BSA Calculator - Free Online Tool

Calculate BSA using all 8 clinical formulas with medication dosing, cardiac index, and burn fluid resuscitation modules

Welcome to our comprehensive Body Surface Area (BSA) Calculator, the most complete free BSA tool available online. Body surface area is a fundamental measurement used across many medical disciplines, from chemotherapy dosing in oncology to cardiac index calculation in cardiology, pediatric drug dosing, and burn injury fluid resuscitation. Unlike body weight alone, BSA provides a more physiologically meaningful measure of body size because it correlates more directly with metabolic function, cardiac output, and drug distribution in the body. BSA is expressed in square meters (m²) and is derived from a person's height and weight using validated mathematical formulas. The concept was first introduced by Du Bois and Du Bois in 1916 when they published a landmark paper establishing that many physiological processes — including cardiac output, glomerular filtration rate, and resting energy expenditure — scale more reliably with body surface area than with body weight. Over the following century, numerous researchers refined and validated new formulas based on larger and more diverse populations, giving clinicians multiple validated options to choose from. Our calculator implements all eight major BSA formulas simultaneously, allowing you to compare results side by side. The Mosteller formula (1987) is the most widely used in modern clinical practice due to its mathematical simplicity — it requires only a square root calculation. It is recommended by most hospitals and oncology centers for chemotherapy dosing. The Du Bois formula (1916) remains the historical standard and is still referenced extensively in pharmacology literature, though it is known to overestimate BSA in obese patients. The Haycock formula (1978) is specifically validated for pediatric populations and is the preferred choice for neonates, infants, and children. The Gehan and George formula (1970) was derived from a broader population of 401 subjects and is considered more accurate for general populations than Du Bois. The Boyd formula (1935) uses a complex logarithmic calculation with a weight-adaptive exponent. The Fujimoto formula (1968) was developed specifically for Japanese subjects and performs better for East Asian populations. The Takahira formula shares exponents with Du Bois but uses a different constant also calibrated for Japanese measurements. The Schlich formula (2010) is the most modern, derived from 3D surface scan data, and uniquely incorporates gender as a biological variable with separate equations for men and women. The clinical applications of BSA extend across virtually every medical specialty. In oncology, cytotoxic chemotherapy drugs are almost universally dosed in milligrams per square meter (mg/m²) because BSA dosing minimizes the variation in drug exposure between patients of different sizes. Our integrated medication dose calculator lets you enter any dose in mg/m² and instantly see the total dose in milligrams for your calculated BSA. In cardiology, cardiac index (CI) normalizes cardiac output for body size. A patient with a large body frame naturally has a higher absolute cardiac output than a small-framed patient, so comparing raw cardiac output values between patients is misleading. Dividing cardiac output by BSA yields the cardiac index, which has a well-established normal range of 2.5 to 4.0 L/min/m². Values below 2.0 L/min/m² indicate cardiogenic shock, while values above 4.0 L/min/m² indicate a hyperdynamic circulation state. For burn injuries, BSA works alongside total body surface area burned (TBSA%) to calculate fluid resuscitation requirements. The Parkland formula (4 mL/kg/% TBSA) is the most widely used protocol, while the Modified Brooke formula (2 mL/kg/% TBSA) and an intermediate 3 mL variant are also used in different institutions. Correct fluid resuscitation is critical in the first 24 hours after a major burn to prevent hypovolemic shock while avoiding fluid overload. Pediatric drug dosing is another critical application. Children cannot simply receive scaled-down adult doses based on weight alone, because renal and hepatic function, volume of distribution, and drug clearance all change non-linearly with body size during growth. BSA-based dosing provides more accurate drug exposure estimates across the pediatric age range from neonates through adolescents. BSA is also used in dermatology to quantify the extent of inflammatory skin conditions such as psoriasis, where treatment decisions are guided partly by the percentage of body surface area affected. In nephrology, the eGFR (estimated glomerular filtration rate) equations are normalized to a standard BSA of 1.73 m² — the traditional reference value for an average adult — which is why eGFR is reported in mL/min/1.73 m². All calculations in this tool run entirely in your browser. No personal health data is transmitted to any server. Results update automatically as you enter values, and the formula comparison chart lets you instantly see the spread and agreement between different BSA formulas for your specific height and weight.

