Body surface area (BSA) estimates total skin area in square metres from height and weight. A 170 cm, 70 kg adult produces a BSA of roughly 1.81 m² using the DuBois formula. This calculator runs five validated formulas side by side so you can compare results and pick the one your clinical context requires.

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Table of Contents

• Why Clinicians Measure Skin Area by the Square Metre
• Six Scenarios Where BSA Drives a Decision
• Five Formulas, Five Decades of Research
• How to Get Your BSA in Under Two Minutes
• Two Worked Examples
• Six Common Mistakes and How to Fix Them
• FAQ
• Assumptions and Notes
• Put Your BSA to Work
• Further Reading

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Why Clinicians Measure Skin Area by the Square Metre

Hospitals around the world dose chemotherapy drugs per square metre of body surface, not per kilogram of weight. The reason is practical: BSA correlates more tightly with metabolic rate, organ size, and blood volume than body weight alone. Two patients at 70 kg with different heights will have meaningfully different cardiac outputs, renal clearance rates, and plasma volumes. BSA captures that difference where weight cannot.

The concept dates to the early 1900s. DuBois and DuBois published their formula in 1916 after measuring cadaver skin by laying it flat and recording the area directly. Later researchers refined the exponents and coefficients using larger, more diverse populations, but the core insight stayed the same: height and weight together predict total skin area with enough precision for clinical dosing.

Outside of pharmacology, BSA matters for burn assessment (the "rule of nines" maps burned regions as percentages of total BSA), cardiac index calculation (cardiac output divided by BSA), and renal function benchmarking. It also appears in pediatric growth monitoring, where BSA-adjusted metabolic rates track development more accurately than weight-for-age charts.

Enter your height and weight into the calculator above and all five formula results appear instantly.

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Six Scenarios Where BSA Drives a Decision

• Chemotherapy dosing. Most cytotoxic agents are prescribed in mg/m². A patient with a BSA of 1.70 m² receiving carboplatin at 400 mg/m² gets 680 mg total. If that patient's BSA were miscalculated as 1.85 m², the dose would jump to 740 mg, a 60 mg overshoot that increases toxicity risk by 8-12% according to oncology safety data.

• Burn severity classification. Emergency departments estimate burn extent as a percentage of total BSA. For an adult with a BSA of 1.90 m², a burn covering 0.19 m² equals 10% TBSA (total body surface area). Burns above 20% TBSA in adults typically require ICU-level fluid resuscitation using the Parkland formula, which itself depends on accurate BSA.

• Cardiac index monitoring. Cardiac index equals cardiac output divided by BSA. Normal cardiac index is 2.5-4.0 L/min/m². A patient with a cardiac output of 4.5 L/min and a BSA of 1.75 m² has a cardiac index of 2.57, barely within normal range. Incorrect BSA inflates or deflates this metric and can influence treatment timing.

• Pediatric drug dosing. Children cannot be dosed by simple weight fractions of adult doses because their organ-to-body-weight ratios differ from adults. A 6-year-old at 20 kg and 115 cm has a BSA near 0.80 m², which is roughly 44% of an average adult's BSA of 1.80 m², not 29% as the weight ratio alone would suggest. BSA-based dosing corrects for this difference.

• Renal function adjustment. Glomerular filtration rate (GFR) is reported per 1.73 m² of BSA by convention. When a patient's actual BSA is 2.10 m², their raw GFR needs to be adjusted downward by a factor of 1.73/2.10 = 0.82 to produce a standardized reading. Skipping this adjustment can mask Stage 2 chronic kidney disease.

• Nutritional support in critical care. Basal energy expenditure equations like the Harris-Benedict formula sometimes underpredict caloric needs in very tall or very short patients. ICU dietitians cross-check calorie targets against BSA-derived estimates, where 25-30 kcal/kg/day is recalibrated to approximately 900-1,000 kcal/m²/day for ventilated patients.

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Five Formulas, Five Decades of Research

Each formula below takes height (in cm) and weight (in kg) and returns BSA in square metres. They differ in their coefficients and exponents because each was derived from a different study population.

