What is the BMI formula?

BMI = weight (kg) ÷ height² (m²). In imperial: BMI = (weight in lbs × 703) ÷ height² (in²). For example, a person weighing 70 kg at 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.

What is a healthy BMI range?

For adults: Under 18.5 = Underweight. 18.5–24.9 = Normal weight. 25–29.9 = Overweight. 30+ = Obese. These ranges apply to adults 18+. Children use age/sex-specific percentile charts.

Is BMI an accurate measure of health?

BMI is a useful screening tool but has limitations. It doesn't account for muscle mass (athletes often score 'overweight'), bone density, age, or fat distribution. Use it as one data point, not a definitive health verdict.

How do I calculate BMI online for free?

Use PickConverter's free BMI Calculator at pickconverter.com/bmi-calculator. Enter your weight and height in metric or imperial and get your BMI and category instantly.

Home› Blog› How to Calculate BMI

Calculators · 5 min read

How to Calculate BMI — Body Mass Index Explained

BMI is one of the most widely used health metrics in the world — and one of the most misunderstood. Here's the formula, what your number means, and why BMI isn't the whole picture.

The BMI Formula

Metric: BMI = weight (kg) ÷ height² (m²)

Imperial: BMI = (weight in lbs × 703) ÷ height² (inches²)

Example: 70 kg, 1.75 m tall
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9

BMI Categories (Adults 18+)

BMI Range	Category
Below 18.5	Underweight
18.5 – 24.9	Normal / Healthy weight ✓
25.0 – 29.9	Overweight
30.0 – 34.9	Obese (Class I)
35.0 – 39.9	Obese (Class II)
40.0 and above	Extremely Obese (Class III)

Note: For children and teens, BMI is interpreted using age and sex-specific growth charts (percentiles), not these fixed ranges.

The Limitations of BMI

BMI is a useful screening tool but it has well-known limitations:

Muscle vs. fat: Muscle is denser than fat. Athletes and bodybuilders often have a BMI in the "overweight" range despite low body fat.
Fat distribution: BMI doesn't tell you where fat is stored. Belly fat carries more health risk than fat stored in hips or thighs.
Age and sex: Older adults tend to have more body fat at the same BMI as younger adults.
Ethnicity: Research suggests some ethnic groups face higher health risks at lower BMI values.

Bottom line: BMI is a starting point, not a diagnosis. Always consult a healthcare professional for a complete health assessment.

Frequently Asked Questions

Metric: weight (kg) ÷ height² (m²). Imperial: (weight in lbs × 703) ÷ height in inches². Example: 70 kg at 1.75 m = BMI 22.9.

For adults, 18.5–24.9 is considered normal/healthy weight. Below 18.5 = underweight; 25–29.9 = overweight; 30+ = obese.

It's a useful screening tool but has limits. It doesn't measure body fat directly, doesn't account for muscle mass, and doesn't reflect fat distribution. Use it as one indicator among many.

Use PickConverter's free BMI Calculator. Enter weight and height in metric or imperial to get your BMI and category instantly.

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