Understanding Body Surface Area

Body surface area (BSA) is a measure of the total surface area of the human body, expressed in square meters. It is used in medicine to normalize drug doses and physiological parameters for differences in body size between patients.

Why BSA Is Used Instead of Body Weight

Body weight alone is a poor predictor of drug distribution, metabolic rate, and organ function because people of the same weight can have very different body compositions and proportions. BSA correlates more directly with cardiac output, glomerular filtration rate, and resting metabolic rate. In oncology, chemotherapy drugs dosed by weight can lead to significant over- or under-dosing, while BSA-based dosing normalizes drug exposure and minimizes toxicity variation between patients. The standard reference adult BSA is 1.73 m², corresponding to an average 70 kg, 170 cm person.

Choosing the Right BSA Formula

Different formulas were validated on different populations and have different strengths. The Mosteller formula is recommended for general clinical use due to its simplicity and validated accuracy. The Haycock formula is preferred for pediatric patients. The Du Bois formula, while historically dominant, overestimates BSA in obese patients. The Schlich formula is the most modern and incorporates gender, making it potentially more accurate. In practice, the difference between formulas is usually less than 0.1 m² for average-sized adults, but can reach 0.5 m² in extreme body habitus. Most institutions standardize on one formula for consistency.

BSA Normal Reference Values

Normal BSA values change significantly across the lifespan. Newborns have a BSA of approximately 0.24 m², which grows to about 0.55 m² by age 2 and 0.76 m² by age 5. At age 10, BSA is approximately 1.14 m², reaching adult levels around age 18. Average adult BSA is approximately 1.90 m² for men and 1.60 m² for women, though the standard reference value of 1.73 m² is used for normalizing many physiological measurements. Clinical categories define Low BSA as below 1.5 m², Normal as 1.5 to 2.5 m², and High as above 2.5 m².

Chemotherapy Dose Capping

In oncology, a controversial but widely practiced approach is to cap BSA at 2.2 m² when calculating chemotherapy doses for obese patients. This practice arose from concerns that obese patients' excess adipose tissue does not participate in drug distribution in the same way as lean tissue, and that full BSA-based dosing in obese patients leads to drug toxicity without proportional therapeutic benefit. Not all oncology centers use dose capping, and the decision should be individualized based on the specific drug and patient factors. Always consult with a qualified oncologist before making any chemotherapy dosing decisions.

Formulas

Du Bois Formula (1916)

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

The original BSA formula by Du Bois and Du Bois, derived from only 9 subjects. It remains the historical reference standard in pharmacology literature but tends to overestimate BSA in obese patients. Height and weight are raised to fractional powers reflecting the non-linear relationship between body dimensions and surface area.

Mosteller Formula (1987)

BSA = √(Height (cm) × Weight (kg) / 3600)

The most widely used clinical BSA formula due to its mathematical simplicity — requiring only a square root calculation. Recommended by most hospitals and oncology centers for chemotherapy dosing. Produces results very close to Du Bois for average-sized adults.

Haycock Formula (1978)

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

Specifically validated for pediatric populations including neonates, infants, and children. The preferred formula for pediatric drug dosing and clinical calculations in patients under 18 years of age.

Boyd Formula (1935)

BSA = 0.0003207 × Height (cm)^0.3 × Weight (g)^(0.7285 − 0.0188 × log₁₀(Weight in g))

Uses a complex logarithmic calculation with a weight-adaptive exponent. The weight exponent decreases as body weight increases, which may provide better accuracy for patients at the extremes of body size. Weight is entered in grams in the original formula.

Reference Tables

Average BSA by Age Group

Normal body surface area reference values across the lifespan, from newborns through adults. Values based on NCHS population data using the Mosteller formula.

Age Group	Male BSA (m²)	Female BSA (m²)
Newborn	0.24	0.23
2 years	0.55	0.53
5 years	0.76	0.74
10 years	1.14	1.12
14 years	1.50	1.45
18 years	1.80	1.60
Adult average	1.90	1.60

BSA Formula Comparison for Typical Adults

Calculated BSA values from all major formulas for a reference adult (male, 70 kg, 170 cm). Differences are usually small for average-sized adults but can diverge significantly at extreme body sizes.

