BMI (Body Mass Index) is a screening measure that relates body weight to height. BMI = weight (kg) ÷ height (m)². It’s commonly used to classify adult weight status in population and clinical screening. It does not directly measure body fat or body composition.

What do BMI categories mean?

For adults, common categories are: <18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, and ≥30 obese. Cutoffs may vary slightly by guideline and population. Interpret BMI alongside clinical context and other measures (e.g., waist circumference).

Is BMI accurate for athletes or very muscular people?

BMI can overestimate fatness in muscular individuals because it doesn’t separate lean mass from fat mass. A high BMI may reflect high muscle mass rather than excess fat. Consider body composition measures if accuracy matters.

Does BMI apply to children?

Children use BMI-for-age percentiles, not adult BMI categories. Pediatric BMI interpretation depends on age and sex. Use growth charts and local pediatric guidance.

BMI Calculator

Q: Is BMI accurate for athletes or very muscular people?

BMI can overestimate fatness in muscular individuals because it doesn’t separate lean mass from fat mass. A high BMI may reflect high muscle mass rather than excess fat. Consider body composition measures if accuracy matters.

Q: Does BMI apply to children?

Children use BMI-for-age percentiles, not adult BMI categories. Pediatric BMI interpretation depends on age and sex. Use growth charts and local pediatric guidance.

Calculate Body Mass Index (BMI) using metric or imperial units, with step-by-step working and examples.

Reviewed by George Lambroglou, RNLast updated 2025-12-23

Calculator How it works Formula Examples Practice questions FAQs References

Unit system

Tip: commas are allowed (e.g. 1,000).

Weight (kg)

Height (cm)

Educational use only. Always interpret results in clinical context and verify with local policies.

How it works

BMI is calculated by dividing weight (in kilograms) by height (in meters) squared. It’s widely used as a screening tool to classify adult weight status and estimate health risk at a population level.

BMI does not measure body fat directly. It can be misleading for very muscular people, older adults, pregnancy, fluid overload, or conditions that change body composition. Use BMI alongside clinical context and additional measures (e.g., waist circumference).

Formula

BMI (kg/m²) = weight (kg) ÷ height (m)²

Worked examples

Example 1: Weight 70 kg, height 170 cm

Height (m) = 170 ÷ 100 = 1.70

Height² = 1.70² = 2.8900

BMI = 70 ÷ 2.8900 = 24.2 kg/m²

Answer: 24.2 kg/m² (Normal weight)

Example 2: Weight 154 lb, height 67 in

Weight (kg) = 154 × 0.453592 = 69.85

Height (m) = 67 × 0.0254 = 1.70

Height² = 1.70² = 2.8900

BMI = 69.85 ÷ 2.8900 = 24.2 kg/m²

Answer: 24.2 kg/m² (Normal weight)

Practice questions

Try these first, then reveal the answer and working to self-check.

Practice 1: Weight 70 kg, height 170 cm. What is BMI?

Show answer + working

BMI = 24.2 kg/m² (Normal weight)

Height (m) = 170 ÷ 100 = 1.70
Height² = 1.70² = 2.8900
BMI = 70 ÷ 2.8900 = 24.2 kg/m²

Practice 2: Weight 154 lb, height 67 in. What is BMI?

Show answer + working

BMI ≈ 24.2 kg/m² (Normal weight)

Weight (kg) = 154 × 0.453592 = 69.85 kg
Height (m) = 67 × 0.0254 = 1.70 m
Height² = 1.70² = 2.8900
BMI = 69.85 ÷ 2.8900 = 24.2 kg/m²

Practice 3: Weight 90 kg, height 180 cm. What is BMI?

Show answer + working

BMI = 27.8 kg/m² (Overweight)

Height (m) = 180 ÷ 100 = 1.80
Height² = 1.80² = 3.2400
BMI = 90 ÷ 3.2400 = 27.8 kg/m²

Clinical safety note

BMI is a screening tool, not a diagnosis. It can misclassify people with high muscle mass or altered body composition. Use clinical judgement and local guidance.

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Body Composition

Browse body composition calculators.

All Calculators

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FAQs

Clinical reminder: Always follow local protocols and consult medication information sheets. These examples are for calculation practice only.

References & Sources

Authoritative references

BMI classification (global guidance)

Web

World Health Organization (WHO)

BMI definition and commonly used adult cutoffs for weight classification.

About BMI for adults

Web

CDC (US Centers for Disease Control and Prevention)

Adult BMI categories and interpretation guidance, including limitations.

BMI healthy weight calculator

Web

NHS (UK)

Public guidance on BMI and how results are interpreted.

Body mass index (BMI)

Web

Better Health Channel (Victoria)

Patient-friendly explanation of BMI, interpretation, and limitations.

References are provided for education. Always interpret BMI alongside clinical context and local guidance.

How it works

Worked examples

Example 1: Weight 70 kg, height 170 cm

Height (m) = 170 ÷ 100 = 1.70

Height² = 1.70² = 2.8900

BMI = 70 ÷ 2.8900 = 24.2 kg/m²

Answer: 24.2 kg/m² (Normal weight)

Example 2: Weight 154 lb, height 67 in

Weight (kg) = 154 × 0.453592 = 69.85

Height (m) = 67 × 0.0254 = 1.70

Height² = 1.70² = 2.8900

BMI = 69.85 ÷ 2.8900 = 24.2 kg/m²

Answer: 24.2 kg/m² (Normal weight)

Practice questions

Try these first, then reveal the answer and working to self-check.

Practice 1: Weight 70 kg, height 170 cm. What is BMI?

Show answer + working

BMI = 24.2 kg/m² (Normal weight)

Height (m) = 170 ÷ 100 = 1.70
Height² = 1.70² = 2.8900
BMI = 70 ÷ 2.8900 = 24.2 kg/m²

Practice 2: Weight 154 lb, height 67 in. What is BMI?

Show answer + working

BMI ≈ 24.2 kg/m² (Normal weight)

Weight (kg) = 154 × 0.453592 = 69.85 kg
Height (m) = 67 × 0.0254 = 1.70 m
Height² = 1.70² = 2.8900
BMI = 69.85 ÷ 2.8900 = 24.2 kg/m²

Practice 3: Weight 90 kg, height 180 cm. What is BMI?

Show answer + working

BMI = 27.8 kg/m² (Overweight)

Height (m) = 180 ÷ 100 = 1.80
Height² = 1.80² = 3.2400
BMI = 90 ÷ 3.2400 = 27.8 kg/m²

How it works

Formula

Worked examples

Practice questions

Clinical safety note

Related calculators

FAQs

QWhat is BMI?

QWhat do BMI categories mean?

QIs BMI accurate for athletes or very muscular people?

QDoes BMI apply to children?

References & Sources

Authoritative references

How it works

Formula

Worked examples

Practice questions

Clinical safety note

Related calculators

FAQs

QWhat is BMI?

QWhat do BMI categories mean?

QIs BMI accurate for athletes or very muscular people?

QDoes BMI apply to children?

References & Sources

Authoritative references