MathIsimple
Clinical Health · 2026

BSA/BMI Calculator

Calculate Body Mass Index (BMI) and Body Surface Area (BSA) with multiple formulas and unit conversions. Essential for health assessment, medical dosing, and fitness monitoring.

100% FreeMultiple FormulasHealth Insights
Body Mass Index & Body Surface Area Calculator
Enter your measurements to calculate BMI and BSA with health interpretations

Normal range: 4-660 lbs

Normal range: 20-98 in

Different formulas may give slightly different BSA values

Healthy range: 18.5 - 24.9 kg/m²

Example Calculations
Sample BMI and BSA values for different body measurements
70 kg, 175 cm
BMI: 22.9 kg/m²
BSA: 1.85 m² (Mosteller)
150 lbs, 68 in
BMI: 22.8 kg/m²
BSA: 1.74 m² (Mosteller)
80 kg, 180 cm
BMI: 24.7 kg/m²
BSA: 2.00 m² (Mosteller)
130 lbs, 64 in
BMI: 22.3 kg/m²
BSA: 1.56 m² (Mosteller)
BMI Calculation Formula
Body Mass Index is calculated using your weight and height

Standard BMI Formula (Metric)

BMI=Weight (kg)Height (m)2\text{BMI} = \frac{\text{Weight (kg)}}{\text{Height (m)}^2}

The most common formula used worldwide

Imperial Units Formula

BMI=Weight (lbs)×703Height (in)2\text{BMI} = \frac{\text{Weight (lbs)} \times 703}{\text{Height (in)}^2}

Multiply by 703 to convert from imperial to metric units

📊 Example Calculation:

Person: 70 kg weight, 1.75 m tall

BMI=701.752=703.0625=22.9 kg/m2\text{BMI} = \frac{70}{1.75^2} = \frac{70}{3.0625} = 22.9 \text{ kg/m}^2

Result: Normal Weight\text{Normal Weight} (18.5-24.9 range)

BSA Calculation Formulas
Three commonly used formulas for Body Surface Area calculation
Mosteller Formula\text{Mosteller Formula}
Most Common
BSA (m2)=Height (cm)×Weight (kg)3600\text{BSA (m}^2\text{)} = \sqrt{\frac{\text{Height (cm)} \times \text{Weight (kg)}}{3600}}

Recommended for general use, simple and accurate

Du Bois Formula\text{Du Bois Formula}
Most Accurate
BSA (m2)=0.007184×Weight (kg)0.425×Height (cm)0.725\text{BSA (m}^2\text{)} = 0.007184 \times \text{Weight (kg)}^{0.425} \times \text{Height (cm)}^{0.725}

Gold standard, widely used in clinical practice

Haycock Formula\text{Haycock Formula}
Pediatric Use
BSA (m2)=0.024265×Weight (kg)0.5378×Height (cm)0.3964\text{BSA (m}^2\text{)} = 0.024265 \times \text{Weight (kg)}^{0.5378} \times \text{Height (cm)}^{0.3964}

Better accuracy for children and adolescents

🧮 Example (Mosteller):

Person: 175 cm tall, 70 kg weight

BSA=175×703600=122503600=3.403=1.845 m2\text{BSA} = \sqrt{\frac{175 \times 70}{3600}} = \sqrt{\frac{12250}{3600}} = \sqrt{3.403} = 1.845 \text{ m}^2
What is BMI and Why It Matters?

Body Mass Index (BMI) is a measure of body fat based on height and weight. It's a screening tool used to categorize weight status and assess health risks.

BMI Categories:

  • Underweight: BMI < 18.5
  • Normal weight: BMI 18.5-24.9
  • Overweight: BMI 25-29.9
  • Obesity: BMI ≥ 30

Limitations:

  • Doesn't distinguish between muscle and fat
  • May not be accurate for athletes or elderly
  • Should be combined with other health metrics
Understanding BSA (Body Surface Area)

Body Surface Area is the total surface area of the human body. It's crucial for medical calculations, especially drug dosing and metabolic assessments.

