BSA Calculator

Free BSA Calculator - Body Surface Area For Clinical Dosing

A BSA calculator works out your Body Surface Area — the total area of your skin, measured in square meters — from just your height and weight. It's one of the most widely used numbers in clinical medicine, especially for working out drug doses that scale with body size more accurately than a flat weight-based dose would.

This calculator doesn't stop at giving you one number from one formula. It runs your height and weight through six different, recognized BSA formulas at the same time — Mosteller, Du Bois, Haycock, Gehan-George, Boyd, and Fujimoto — and shows you how closely they agree. You also get an optional dosing calculator, so if you already know a dose per square meter, you can see the total dose in seconds.

Nurses, pharmacy students, caregivers, and patients preparing for a hospital visit all use tools like this to double-check a number or understand where a dose comes from. This page is built to make that process fast, transparent, and easy to follow — not to replace a clinician's judgment.

What Is Body Surface Area (BSA)?

Body Surface Area is exactly what it sounds like: the total surface area of a person's body, usually expressed in square meters (m²). An average adult has a BSA of somewhere around 1.6 to 2.0 m², though this varies a lot with height and weight.

BSA matters in medicine because many physiological processes — blood volume, cardiac output, kidney filtration rate, and metabolic rate — scale more closely with body surface area than with weight alone. Two people can weigh the same but have very different heights, and BSA captures that difference in a way a simple kilogram number can't.

How This BSA Calculator Works

Enter your height and weight in either metric or imperial units. The calculator instantly works out your BSA using whichever formula you've selected, and shows the result in both square meters and square feet.

Underneath the main result, every other recognized formula runs on the same inputs, so you get an at-a-glance comparison chart and table. This is useful because different formulas can produce slightly different numbers, and seeing them side by side — along with the average and the spread between the highest and lowest — gives a much fuller picture than a single output ever could.

The Six BSA Formulas Explained

Each formula below takes height in centimeters and weight in kilograms, and returns BSA in square meters:

• Mosteller (1987) — BSA = √[(height × weight) / 3600]. The simplest formula to compute by hand, and the one most modern hospital systems and drug-dosing calculators default to.
• Du Bois & Du Bois (1916) — BSA = 0.007184 × height^0.725 × weight^0.425. The original formula, still cited as a historical reference standard.
• Haycock (1978) — BSA = 0.024265 × height^0.3964 × weight^0.5378. Developed and validated specifically across infants, children, and adults, so it's often favored in pediatrics.
• Gehan & George (1970) — BSA = 0.0235 × height^0.42246 × weight^0.51456. Built from a larger dataset than Du Bois, sometimes used in oncology protocols.
• Boyd (1935) — an older formula derived from cadaver-based surface measurements, using a weight-dependent exponent.
• Fujimoto (1968) — BSA = 0.008883 × height^0.663 × weight^0.444. Derived specifically from measurements of Japanese adults.

Why Do The Formulas Give Slightly Different Answers?

Each formula was built from a different, limited group of people measured decades apart, using different techniques to physically map body surface area. Because of that, no two formulas are ever perfectly identical — though for most adults of average build, they typically land within about 0.05 to 0.15 m² of each other.

In practice, the Mosteller formula is the most commonly used today simply because it's simple to calculate and validated well against more complex formulas. Du Bois remains a common historical reference point in pharmacology texts. This calculator shows you all six side by side specifically so you're not left guessing which one a particular chart or protocol might be using.

BSA-Based Drug Dosing Explained

Many drugs — most notably chemotherapy agents, but also some antibiotics, immunosuppressants, and pediatric medications — are dosed per square meter of body surface area rather than by weight alone. A prescribing order might read, for example, '75 mg/m²', which means the total dose is calculated by multiplying that number by the patient's BSA.

This calculator includes an optional dose field: enter a dose per m² and it multiplies that by your calculated BSA to show the total dose instantly. This is meant as a quick verification tool, not a substitute for a pharmacist or prescribing clinician's own calculation — always cross-check any real dosing decision against the actual prescribing order and current product labeling.

Why Is BSA Used Instead Of Just Body Weight?

Weight-based dosing (mg per kg) is simpler, but it doesn't scale perfectly with how the body actually processes many drugs. Blood volume, cardiac output, and metabolic rate all correlate more closely with body surface area than with total body mass, especially across a wide range of heights and body types.

BSA-based dosing was originally adopted largely for chemotherapy drugs, where the therapeutic window between an effective dose and a toxic one is narrow. Using BSA rather than raw weight helps normalize doses across patients of very different builds, reducing the risk of significant under- or over-dosing.

BSA In Children Versus Adults

Children have a notably larger BSA relative to their body weight than adults do — a young child's surface-area-to-weight ratio is roughly two to three times that of an adult. This is one reason pediatric drug doses are so often calculated per m² of BSA instead of simply scaling an adult dose down by weight.

The Haycock formula in particular was developed and validated with pediatric and infant measurements in mind, which is why it's frequently the preferred formula in pediatric and neonatal settings. This calculator supports height and weight ranges spanning from small infants through large adults, and flags roughly which age group a given BSA is typical for.