DuBois & DuBois (1916):    BSA = 0.007184 × height^0.725 × weight^0.425
Fujimoto (1968):           BSA = 0.008883 × height^0.663 × weight^0.444
Gehan & George (1970):     BSA = 0.0235 × height^0.42246 × weight^0.51456
Haycock (1978):            BSA = 0.024265 × height^0.3964 × weight^0.5378
Mosteller (1987):          BSA = √(height × weight / 3600)

Mosteller's formula stands out for simplicity. It requires only a square root and basic multiplication, making it easy to compute by hand or mental math in clinical settings without a calculator nearby.

BSA Comparison at Common Profiles

Typical Adult BSA Ranges

Genetic and ethnic variation affects BSA independently of height and weight. The Fujimoto formula was developed on a Japanese population and produces slightly lower values for the same inputs compared to DuBois, which was calibrated primarily on Western European subjects. For East Asian patients, some oncologists prefer Fujimoto; for pediatric patients, Haycock is the most commonly recommended because its validation cohort included children and infants down to 3.5 kg.

The formulas converge most tightly in the 160-180 cm, 55-85 kg range. They diverge at extremes: for a 200 cm, 130 kg patient, the spread between the lowest and highest formula result can exceed 0.15 m², enough to shift a chemotherapy dose by 5-8%.

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How to Get Your BSA in Under Two Minutes

1. Enter your weight in kilograms. Use a recently calibrated scale. Clinical pharmacists recommend recording weight on the day of treatment, not relying on a weight taken days or weeks earlier. Fluid shifts from illness or medication can change weight by 2-4 kg within a week.

2. Enter your height in centimetres. Measure standing upright without shoes against a flat wall. Rounding height up by even 2 cm changes BSA by approximately 0.01-0.02 m², which seems small but compounds across dose calculations.

3. Review all five formula outputs. When four or five formulas agree within 0.03 m², the consensus value is reliable for clinical or personal use. Disagreement greater than 0.08 m² usually signals a data entry error.

4. Identify which formula your context requires. Oncology protocols in North America typically specify DuBois or Mosteller. Pediatric units often default to Haycock. If your hospital or prescriber specified a formula, use that one exclusively.

5. Record the result with its formula name. Writing "BSA = 1.82 m² (Mosteller)" is meaningful. Writing "BSA = 1.82 m²" alone is not, because a clinician comparing across visits needs formula consistency.

6. Cross-check against the typical range table. An adult male getting 1.45 m² or 2.55 m² should double-check inputs. Results far outside the expected range almost always trace back to a unit error or digit transposition.

Non-obvious insight: The Mosteller formula is the only one you can verify by mental math. Multiply height by weight, divide by 3600, and take the square root. For 170 cm and 70 kg: 170 × 70 = 11,900; 11,900 / 3600 = 3.306; square root of 3.306 is approximately 1.82 m². This mental check catches data entry errors before they reach a prescription.

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Two Worked Examples

Example 1: 58-Year-Old Retired Nurse Preparing for Chemotherapy

Margaret is 163 cm tall and weighs 72 kg. Her oncologist has prescribed carboplatin dosed at 400 mg/m² and needs her BSA to calculate the total dose. She wants to verify the number herself before her appointment.

DuBois Calculation:

BSA = 0.007184 × 163^0.725 × 72^0.425
    = 0.007184 × 46.57 × 7.00
    = 0.007184 × 326.0
    = 1.78 m²

Mosteller Calculation:

BSA = √(163 × 72 / 3600)
    = √(11,736 / 3600)
    = √3.26
    = 1.81 m²

The 0.03 m² gap between the two formulas translates to a 12 mg dose difference. For carboplatin, this falls within the acceptable rounding range at most oncology centres, but Margaret should confirm which formula her prescribing oncologist uses. If the hospital protocol specifies Mosteller, the 1.81 m² figure is the one that goes on the chart. She should also have her weight re-measured on the day of infusion, as even 1-2 kg of fluid retention from pre-treatment steroids would shift BSA by about 0.01 m².