BSA Formulas:

  • Mosteller: Most commonly used, simple calculation
  • Du Bois: Most accurate, gold standard
  • Haycock: Better for pediatric patients

Clinical Applications:

  • Chemotherapy drug dosing
  • Burn treatment calculations
  • Cardiac output assessment
  • Renal function evaluation
How to Use BMI/BSA for Health Management

Quick interpretation: BMI gives a broad weight-status screen, while BSA helps clinical dosing and metabolic estimation. When both values are reviewed together, clinicians can separate lifestyle coaching goals from medication and treatment calculations. Use trends over time, not one isolated measurement, and combine results with blood pressure, glucose, lipid markers, and waist circumference for a fuller risk profile.

Medical Applications

  • • BMI for health risk assessment
  • • BSA for precise drug dosing
  • • Combined metrics for comprehensive evaluation
  • • Regular monitoring for health tracking

Fitness Goals

  • • BMI trends for weight management
  • • BSA affects metabolic rate calculations
  • • Combine with body composition analysis
  • • Set realistic target ranges

Combined Metrics

  • • Use with waist-hip ratio (WHR)
  • • Include body fat percentage
  • • Consider muscle mass distribution
  • • Regular health check-ups
BMI: A Flawed Standard That Won't Die — Why Doctors Still Use It

BMI was invented in 1832 by Belgian mathematician Adolphe Quetelet — not as a health tool, but to describe the "average man." He explicitly said it should not be used to judge individuals. Yet here we are, 190+ years later.

❌ Why BMI Fails

  • • LeBron James (6'9", 250 lbs) has BMI = 250×703812=26.8\frac{250 \times 703}{81^2} = 26.8 — technically "overweight"
  • • Doesn't distinguish muscle from fat
  • • Ignores fat distribution (visceral vs subcutaneous)
  • • Was calibrated on white European men only

✅ Why It Still Matters

  • • Free, instant, no equipment needed
  • • Strong population-level correlation with health outcomes
  • • CDC uses it for epidemiological tracking
  • • Insurance companies use BMI for policy pricing

Better alternatives exist: Waist-to-hip ratio (WHR), body fat percentage via DEXA scan, and the newer Body Roundness Index (BRI) may be more accurate health predictors. A 2024 Nature review found BRI outperformed BMI in predicting diabetes risk.

📖 CDC: About Adult BMI

Health disclaimer: this BMI/BSA calculator is for educational screening and is not a substitute for medical advice, diagnosis, or treatment. BMI and BSA do not replace physical exams, lab work, or clinician judgment. If results suggest underweight, obesity, or rapid changes, discuss next steps with a licensed healthcare professional.

The Muscle Lie: Why The Rock is "Obese"

Plug Dwayne "The Rock" Johnson's stats into any BMI calculator — 6'5", 260 lbs — and watch what happens.

The Result

BMI = 30.7

Classification: OBESE

Ridiculous, right? Anyone with eyes can see he's packed with muscle, not fat. But BMI doesn't have eyes. It's just a math formula — one that a Belgian astronomer named Adolphe Quetelet invented in 1832 to study population trends, not individual health.

I've seen friends who lift five days a week get flagged as "overweight" at their annual checkup. The doctor glances at the chart, not the patient. That's the frustrating truth for anyone who's built muscle: the fitter you get, the more BMI says you're unhealthy.

BMI Can't Tell Muscle From Fat

BMI is like judging a suitcase by its weight — you've got no idea what's inside. A 200-pound person could be carrying dense muscle (the encyclopedia) or fluffy visceral fat (the styrofoam). Same weight, same height, wildly different health outcomes.

The Formula (Since 1832)

BMI=Weight (kg)Height (m)2\text{BMI} = \frac{\text{Weight (kg)}}{\text{Height (m)}^2}

Divide your weight in kilograms by your height in meters, squared. That's it — no body composition, no muscle-to-fat ratio, no context.

