How is BMI calculated?

BMI is calculated by dividing your weight in kilograms by the square of your height in metres: BMI = kg / m². For imperial units, the formula is: BMI = (lbs / in²) × 703.

BMI is a useful screening tool but has limitations. It does not directly measure body fat, and factors like muscle mass, bone density, age, sex, and ethnicity can affect its accuracy. Athletes may have a high BMI despite low body fat. Always consult a healthcare professional for a full assessment.

What BMI is considered obese?

A BMI of 30.0 or higher is classified as obese. Class I obesity is 30–34.9, Class II is 35–39.9, and Class III (severe or morbid obesity) is 40 or above.

BMI Calculator – Body Mass Index Calculator (Metric & Imperial)

BMI Range	Category
0 – 16	Severely Underweight
16 – 18.5	Underweight
18.5 – 25	Normal weight
25 – 30	Overweight
30 – 35	Obese (Class I)
35 – 40	Obese (Class II)
≥ 40	Obese (Class III)

What is BMI?

Body Mass Index (BMI) is a simple numerical measure calculated from a person's weight and height. It provides a quick snapshot of whether a person is underweight, normal weight, overweight, or obese. Developed in the 19th century by Belgian statistician Adolphe Quetelet, BMI is now the most widely used screening tool for weight-related health issues worldwide.

BMI Formula

The formula differs slightly depending on which unit system you use:

Metric: BMI = weight (kg) ÷ height² (m²)
Imperial: BMI = [weight (lbs) ÷ height² (in²)] × 703

For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of: 70 ÷ (1.75 × 1.75) = 22.9 — Normal weight.

BMI Categories (WHO)

The World Health Organization defines the following BMI classifications for adults:

Below 16.0 — Severely Underweight
16.0 – 18.4 — Underweight
18.5 – 24.9 — Normal weight ✅
25.0 – 29.9 — Overweight
30.0 – 34.9 — Obese (Class I)
35.0 – 39.9 — Obese (Class II)
40.0 and above — Obese (Class III)

Limitations of BMI

BMI is a useful population-level screening metric, but it has well-known limitations for individuals:

Athletes & muscle mass: Highly muscular people may have a high BMI but very low body fat.
Older adults: BMI may underestimate body fat in older adults who have lost muscle mass.
Ethnicity: Some studies suggest that Asian populations may face health risks at lower BMI thresholds.
Children: For people under 18, BMI is age- and sex-adjusted (BMI-for-age percentile), not the same scale as adults.

Always use BMI alongside other health indicators — waist circumference, body fat percentage, blood pressure, and cholesterol — for a complete picture.

Frequently Asked Questions

What is a healthy BMI for adults?

For most adults, a BMI between 18.5 and 24.9 is considered healthy. However, "healthy" also depends on your age, sex, ethnicity, and lifestyle factors.

Can BMI be too low?

Yes. A BMI below 18.5 is classified as underweight, which can be associated with nutritional deficiencies, weakened immune function, and bone density loss. Below 16 is considered severely underweight.

How do I calculate BMI for children?

For children and teenagers (ages 2–19), the CDC recommends using BMI-for-age percentile charts, which account for growth and development patterns. A BMI at or above the 95th percentile for age and sex is considered obese for children.

How often should I check my BMI?

Most health professionals suggest checking BMI every 1–2 years as part of a routine health assessment, unless you are actively managing your weight, in which case monthly tracking may be helpful.

⚖️ BMI Calculator