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Example 2: 6-Year-Old Pediatric Patient

Lucas is 6 years old, 118 cm tall, and weighs 22 kg. His pediatrician needs a BSA figure to dose methotrexate at 3.3 mg/m² for a standard protocol.

Haycock Calculation (preferred for pediatric patients):

BSA = 0.024265 × 118^0.3964 × 22^0.5378
    = 0.024265 × 9.86 × 5.08
    = 0.024265 × 50.09
    = 0.84 m²

Mosteller Calculation:

BSA = √(118 × 22 / 3600)
    = √(2,596 / 3600)
    = √0.721
    = 0.85 m²

The two formulas agree closely for Lucas, producing a dose difference under 0.05 mg. For pediatric dosing, Haycock is the standard reference because its derivation dataset included children from infancy through adolescence. The Mosteller result serves as a quick confirmation. Lucas's parents should know that his BSA will increase as he grows: by age 10, a typical child of average height and weight reaches roughly 1.10 m², a 30% increase from the current value. Dose recalculation at every visit is standard pediatric practice for this reason.

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Six Common Mistakes and How to Fix Them

Mixing up centimetres and inches. Entering 68 (inches) instead of 173 (cm) for height produces a BSA of roughly 1.15 m² instead of 1.85 m² using DuBois. That is a 38% underestimate. Always confirm the unit label before pressing calculate.

Using a stale weight. A weight recorded 3 weeks ago may differ from today's weight by 2-5 kg due to fluid changes, medication effects, or dietary shifts. In oncology, same-day weight is the standard. Even for personal tracking, a weight older than 7 days reduces accuracy.

Applying the wrong formula for the patient population. DuBois was validated primarily on adults of European descent. Using it for a 4-year-old or an elderly East Asian patient introduces bias. Haycock is better for children; Fujimoto better reflects body proportions common in Japanese populations. The formula choice is not arbitrary.

Rounding BSA before calculating the drug dose. Rounding 1.78 m² to 1.80 m² before multiplying by a dose rate of 400 mg/m² adds 8 mg. For drugs with narrow therapeutic windows like vincristine or doxorubicin, that 8 mg can push the dose above the toxicity threshold. Carry at least 2 decimal places through the dose calculation, then round the final dose.

Assuming all five formulas should give the same answer. They will not. Each was derived from a different dataset with different regression methods. A spread of 0.03-0.05 m² across all five is normal and expected. A spread above 0.10 m² suggests an input error or a patient at the extreme edge of the formulas' validated ranges.

Ignoring BSA in non-oncology contexts. BSA is not only for cancer treatment. Burn surgeons use it daily. Cardiologists depend on it for cardiac index. Nephrologists normalize GFR to 1.73 m² of BSA. Treating it as an "oncology-only" number means missing its value in at least 3 other major clinical areas.

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Assumptions and Notes

• Margin of error: All five formulas carry a reported estimation error of approximately ±5-10% compared to direct measurement methods such as 3D body scanning or coating-and-weighing techniques. For clinical drug dosing, this margin is generally accepted as tolerable because therapeutic windows are designed with it in mind.
• Professional disclaimer: This calculator is for informational and educational purposes only and does not replace medical advice. BSA results should not be used to self-adjust medication doses. Always consult your prescribing physician or clinical pharmacist before making dosing decisions.

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Put Your BSA to Work

Margaret's story from the examples above captures the practical value here. She did not just want a number; she wanted to arrive at her oncology appointment with a verified BSA so she could ask informed questions about her carboplatin dose. Lucas's parents needed to understand why their son's medication changes every few months as he grows. Both cases come back to the same action: enter two measurements, read five results, and pick the formula that matches your clinical context.

Use the calculator at the top of this page to get your BSA now.

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Further Reading

• BMI Calculator: Check Your Body Mass Index Alongside BSA
• TDEE Calculator: Estimate Daily Calorie Needs Using Metabolic Scaling
• Lean Body Mass Calculator: Separate Lean Tissue From Fat Mass