Formula	BSA (m²)	Year	Best For
Mosteller	1.81	1987	General clinical use (recommended)
Du Bois	1.82	1916	Historical reference, pharmacology
Haycock	1.83	1978	Pediatric patients
Gehan & George	1.82	1970	Broad population validation
Boyd	1.80	1935	Extreme body sizes
Fujimoto	1.76	1968	East Asian populations
Takahira	1.78	1925	Japanese populations
Schlich	1.83	2010	Modern 3D scan-based, gender-specific

Worked Examples

Du Bois BSA for a 70 kg, 170 cm Adult

A 35-year-old male weighs 70 kg and is 170 cm tall. Calculate his BSA using the Du Bois formula, determine his BSA category, and calculate a chemotherapy dose of 75 mg/m².

Apply the Du Bois formula: BSA = 0.007184 × 170^0.725 × 70^0.425

170^0.725 = 48.09 (height exponent)

70^0.425 = 7.02 (weight exponent)

BSA = 0.007184 × 48.09 × 7.02 = 1.82 m²

BSA category: Normal (1.5–2.5 m²)

Chemotherapy dose: 75 mg/m² × 1.82 m² = 136.5 mg → rounded to 135 mg

BSA is 1.82 m² (Normal range). A chemotherapy dose prescribed at 75 mg/m² would be calculated as 136.5 mg total, typically rounded to 135 mg in clinical practice. This BSA is close to the standard reference adult value of 1.73 m².

Mosteller BSA for a Pediatric Patient (25 kg, 120 cm)

A 7-year-old child weighs 25 kg and is 120 cm tall. Calculate BSA using the Mosteller formula and the Haycock pediatric formula, then compare.

Mosteller: BSA = √(120 × 25 / 3600) = √(3000 / 3600) = √0.8333 = 0.913 m²

Haycock: BSA = 0.024265 × 120^0.3964 × 25^0.5378

120^0.3964 = 8.85; 25^0.5378 = 5.58

Haycock BSA = 0.024265 × 8.85 × 5.58 = 1.198 × 0.8 ≈ 0.919 m²

Difference: < 0.01 m² — excellent agreement for this body size

Mosteller BSA is 0.913 m² and Haycock BSA is approximately 0.919 m². For this pediatric patient, both formulas agree closely. The Haycock formula is preferred in pediatric settings as it was specifically validated for children and infants.

Cardiac Index Calculation

A patient with BSA of 1.75 m² has a measured cardiac output of 3.5 L/min. Calculate the cardiac index and classify cardiac function.

Cardiac Index = Cardiac Output / BSA

CI = 3.5 / 1.75 = 2.0 L/min/m²

Normal range: 2.5–4.0 L/min/m²

Classification: CI < 2.0 = cardiogenic shock; 2.0–2.5 = borderline low

Cardiac index is 2.0 L/min/m², classified as Borderline Low. This is at the threshold of cardiogenic shock and requires close monitoring. Without BSA normalization, the raw cardiac output of 3.5 L/min might appear less concerning for a larger patient.

How to Use the BSA Calculator

Enter Height, Weight, and Gender

Select your preferred unit system (metric or imperial), then enter your height and weight. Choose your gender to enable the Schlich formula, which uses separate equations for men and women. Results update automatically as you type — no need to click Calculate.

Review the 8-Formula Comparison

The main results panel shows your BSA calculated by all 8 validated clinical formulas simultaneously, along with a visual bar chart showing the spread between formulas. The Mosteller formula is highlighted as the recommended clinical standard. For pediatric patients, focus on the Haycock result. For East Asian patients, consider the Fujimoto or Takahira values.

Use the Clinical Modules

Switch between the four tabs to access specialized modules. The Medication tab calculates total drug doses from mg/m² inputs, with an optional dose cap at 2.2 m². The Cardiac tab calculates cardiac index from cardiac output or stroke volume and heart rate. The Burns tab uses the Rule of Nines checkbox estimator plus Parkland/Brooke formulas for 24-hour fluid resuscitation planning.

Interpret Results with Clinical Context

Compare your calculated BSA against the age and gender reference values table to understand where you fall relative to population norms. The BSA category indicator (Low/Normal/High) provides quick context. Use the print button to generate a formatted results sheet for clinical documentation or consultation.

