BSA Calculator

Comprehensive Guide to Body Surface Area (BSA)

Body Surface Area (BSA) is a measurement of the total skin surface area of a human body, expressed in square meters (m²). While not directly used in fitness training, BSA is a critical metric in medical practice used for:

• Medication dosing: Many drugs (especially chemotherapy) are dosed based on BSA rather than body weight
• Cardiac assessment: Used to evaluate heart function and diagnosis of cardiovascular conditions
• Fluid management: Helps determine appropriate IV fluid resuscitation in burns and critical illness
• Metabolic calculations: More accurate than body weight alone for certain metabolic predictions
• Clinical evaluation: Standardizes comparisons across patients of different sizes

Four main formulas are used to calculate BSA, each developed through different research populations.

How to Use the BSA Calculator

Our BSA calculator determines your body surface area using multiple formulas:

1. Enter Your Height

   • In feet/inches or centimeters
   • Accurate measurement important

2. Enter Your Weight

   • In pounds or kilograms
   • Current body weight

3. View BSA Estimates

   • Results from Mosteller formula (most common)
   • Results from Du Bois formula (original)
   • Results from Haycock formula (pediatric focus)
   • Results from Gehan & George formula
   • Average and median values

4. Understand Clinical Applications

   • Why BSA matters
   • How doctors use it
   • Clinical dosing examples

The Four BSA Formulas

The Mosteller Formula (1987)

The Mosteller formula is the simplest and most widely used in modern clinical practice.

Formula:

BSA (m²) = √ [Height (cm) × Weight (kg) ÷ 3600]

Or in imperial units:

BSA (m²) = √ [Height (in) × Weight (lbs) ÷ 3131]

Advantages:

• Simplest formula mathematically
• Readily available on calculators
• Good accuracy across patient populations
• Recommended by many clinical guidelines

Example: 70 kg person, 180 cm tall

• BSA = √ [180 × 70 ÷ 3600]
• BSA = √ [12,600 ÷ 3600]
• BSA = √ 3.5
• BSA = 1.87 m²

The Du Bois Formula (1916)

The Du Bois formula was the first widely used BSA formula and remains standard in some contexts.

Formula:

BSA (m²) = 0.007184 × Weight (kg)^0.425 × Height (cm)^0.725

Advantages:

• Original validated formula
• Good accuracy for adult patients
• Widely recognized historically
• Still preferred in some countries

Example: 70 kg person, 180 cm tall

• BSA = 0.007184 × 70^0.425 × 180^0.725
• BSA = 0.007184 × 5.39 × 25.56
• BSA ≈ 1.92 m²

The Haycock Formula (1978)

The Haycock formula was specifically developed for pediatric (child) patients.

Formula:

BSA (m²) = 0.024265 × Weight (kg)^0.5378 × Height (cm)^0.3964

Advantages:

• More accurate for children and adolescents
• Accounts for different body proportions in children
• Recommended for pediatric dosing
• Used in pediatric hospitals

Example: 15 kg child, 100 cm tall

• BSA = 0.024265 × 15^0.5378 × 100^0.3964
• BSA = 0.024265 × 3.44 × 15.85
• BSA ≈ 0.61 m²

The Gehan & George Formula (1970)

The Gehan & George formula was developed through extensive research in patient populations.

Formula:

BSA (m²) = 0.010 × Weight (kg)^0.67 × Height (cm)^0.33

Advantages:

• Strong empirical basis
• Good for general population
• Useful for comparing to historical data
• Used in some research contexts

Example: 70 kg person, 180 cm tall

• BSA = 0.010 × 70^0.67 × 180^0.33
• BSA = 0.010 × 18.6 × 5.65
• BSA ≈ 1.05 m² (note: this formula tends to underestimate)

Comparing the Formulas

Key Point: For most clinical applications, Mosteller and Du Bois produce nearly identical results in adults (within 2-5% difference). The choice between them rarely affects clinical decisions.

BSA Categories and Clinical Significance

BSA Applications in Clinical Practice

Medication Dosing

Many chemotherapy drugs and some other medications are dosed based on BSA rather than body weight because it provides more accurate dosing across different body sizes.

Example: Chemotherapy Dosing

A drug prescribed at 100 mg/m²:

• Patient with BSA 1.8 m²: 180 mg dose
• Patient with BSA 2.2 m²: 220 mg dose

Why use BSA?

