How to Calculate Body Surface Area — BSA Formulas, Uses in Medicine and What Your Result Means
Body Surface Area — or BSA — is one of those medical measurements most people have never heard of until a doctor mentions it. Yet it plays a surprisingly important role in clinical medicine, from calculating chemotherapy doses to assessing burn injuries and normalizing cardiac measurements.
Unlike body weight or BMI, BSA measures the total external surface of your body in square metres. It accounts for both your height and weight together, which makes it a more precise way to scale drug doses and medical measurements to individual body size.
In this guide we explain exactly what BSA is, walk through the main formulas used to calculate it, cover its key medical applications, and show you how to get your result instantly using our free BSA calculator.
What Is Body Surface Area (BSA)?
Body Surface Area is a measurement of the total surface of the human body expressed in square metres (m²). It is not the same as body weight and it is not the same as BMI. Where BMI uses height and weight to estimate body fatness, BSA is purely a geometric measurement — the area of your body’s outer surface.
For most adults, BSA falls between 1.6 m² and 2.0 m². The average adult male has a BSA of approximately 1.9 m² and the average adult female approximately 1.6 m². Children have significantly lower BSA values — a newborn has a BSA of around 0.25 m².
The reason BSA matters in medicine is that many physiological processes — kidney filtration, cardiac output, drug metabolism — scale more reliably with body surface area than with body weight alone. A person who is very tall and lean and a person who is short and heavy might have the same weight but very different BSA values, and their medical needs differ accordingly.
How to Calculate Body Surface Area — The Main Formulas
There is no single universally accepted BSA formula. Over the past century researchers have developed multiple formulas using different mathematical approaches and different study populations. Each has its strengths and is preferred in different clinical contexts.
All formulas use two inputs: body weight in kilograms and height in centimetres.
1. The Mosteller Formula — Simplest and Most Widely Used
The Mosteller formula was published in 1987 and is now the most commonly used formula in clinical practice because of its simplicity and accuracy.
Formula: BSA (m²) = √ (Height cm × Weight kg / 3600)
Example: A person weighing 70 kg and 170 cm tall has a BSA of √ (170 × 70 / 3600) = √ 3.306 = 1.82 m²
2. The Du Bois Formula — The Original Standard
The Du Bois formula was developed in 1916 and was the standard for most of the 20th century. It is still widely cited and used in many clinical guidelines.
Formula: BSA (m²) = 0.007184 × Weight kg⁰·⁴²⁵ × Height cm⁰·⁷²⁵
The Du Bois formula tends to underestimate BSA in obese individuals compared to more modern formulas.
3. The Haycock Formula — Best for Children
The Haycock formula was developed specifically for pediatric patients and is the preferred formula for calculating BSA in children and infants.
Formula: BSA (m²) = 0.024265 × Weight kg⁰·⁵³⁷⁸ × Height cm⁰·³⁹⁶⁴
4. Other Formulas in This Calculator
Our BSA calculator includes five additional formulas — Gehan & George, Boyd, Fujimoto, Takahira, and Schlich. Each was developed for specific populations or body types:
- Gehan & George (1970) — general adult and pediatric use
- Boyd (1935) — handles extreme weight ranges better than most formulas
- Fujimoto (1968) — developed on Japanese subjects, preferred for Asian populations
- Takahira (1925) — another formula developed for Japanese adults
- Schlich (2010) — uses separate equations for males and females for greater precision
BSA Formula Comparison
| Formula | Developed By | Best Used For | Complexity |
| Du Bois | Du Bois & Du Bois, 1916 | General adult use — most widely cited in medicine | Standard |
| Mosteller | Mosteller, 1987 | Clinical settings — simplest and fastest to calculate | Very Simple |
| Haycock | Haycock et al, 1978 | Pediatric patients — children and infants | Standard |
| Gehan & George | Gehan & George, 1970 | General adult and pediatric use | Standard |
| Boyd | Boyd, 1935 | Wide weight ranges — handles extremes well | Complex |
| Fujimoto | Fujimoto et al, 1968 | Asian populations — developed on Japanese subjects | Standard |
| Takahira | Takahira, 1925 | Japanese adult populations | Standard |
| Schlich | Schlich et al, 2010 | Gender-specific calculation — separate male/female formulas | Standard |
Medical Uses of Body Surface Area
BSA is used across multiple medical specialties. Here is a breakdown of the most important clinical applications:
| Medical Use | Why BSA Matters |
| Chemotherapy dosing | Most chemo drugs are dosed in mg per m² of BSA to avoid under or overdosing based on body size |
| Burn assessment | The Rule of Nines uses BSA to estimate the percentage of body surface burned — critical for treatment planning |
| Cardiac output | Cardiac index is calculated by dividing cardiac output by BSA to normalize results across different body sizes |
| Kidney function | GFR (glomerular filtration rate) is often adjusted for BSA to give a standardized measure of kidney function |
| Pediatric drug dosing | Children’s medication doses are frequently calculated per m² of BSA rather than body weight alone |
| Radiation therapy | Some radiation treatment plans use BSA to determine appropriate dosing fields |
BSA vs BMI — What Is the Difference?