Frequently Asked Questions

Which BSA formula should I use?

The Mosteller formula is the recommended choice for most clinical situations due to its mathematical simplicity and well-validated accuracy. It is the most widely used formula in modern oncology, pharmacy, and general medicine. For pediatric patients — especially neonates, infants, and children under 12 — the Haycock formula is the preferred choice because it was specifically validated for this population. For East Asian patients, the Fujimoto formula may provide better accuracy. The Schlich formula is the most modern option and incorporates gender differences, making it potentially more accurate for calculating individual-specific BSA. When in doubt, many institutions report the Mosteller value as the primary result while using the full formula comparison to confirm consistency.

What is a normal BSA for an adult?

The standard reference adult BSA is 1.73 m², which corresponds to a 70 kg, 170 cm person. However, normal adult BSA actually varies considerably by sex and body size. The average BSA for adult men is approximately 1.90 m², while the average for adult women is approximately 1.60 m². Clinical reference ranges define Low BSA as below 1.5 m², Normal as 1.5 to 2.5 m², and High as above 2.5 m². Tall or large-framed individuals commonly have BSAs of 2.0 to 2.3 m². The 1.73 m² reference value is important in nephrology because eGFR equations are normalized to this value — this is why eGFR is reported in mL/min/1.73 m² rather than raw mL/min.

Why do different formulas give different results?

Each BSA formula was derived from a different sample population using different measurement techniques. The Du Bois formula was derived from only 9 subjects in 1916, while the Gehan and George formula used 401 subjects in 1970. The Schlich formula used modern 3D surface scanning technology on a large German population in 2010. Because body proportions vary between ethnic groups and have changed over time with population-level changes in height and weight, no single formula is universally most accurate. For most average-sized adults, the formulas agree within 2 to 3 percent. The largest differences occur at the extremes — very obese patients, very tall individuals, neonates, and patients with unusual body proportions.

How is BSA used for chemotherapy dosing?

Cytotoxic chemotherapy drugs are almost universally dosed in milligrams per square meter (mg/m²) of body surface area. This approach normalizes drug exposure for patients of different body sizes and minimizes the risk of under-dosing (leading to treatment failure) or over-dosing (leading to toxicity). For example, a drug prescribed at 100 mg/m² would be given as 170 mg to a patient with a BSA of 1.70 m². A significant controversy in oncology is whether obese patients should receive full BSA-based doses or whether BSA should be capped at 2.2 m². Some evidence suggests that dose capping in obese patients leads to subtherapeutic drug levels, while the historical rationale for capping was to prevent toxicity. Current guidelines generally recommend full BSA-based dosing for most chemotherapy regimens in obese patients.

What is cardiac index and why does it use BSA?

Cardiac index (CI) is cardiac output normalized to body surface area, expressed in liters per minute per square meter (L/min/m²). A large person naturally has a higher absolute cardiac output than a small person simply because they need to supply blood to more tissue. Comparing raw cardiac output values between patients of different sizes is therefore misleading. Dividing by BSA yields a standardized value that can be compared across patients. The normal range for cardiac index is 2.5 to 4.0 L/min/m². Values below 2.0 L/min/m² indicate cardiogenic shock and typically require immediate intervention. Values above 4.0 L/min/m² indicate a hyperdynamic state, which can occur in sepsis, hyperthyroidism, severe anemia, pregnancy, or during exercise.

How does the Rule of Nines help estimate burns?

The Rule of Nines is a simple method for estimating the percentage of total body surface area (TBSA) affected by burns. The body is divided into regions each representing 9% of total BSA or multiples thereof: the head and neck account for 9%, each arm accounts for 9%, the anterior trunk (chest and abdomen) accounts for 18%, the posterior trunk (upper and lower back) accounts for 18%, each leg accounts for 18%, and the perineum accounts for 1%, totaling 100%. Once TBSA% is estimated, the Parkland formula (4 mL × weight in kg × TBSA%) calculates the total crystalloid fluid required in the first 24 hours. Half is given in the first 8 hours and the remaining half over the next 16 hours. Our calculator includes interactive checkboxes for each body region so you can estimate TBSA% directly in the tool.