BSA Calculator

Giving the same milligram-per-kilogram dose to a 50 kg patient and a 110 kg patient can lead to toxicity or under-treatment. Body surface area (BSA) solves this by scaling drug doses to metabolic mass rather than total weight. A BSA calculator converts height and weight into a single figure in square metres, supporting precise chemotherapy dosing, cardiac index measurement, and burn assessment.

This tool is for educational purposes and does not replace clinical judgment or pharmacist verification of chemotherapy protocols.

What Is Body Surface Area?

Body surface area is the total area of the human skin, expressed in m². Average BSA is roughly 1.9 m² for adult men and 1.6 m² for adult women, but individual values depend on frame size. Unlike body-mass index, which describes adiposity, BSA reflects metabolic mass and is the standard normalising factor in oncology and critical care.

How the BSA Calculator Works

Parameters

The calculator above accepts metric or imperial measurements and applies several validated equations at once. It returns results from the Du Bois, Mosteller, and Haycock formulas simultaneously, letting you compare the values used by different hospital protocols. Most chemotherapy order sets default to the Mosteller or Du Bois result.

BSA Calculator Formulas

Du Bois formula BSA = 0.007184 × W^0.425 × H^0.725 This 1916 equation remains the reference standard in many oncology departments.

Mosteller formula BSA = √([W × H] / 3600) Because it requires only basic arithmetic, this 1987 formula dominates modern electronic health records.

Haycock formula BSA = 0.024265 × W^0.5378 × H^0.3964 Commonly cited in paediatric dosing and intensive-care literature.

Alternative equations by Gehan & George and Boyd introduce logarithmic terms for patients at extreme weights, though they are encountered less frequently in routine practice.

Which BSA Formula Is Most Accurate?

No single equation is universally superior. For adults of average build, Du Bois and Mosteller differ by only 2–3%. The Haycock formula may perform marginally better in neonates and in patients with extreme obesity or severe malnutrition. Because consistency matters more than absolute precision in chemotherapy, each oncology unit selects one formula and documents it in the patient record to avoid dosing discrepancies.

Clinical Uses of BSA

Chemotherapy dosing Agents such as cisplatin, doxorubicin, and paclitaxel are prescribed in mg/m². Using BSA reduces the risk of overdosing a slightly built patient or under-dosing a larger one.

Cardiac index Dividing cardiac output by BSA yields the cardiac index, normally 2.5–4.0 L/(min·m²). This lets intensivists judge perfusion independently of body size.

Burn assessment The Rule of Nines and Lund-Browder charts express burned area as a percentage of total BSA, guiding fluid resuscitation in the first 24 hours.

Glomerular filtration rate Some GFR estimates are indexed to 1.73 m². Knowing a patient’s actual BSA allows conversion between indexed and non-indexed renal function values.

Example Calculation

For a patient who is 175 cm tall and weighs 70 kg:

  • Mosteller: √(70 × 175 / 3600) = 1.84 m²
  • Du Bois: 0.007184 × 70^0.425 × 175^0.725 ≈ 1.82 m²
  • Haycock: 0.024265 × 70^0.5378 × 175^0.3964 ≈ 1.85 m²

The three results cluster within 0.03 m², illustrating the close agreement seen in typical adult outpatients.

When to Recalculate BSA

Recalculate body surface area whenever body weight shifts by more than 5–10%, before each chemotherapy cycle during active treatment, or during prolonged critical illness where fluid and muscle losses are significant. For patients with extreme obesity, some protocols cap the weight used in the formula or apply an adjusted body weight to prevent excessive drug exposure.

Frequently Asked Questions

What is a normal BSA for adults?
Average BSA is about 1.9 m² for adult men and 1.6 m² for adult women, though individual values vary with height and weight. Clinicians calculate personalised BSA rather than rely on averages because body proportions differ significantly.
How does BSA differ from BMI?
BMI estimates body fat based on weight and height, while BSA measures total skin surface area. Doctors use BSA to dose medications such as chemotherapy, whereas BMI is used to assess obesity and metabolic risk.
Why is chemotherapy dosed by BSA instead of body weight?
Drug metabolism and distribution correlate more closely with body surface area than with total body mass. Dosing by BSA reduces toxicity risk in smaller patients and prevents under-dosing in larger patients, leading to safer chemotherapy protocols.
Can children use the same BSA formulas?
The Mosteller and Du Bois formulas work for both children and adults when accurate height and weight are available. For infants under 10 kg, the Haycock formula is sometimes preferred because it was developed using data that included very small subjects.
How often should BSA be recalculated?
Recalculate BSA whenever body weight changes by more than 5–10%, during prolonged hospitalisation, or before each chemotherapy cycle if significant weight loss or gain has occurred. Keeping the value current prevents dosing errors.
Which BSA formula gives the highest value?
The Du Bois formula usually yields slightly higher results for adults than Mosteller, but the difference is typically within 2–3%. Most oncology units standardise on one formula for consistency rather than switching between equations.
  1. Body Surface Area Calculator