Normal BSA Ranges By Life Stage

As a general reference — actual values vary a great deal by individual height and weight:

• Newborn infant: roughly 0.2 - 0.25 m²
• 1-year-old: roughly 0.4 - 0.5 m²
• 10-year-old child: roughly 1.0 - 1.2 m²
• Average adult: roughly 1.6 - 2.0 m²
• Larger adult: can exceed 2.2 - 2.5 m²

Other Places BSA Shows Up In Medicine

Beyond drug dosing, BSA is used to normalize a number of other clinical measurements so they can be fairly compared between people of different sizes.

• Cardiac index — cardiac output divided by BSA, used to assess heart function relative to body size.
• Kidney function — glomerular filtration rate (GFR) is often reported per 1.73 m² (an average adult BSA) to allow fair comparison across patients.
• Burn assessment — estimating what percentage of total BSA has been affected helps guide fluid resuscitation and treatment planning.
• Nutrition and metabolic calculations — some basal metabolic rate and nutritional formulas use BSA as an input.

How Accurate Is A BSA Calculator?

BSA formulas are estimates, not direct measurements. They were derived from studies that physically measured surface area on a limited number of people and then fit a mathematical formula to those data points. For the vast majority of adults of typical build, these estimates land within a few percent of true measured surface area.

Accuracy tends to decrease at the extremes — for people who are very tall, very short, or at very high or very low body weight, different formulas can diverge more noticeably from each other and from directly measured values. That's exactly why this calculator shows all six formulas together: if your inputs sit at an unusual combination of height and weight, you'll see the formulas spread further apart, which is itself useful information.

Limitations To Keep In Mind

A BSA calculator like this one is a fast reference tool, not a clinical decision on its own. A few things worth remembering:

• It cannot account for edema, amputation, unusual body composition, or other factors that change true surface area independent of height and weight.
• Different institutions and different drug protocols may specify a particular formula — always confirm which one applies before using a BSA value for an actual dosing decision.
• BSA-based dosing calculations for real patients should always be verified against the original prescribing order, current drug labeling, and institutional protocol, not sourced from a general web calculator.
• This tool is intended for reference, learning, and quick double-checking — not as a substitute for pharmacist or physician verification.

Who Uses A BSA Calculator?

Nursing and pharmacy students use BSA calculators constantly while learning dosing calculations, since working through the same problem across multiple formulas is one of the fastest ways to understand where the numbers come from. Practicing clinicians use them as a quick sanity check alongside an electronic health record's built-in calculation. Patients and caregivers sometimes use a tool like this simply to understand a number that's already appeared on a treatment plan or lab report.

BSA vs BMI vs Body Weight: What's The Difference?

It's easy to mix up BSA with other common health metrics, but each one measures something different and gets used for a different purpose. Body weight alone is the simplest measure, but it doesn't account for height at all — two people of very different heights can weigh the same and have very different bodies.

BMI (Body Mass Index) combines height and weight into a single number used mainly to screen for whether someone falls into an underweight, healthy, overweight, or obese category. It's a useful general health screening tool, but it isn't used for drug dosing because it doesn't correlate as directly with blood volume or metabolic processes as BSA does.

BSA also combines height and weight, but through a different set of formulas designed specifically to estimate total skin surface area. That specific focus is what makes it the preferred measurement for calculating chemotherapy doses, cardiac index, kidney filtration rates, and burn severity — situations where how the body physically processes a substance or condition matters more than a general weight category.

Advanced Features In This Calculator

This BSA calculator goes further than a single-formula tool in several ways, aimed at students, clinicians, and anyone who wants to double-check a number rather than take it at face value.

• Six-formula comparison — Mosteller, Du Bois, Haycock, Gehan-George, Boyd, and Fujimoto all calculate at once from the same height and weight, with the average and the spread between the highest and lowest shown automatically.
• Metric and imperial support — switch instantly between centimeters/kilograms and feet-inches/pounds without re-entering your data.
• Built-in dosing calculator — enter any dose per m² (such as a chemotherapy protocol's mg/m² figure) and see the total dose calculated instantly against your chosen formula's BSA.
• Full step-by-step working — every calculation is broken down into individual steps with plain-language explanations, useful for coursework, exam prep, or simply understanding where a number came from.
• Bar chart and data table views — compare all six formulas visually or in a sortable reference table, and download the full comparison as a CSV file for offline reference or record-keeping.
• Common reference dose table — a quick-reference table of typical BSA-based dosing ranges for several well-known chemotherapy drugs, for educational context.

How To Use Your Result

Start by picking the formula your reference material, professor, or clinical protocol specifies — Mosteller is the most common modern default, but always check the source if it matters for an actual decision. Compare your chosen formula's result against the average of all six to see how much it might differ from an alternative formula.

If you're working through a dosing problem, use the optional dose-per-m² field to see the full calculation worked out step by step, then always verify against the original order and current labeling before applying it to a real situation.