What is BMI and how is it calculated?

BMI is weight divided by height squared. Metric: BMI = kg / m^2. US: BMI = lb x 703 / in^2.

What are the BMI categories?

BMI categories for adults: Underweight ( =40). These 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?

Du Bois: BSA = 0.007184 x weight(kg)^0.425 x height(cm)^0.725.

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 doesn't account for fat distribution, bone density, or ethnicity. Use it as a screening tool, not a diagnostic measure.

What is a healthy BMI range?

For most adults, BMI categories define underweight, normal, overweight, and obese ranges. Use BMI as a screening tool, not a diagnosis.

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

Weight *

Normal range: 4-660 lbs

Height *

Normal range: 20-98 in

BSA Formula

Different formulas may give slightly different BSA values

Target BMI (Optional)

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)

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

The most common formula used worldwide

Imperial Units Formula

\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

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

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

BSA Calculation Formulas

Three commonly used formulas for Body Surface Area calculation

\text{Mosteller Formula}

Most Common

\text{BSA (m}^2\text{)} = \sqrt{\frac{\text{Height (cm)} \times \text{Weight (kg)}}{3600}}

Recommended for general use, simple and accurate

\text{Du Bois Formula}

Most Accurate

\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

\text{Haycock Formula}

Pediatric Use

\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

\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 = $\frac{250 \times 703}{81^2} = 26.8$ — 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.

Frequently Asked Questions

BMI (Body Mass Index) measures body fat based on height and weight. Formula: BMI = weight(kg) / height(m)². For US units: BMI = weight(lb) × 703 / height(in)². A BMI of 18.5-24.9 is considered healthy for most adults.

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