BSA and BMI are both calculated from height and weight but they measure completely different things and are used for different purposes.
- BMI estimates body fatness — it is a screening tool for underweight, normal weight, overweight, and obesity categories
- BSA measures total body surface area — it is a clinical measurement used to scale drug doses and normalize physiological measurements
- BMI does not account for body composition — a muscular athlete and a sedentary person of the same height and weight have the same BMI but different health profiles
- BSA is less affected by body composition variation — it scales with overall body size regardless of whether mass is fat or muscle
In practice most people will encounter BMI in general health screening and BSA in clinical or hospital settings — particularly in oncology, cardiology, and nephrology.
What Is a Normal BSA Value?
BSA varies significantly by age, sex, and body size. Here are general reference ranges:
- Newborn infant — approximately 0.25 m²
- 2-year-old child — approximately 0.5 m²
- 10-year-old child — approximately 1.1 m²
- Average adult female — approximately 1.6 m²
- Average adult male — approximately 1.9 m²
- Larger adults — can reach 2.5 m² or higher
There is no universally ‘ideal’ BSA value the way there is for BMI. BSA is used to normalize other measurements rather than as a standalone health indicator. A higher or lower BSA is simply a reflection of your body size.
For clinical reference on BSA formulas and their applications in drug dosing, visit the National Institutes of Health research on BSA calculation methods.
How to Use the Free BSA Calculator
Our free Body Surface Area Calculator supports all 8 major BSA formulas and calculates all of them simultaneously so you can compare results. Here is how to use it:
- Enter your body weight and select your unit — pounds or kilograms
- Enter your height in cm, metres, or feet — or use the feet and inches input directly below
- Select your gender — required for the Schlich formula which uses separate male and female equations
- Click Calculate BSA to see all 8 formula results side by side in m², ft², and in²
- Your BMI is also calculated and displayed automatically at the bottom of the results
Try it here: Free BSA Calculator — Calculate Your Body Surface Area
Frequently Asked Questions
| Which BSA formula is most accurate? | No single formula is universally most accurate. The Mosteller formula is the most widely used in clinical practice due to its simplicity. The Du Bois formula is historically the most cited. Haycock is preferred for children. For most adults the differences between formulas are small — typically within 2 to 5 percent of each other. |
| Why do different BSA formulas give different results? | Each formula was developed on a different study population and uses a different mathematical approach. Formulas developed on specific ethnic groups or age groups may perform better for those populations. Our calculator shows all 8 formulas simultaneously so you can compare results and use the one most appropriate for your situation. |
| Is BSA used for chemotherapy dosing? | Yes. Most chemotherapy drugs are dosed in milligrams per square metre of BSA (mg/m²). This approach accounts for differences in body size between patients and helps oncologists deliver the correct therapeutic dose while minimizing toxicity risk. It is one of the most important clinical applications of BSA. |
| Can I calculate BSA for a child using this calculator? | Yes. The calculator works for any age. For children and infants the Haycock formula is generally recommended as it was specifically developed and validated for pediatric patients. Enter the child’s weight and height and the calculator will display all 8 formula results including Haycock. |
| What is the difference between BSA and body surface area percentage in burns? | BSA in square metres is the total surface area of the body used for drug dosing and clinical normalization. BSA percentage in burn assessment refers to the percentage of the body surface that has been burned — calculated using the Rule of Nines or Lund-Browder chart. Both concepts use body surface area but for different purposes. |
Conclusion
Body Surface Area is a straightforward measurement once you understand what it represents — the total external surface of your body in square metres. It is calculated from your height and weight using one of several established formulas, each developed for slightly different populations and clinical contexts.
The most important thing to know is that BSA is primarily a clinical tool. It helps doctors scale drug doses, normalize physiological measurements, and assess conditions like burns more accurately than body weight alone. For most people it is not a routine health metric — but understanding what it is and how it is calculated puts you in a better position to understand your own medical care.
Calculate your BSA instantly using all 8 formulas with our free Body Surface Area Calculator.