The formula divides mass by height squared. It doesn't know — and can't know — what that mass is made of. Muscle is about 18% denser than fat, according to research published in the British Journal of Nutrition. Two people with identical BMIs can have completely different body fat percentages, waist-to-hip ratios, and metabolic health profiles.

Why Doctors Still Use a Formula From 1832

If it's so flawed, why hasn't medicine moved on?

It's cheap and fast. A DEXA scan costs $75–$300 and requires a clinic visit. Underwater weighing is even more involved. BMI needs a tape measure and a bathroom scale. For large population studies — tracking obesity trends across millions of people — that simplicity is genuinely useful. For the general population who don't lift weights, BMI correlates "well enough" with health risk, according to CDC guidelines.

The problem isn't that BMI exists. It's that people treat a population-level screening tool as an individual diagnosis. Your doctor should be using it as a starting point, not a verdict.

The Athlete and the 'Skinny Fat' Paradox

The Athlete (BMI Overestimates Risk)

BMI says: Overweight or Obese
Reality: Low body fat, high muscle mass, excellent metabolic health
Better metric: Body fat percentage via calipers, DEXA scan, or our body fat calculator

The "Skinny Fat" Person (BMI Underestimates Risk)

BMI says: Normal / Healthy
Reality: Low muscle, high visceral fat around organs, elevated metabolic risk
The danger: A "normal" BMI with high body fat carries similar cardiovascular risks as clinical obesity. Don't celebrate a number — check your body composition.

BMI isn't the only health metric with a misleading origin story. The "10,000 steps a day" target? That came from a 1964 Japanese marketing campaign, not medical research.

Frequently Asked Questions

Is BMI accurate for muscular people?

No. BMI can't distinguish muscle from fat. Anyone with significant muscle mass — athletes, bodybuilders, manual laborers — will likely score higher than their actual health risk warrants. Body fat percentage or a DEXA scan gives a much clearer picture.

What BMI is considered obese?

A BMI of 30 or above is classified as obese under WHO and CDC standards. 25–29.9 is "overweight," 18.5–24.9 is "normal." But these thresholds were designed for population screening, not individual diagnosis — context matters enormously.

What should I use instead of BMI?

Waist-to-hip ratio is a quick free alternative that better predicts cardiovascular risk. For more precision, body fat percentage (via calipers or DEXA scan) and FFMI (fat-free mass index) account for body composition. No single number tells the whole story — use BMI as one data point, not the only one.

Check Your Score (But Don't Panic)

Use BMI as a rough guide, not a judge. If it says "Overweight" but you can see your abs, ignore it. If it says "Normal" but you can't climb stairs, pay attention.

Frequently Asked Questions

What is BMI and how is it calculated?
BMI (Body Mass Index) measures body fat based on height and weight. Metric: BMI = kg / m^2. US: BMI = lb x 703 / in^2. A BMI of 18.5-24.9 is considered healthy for most adults.
What are the BMI categories?
BMI categories for adults: Underweight (<18.5), Normal weight (18.5-24.9), Overweight (25-29.9), Obese Class I (30-34.9), Obese Class II (35-39.9), Obese Class III (>=40). Ranges may vary for athletes, elderly, and different ethnic groups.
What is BSA and why is it important?
BSA (Body Surface Area) measures the total skin area of a body. It's crucial in medicine for calculating drug dosages (especially chemotherapy), determining cardiac output, and assessing burn injuries. Average adult BSA is about 1.7 m^2.
What is the Du Bois BSA formula?
The Du Bois formula (1916): BSA(m^2) = 0.007184 x weight(kg)^0.425 x height(cm)^0.725. It is one of the most widely used formulas in clinical practice for calculating body surface area.
Is BMI accurate for everyone?
BMI has limitations. It may overestimate body fat in athletes with high muscle mass and underestimate it in older adults who have lost muscle. It does not account for fat distribution, bone density, or ethnicity. Use it as a screening tool, not a diagnostic measure.
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Disclaimer: This calculator is for general educational purposes only and is not medical advice. It does not diagnose, treat, or replace consultation with a qualified healthcare professional. Always discuss your results with your doctor.