• Two patients weighing 200 lbs may have very different body compositions
• One might be muscular (lower BSA), one obese (higher BSA)
• BSA captures this difference better than weight alone

Cardiac Assessment

BSA is used to calculate several important cardiac metrics:

Cardiac Index:

Cardiac Index = Cardiac Output (L/min) ÷ BSA (m²)

Normal: 2.5-4.0 L/min/m²

This standardizes heart function across different patient sizes.

Burn Assessment

In severe burn cases, BSA of burned skin determines fluid resuscitation strategy (Parkland formula):

24-hour fluid (mL) = 4 × BSA burned (m²) × Weight (kg)

Example: 30 kg child with 50% BSA burn (BSA = 1.0 m²)

• 24-hour fluid = 4 × 1.0 × 30 = 120 mL/hour average

Metabolic Rate Estimation

A more body-composition-independent estimate of metabolic rate:

Resting Metabolic Rate ≈ 3,500 × BSA (kcal/day)

This is more accurate than some weight-based formulas for obese individuals.

BSA Across Different Body Types

Ectomorph (Thin)

• Example: 150 lb (68 kg), 6'0" (183 cm)
• Mosteller BSA: 1.77 m²
• Interpretation: Lower BSA relative to weight due to thin, linear frame

Mesomorph (Athletic)

• Example: 190 lb (86 kg), 6'0" (183 cm)
• Mosteller BSA: 2.05 m²
• Interpretation: BSA proportional to height and weight; athletic composition

Endomorph (Heavier)

• Example: 240 lb (109 kg), 5'9" (175 cm)
• Mosteller BSA: 2.27 m²
• Interpretation: Higher BSA relative to height; more carrying body mass

Key insight: Two people of the same weight and height have the same BSA regardless of body composition (muscle vs. fat), which is why BSA is so useful for standardizing medical dosing.

Pediatric BSA Examples

BSA and Drug Dosing Examples

Example 1: Chemotherapy (Vincristine)

Dose: 1.4 mg/m²

Patient: 80 kg, 180 cm → BSA = 1.95 m²

• Dose = 1.4 × 1.95 = 2.73 mg

Example 2: Pediatric Antibiotic (Based on BSA)

Dose: 50 mg/m²/day

Child: 20 kg, 110 cm → BSA = 0.80 m² (Haycock)

• Daily dose = 50 × 0.80 = 40 mg/day

Example 3: Immunosuppressant Drug

Dose: 4 mg/kg/BSA

Patient: 70 kg, 175 cm → BSA = 1.85 m²

• Dose = (4 × 70) ÷ 1.85 = 280 ÷ 1.85 = 151 mg

Limitations of BSA

What BSA Doesn't Account For

• Organ function: BSA-based dosing assumes organs work proportionally to surface area, which isn't always true
• Age and metabolism: Pediatric patients have different metabolism than adults at the same BSA
• Genetic variation: Individual responses to drugs vary beyond what BSA predicts
• Body composition: Very obese or very lean patients may metabolize drugs differently

When Doctors Adjust BSA-Based Dosing

• Severe liver or kidney disease
• Extreme obesity
• Genetic metabolic differences
• Drug interactions
• Age (pediatric/geriatric)
• Pregnancy

Frequently Asked Questions

Which BSA formula should I use?

For most purposes, use Mosteller—it's the simplest, most commonly used, and recommended by major medical organizations. Du Bois is acceptable and historically established. Haycock for pediatric patients.

How accurate are BSA calculations?

All major formulas are accurate within ±10% for most populations. Accuracy decreases in extreme cases (very obese, very lean, very tall/short). When exact accuracy matters clinically, doctors may adjust based on other factors.

Does my BSA change if I gain or lose weight?

Yes. BSA directly incorporates weight, so gaining 10 pounds increases BSA, losing 10 pounds decreases it. Height is constant, so weight changes are the main factor affecting BSA over time.

How much does one pound of weight change affect BSA?

Roughly 0.01-0.015 m² change per 10 lbs gained or lost, depending on height (ranges from ~0.001 to ~0.02 m² per pound depending on your current BSA and body dimensions).

Is BSA used in fitness?

Not routinely. Fitness typically uses body weight, BMI, body fat percentage, and lean body mass. However, some research uses BSA for comparing metabolic rates across individuals.

Can very obese patients have lower BSA than expected?

Surprisingly, sometimes yes. BSA is a function of height and weight, but very obese people often have less surface area relative to their volume than expected (due to rounded, compact shape). This is one reason BSA-based dosing can be inaccurate in severe obesity.