Body Surface Area Calculator

Body Surface Area Guide

Why BSA Is Used Instead of Weight?

Many physiological processes scale with BSA rather than body weight — including cardiac output, glomerular filtration rate, and metabolic rate. In drug dosing, particularly for chemotherapy and immunosuppressants, BSA-based dosing is used to account for the fact that drug distribution, metabolism, and clearance depend on body surface rather than mass alone. A 50kg and 100kg person have very different surface areas, which affects how drugs should be dosed for optimal effect without toxicity.

What should I know about Formula Comparison?

Mosteller (1987): BSA = √(height cm × weight kg / 3600). Simplest and widely used in clinical practice. DuBois & DuBois (1916): BSA = 0.007184 × height^0.725 × weight^0.425. The original BSA formula, derived from nine subjects. Haycock (1978): BSA = 0.024265 × height^0.3964 × weight^0.5378. Most accurate for children and neonates. Boyd: uses weight only, suitable when height is unknown. For most clinical purposes, the differences between formulas are small (under 5%) and do not affect treatment for the vast majority of patients.

What do I need to know about Average BSA Values?

Average adult male: approximately 1.9 m². Average adult female: approximately 1.6 m². Average 10-year-old: approximately 1.14 m². Average newborn: approximately 0.25 m². The original BSA-based drug dosing assumed a standard 1.73 m² reference value — this is still used as the normalisation factor in eGFR (estimated glomerular filtration rate) calculations on kidney function blood tests.

What's the key thing to understand about Burn Surface Area Estimation?

In burn injury assessment, the Rule of Nines estimates which percentage of BSA is affected: head and neck 9%, each arm 9%, each leg 18% (front 9%, back 9%), anterior trunk 18%, posterior trunk 18%, perineum 1%. For children, the Lund and Browder chart adjusts these percentages because children have proportionally larger heads. Burns covering more than 15% BSA in adults (10% in children) are considered major burns requiring specialist burns unit care.

What's the key thing to understand about Why BSA Matters in Medicine?

Body surface area is used clinically far more than most people realise, mainly because it predicts physiological processes (like blood volume, cardiac output, and kidney filtration rate) more accurately than body weight alone, especially in people whose weight doesn't reflect their overall body size proportionally. Its best-known use is dosing chemotherapy drugs: most chemotherapy regimens are prescribed in milligrams per square metre of BSA rather than per kilogram of body weight, because it gives a dose that scales more safely and predictably with a person's actual metabolic capacity and organ size across a wide range of body types. BSA is also used to calculate the cardiac index (cardiac output divided by BSA) in assessing heart function, to normalise kidney function tests like the glomerular filtration rate (GFR) for comparison between people of different sizes, and in burns medicine to estimate what percentage of total skin surface has been affected and how much intravenous fluid replacement is needed, since fluid loss through burned skin scales with the area affected. Because small errors in BSA can translate into meaningful dosing errors for drugs with a narrow safety margin, clinical BSA calculations are typically cross-checked and rounded conservatively, unlike a general-purpose estimate like this calculator.

What do I need to know about Different BSA Formulas?

Several formulas exist to estimate body surface area from height and weight, and they can give slightly different results. The Du Bois formula, published in 1916 and derived from direct surface measurements of just nine people, was the clinical standard for most of the 20th century: BSA = 0.007184 × weight^0.425 × height^0.725 (weight in kg, height in cm). The Mosteller formula, introduced in 1987, is simpler to calculate by hand — BSA = √(height × weight / 3600) — and tends to agree closely with Du Bois for average adult body types while being easier to compute without a calculator, which made it popular in clinical settings before computers were standard. Other formulas (Haycock, Gehan and George, Boyd) were developed with more diverse study populations, including specific formulas optimised for infants and children, whose body proportions differ substantially from adults. Differences between formulas are generally small (within a few percent) for an average-sized adult but can diverge more for people at the extremes of height or weight, which is one reason different hospitals and drug manufacturers sometimes specify which formula to use for a given medication's dosing calculation rather than leaving it to clinician preference.

What should I know about BSA-Based Chemotherapy Dosing?

Many chemotherapy drugs, along with some other medicines with a narrow safety margin, are dosed per square metre of body surface area rather than per kilogram of body weight. This convention traces back to early 20th-century pharmacology research showing that BSA correlated more closely than body weight with metabolic rate and with how various mammals — used in early drug toxicity and dosing studies — processed certain drugs, under the reasoning that BSA scales more closely with blood volume, cardiac output and kidney filtration rate than weight alone does. For chemotherapy specifically, where the gap between an effective dose and a dangerously toxic dose can be narrow, BSA-based dosing became the established convention through decades of clinical trials calculating safe and effective doses in BSA terms, and changing the convention now would require extensive re-validation against that existing body of evidence. It's worth noting that this approach has known limitations — it doesn't fully account for variation in body composition, kidney or liver function, or genetic differences in drug metabolism — which is why oncology dosing also incorporates blood test results and dose adjustments for individual patients, rather than relying on a BSA calculation in isolation.

Worked Example: Comparing BSA Formulas for the Same Person

Take an adult who is 170cm tall and weighs 70kg. Running this through three commonly used formulas shows how closely they typically agree, even though they were derived differently.

Working through Mosteller specifically: (170 × 70) ÷ 3600 = 11,900 ÷ 3600 = 3.306, and the square root of 3.306 is approximately 1.82 m². All three formulas land within about 0.02 m² of each other for this person — a difference of roughly 1%, which is well within the margin that matters clinically. This close agreement is normal for adults of average build; the formulas diverge more noticeably at the extremes of body size, which is one reason Mosteller (the simplest to calculate) remains popular for quick clinical use despite Du Bois historically being the most-cited reference formula.

Worked Example: BSA in a Paediatric Case

BSA-based dosing is especially important in children, since body proportions and metabolic rate differ substantially from adults relative to body weight alone. Take a 6-year-old child weighing 20kg and 115cm tall, using the Mosteller formula: BSA = √((height(cm) × weight(kg)) ÷ 3600) = √((115 × 20) ÷ 3600) = √(2,300 ÷ 3,600) = √0.639 ≈ 0.80 m². Compare this with the adult example earlier (70kg/170cm, BSA ≈ 1.82 m²): the child weighs only 29% as much as the adult (20kg vs 70kg) but has 44% of the adult's BSA (0.80 vs 1.82 m²) — a clear illustration of why BSA and body weight scale differently with size, and why a medication dose calculated purely as 'mg per kg' can under- or over-shoot the appropriate dose for a child compared with a dose calculated as 'mg per m² of BSA', which is why BSA-based dosing is standard for many paediatric chemotherapy and other weight-sensitive drug protocols.

BSA and Fluid Resuscitation in Burns

Beyond chemotherapy dosing, BSA calculations are also central to planning fluid replacement in burns care, closely linked to the burn surface area estimation already covered. The Parkland formula, a widely used burns fluid resuscitation guide, calculates total fluid needed in the first 24 hours as 4mL × body weight (kg) × percentage of body surface area burned, with half given in the first 8 hours. This again illustrates why burns teams need an accurate percentage of total BSA affected, not just a weight-based estimate, since fluid loss through burned skin scales with the proportion of the body's surface area that has lost its normal skin barrier.

Push-Ups as a Minimal-Equipment Fitness Standard

Clinicians also use BSA to normalise other measurements for comparison across people of different sizes — for example, cardiac index (cardiac output divided by BSA) and glomerular filtration rate (a kidney function measure) are both commonly reported per square metre of body surface area rather than as raw totals, precisely because raw totals scale with body size and would otherwise make comparison between a small and a large person misleading.