How is adult BMI calculated?

Adult BMI is calculated as weight in kilograms divided by height in meters squared.

How is body surface area calculated?

The default displayed body surface area uses the Mosteller equation: the square root of height in centimeters multiplied by weight in kilograms divided by 3600.

Can this calculator be used for children or adolescents?

No. This calculator uses adult BMI categories and is intended for adults 18 years and older. Pediatric and adolescent BMI assessment requires age- and sex-specific percentiles.

Adult Body Mass Index (BMI) and Body Surface Area (BSA) Calculator

Enter adult weight and height for live BMI, BSA, ideal body weight, and adjusted body weight results.

No submit is required. Results update automatically as values are entered.

Calculator

Adult BMI category tables

Adult-only: these categories apply to adults. For children and adolescents, BMI must be interpreted using age- and sex-specific percentile charts.

BMI	CDC adult category
<18.5	Underweight
18.5 to <25	Healthy weight
25 to <30	Overweight
30 to <35	Class 1 obesity
35 to <40	Class 2 obesity
≥40	Class 3 obesity / severe obesity

WHO obesity-related health-risk classification

BMI	WHO nutritional status	WHO obesity-related comorbidity risk
<18.5	Underweight	Low for obesity-related comorbidity, but other clinical risk may be increased.
18.5–24.9	Normal range	Average risk.
25.0–29.9	Pre-obese / overweight	Increased risk.
30.0–34.9	Obesity class I	Moderate risk.
35.0–39.9	Obesity class II	Severe risk.
≥40	Obesity class III	Very severe risk.

Formulas used

BMI

BMI = weight(kg) / height(m)²

Adult BMI category is displayed using standard adult cut points.

BSA: Mosteller

BSA = √[(height(cm) × weight(kg)) / 3600]

Displayed as the primary BSA result.

BSA: Du Bois

BSA = 0.007184 × height(cm)^0.725 × weight(kg)^0.425

Shown for comparison when a protocol specifies Du Bois.

BSA: Haycock

BSA = 0.024265 × height(cm)^0.3964 × weight(kg)^0.5378

Shown for comparison when a protocol specifies Haycock.

Ideal body weight: Devine

Female: 45.5 kg + 2.3 kg per inch over 5 ft
Male: 50 kg + 2.3 kg per inch over 5 ft

Sex entry is used only for IBW and adjusted body weight.

Adjusted body weight

AdjBW = IBW + 0.4 × (actual weight − IBW)

Displayed only as a support value. Confirm medication- or protocol-specific requirements before use.

Clinical summary

Copy/paste summary for documentation.

Adult BMI and BSA summary will appear after calculation.

Use notes

This calculator is for adults 18 years and older. It does not calculate pediatric or adolescent BMI percentiles.

BMI is a screening measure and does not directly measure body composition or metabolic risk. BSA formulas are estimates and may differ slightly by formula, especially at body-size extremes.

Adjusted body weight is displayed for protocols where it is clinically appropriate; confirm medication-specific or institutional formulas before use.

References

Centers for Disease Control and Prevention. Adult BMI Categories. CDC BMI categories.
World Health Organization. Obesity: Preventing and Managing the Global Epidemic. WHO Technical Report Series 894. Geneva: WHO; 2000.
Mosteller RD. Simplified calculation of body-surface area. N Engl J Med. 1987;317:1098.
Du Bois D, Du Bois EF. A formula to estimate the approximate surface area if height and weight be known. Arch Intern Med. 1916;17:863–871.
Haycock GB, Schwartz GJ, Wisotsky DH. Geometric method for measuring body surface area: a height-weight formula validated in infants, children, and adults. J Pediatr. 1978;93:62–66.
Devine BJ. Gentamicin therapy. Drug Intell Clin Pharm. 1974;8:650–655.

Disclaimer: All calculations should be confirmed before clinical use. This adult BMI and BSA calculator is for education and documentation support and does not replace clinical judgment.