BMI Calculator

Calculate your Body Mass Index with metric or imperial units

What is BMI?

Body Mass Index (BMI) is a simple measure that uses your height and weight to estimate whether you are underweight, normal weight, overweight, or obese. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it remains one of the most widely used screening tools for weight-related health risks around the world.

How is BMI Calculated?

BMI is calculated by dividing your weight in kilograms by the square of your height in meters: BMI = weight (kg) / height (m)². For imperial units, the formula is: BMI = weight (lb) / height (in)² × 703.

BMI Categories

The World Health Organization (WHO) defines the following BMI categories:

Underweight: BMI < 18.5
Normal weight: BMI 18.5 – 24.9
Overweight: BMI 25 – 29.9
Obese: BMI ≥ 30

Common Mistakes

✗Using pounds and inches directly without converting
✗Ignoring that BMI doesn’t account for muscle mass
✗Comparing BMI across different age groups without context

Pro Tips

✓BMI is a screening tool, not a diagnostic — consult a doctor for health assessments
✓Athletes may have a high BMI due to muscle mass, not excess fat
✓Waist-to-height ratio can be a complementary measurement

Real-World Examples

Health screening

BMI 24.9 is the upper limit of the ‘normal’ range

Athlete assessment

A muscular athlete may have BMI 27 but very low body fat

Child growth tracking

BMI percentiles are used differently for ages 2-20

Weight (kg)

Height (cm)

BMI = weight (kg) / height (m)². For imperial: BMI = weight (lb) / height (in)² × 703.

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What is BMI?
Limitations of BMI

About BMI Calculator

What is BMI?

Body Mass Index (BMI) is a simple measure that uses your height and weight to estimate whether you are underweight, normal weight, overweight, or obese. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and remains one of the most widely used screening tools for weight-related health risks.

The formula for BMI is weight in kilograms divided by height in meters squared (kg/m²). For imperial units, multiply the result by 703. A BMI between 18.5 and 24.9 is generally considered normal weight. Below 18.5 is underweight, 25-29.9 is overweight, and 30 or above is classified as obese. These categories are used by the World Health Organization (WHO) and healthcare providers worldwide as a quick screening tool.

Limitations of BMI

While BMI is a useful screening tool, it has significant limitations. It does not distinguish between muscle mass and fat mass, so athletes or individuals with high muscle mass may be classified as overweight or obese despite having low body fat. Similarly, BMI may underestimate body fat in older adults who have lost muscle mass.

BMI also does not account for fat distribution, which is an important health indicator. Visceral fat (around the waist) is associated with higher health risks than subcutaneous fat (under the skin). For a more complete assessment, BMI should be used alongside other measurements such as waist circumference, body fat percentage, or waist-to-hip ratio. Always consult a healthcare professional for a comprehensive health evaluation.

This utility is provided for informational purposes only. KnowKit is not responsible for any errors in the output.

BMI Calculator FAQ

Is BMI accurate for everyone?

BMI is a useful screening tool but has limitations. It doesn't distinguish between muscle and fat mass, so athletes may be classified as overweight despite having low body fat. It also doesn't account for age, gender, or body fat distribution. For a complete health assessment, BMI should be used alongside other measurements like waist circumference and body fat percentage.

What's a healthy BMI range?

According to the WHO, a BMI between 18.5 and 24.9 is considered normal weight. However, optimal BMI can vary based on age, gender, ethnicity, and individual health factors. Some research suggests that a slightly higher BMI (23-27) may be optimal for older adults. Consult a healthcare provider for personalized guidance.

Does BMI work for children?

Standard BMI categories apply to adults (20+ years). For children and teens (2-19 years), BMI is calculated the same way but interpreted using age- and sex-specific growth charts from the CDC. A child's BMI percentile shows how their BMI compares to other children of the same age and sex.

How often should I check my BMI?

BMI can fluctuate with weight changes. For general health monitoring, checking once a month is sufficient. If you're actively working on weight management through diet or exercise, weekly checks can help track progress. Remember that daily weight fluctuations of 1-2 pounds are normal and don't necessarily reflect changes in body composition.

What are better alternatives to BMI?

While BMI is convenient, more accurate measures include: waist-to-hip ratio (assesses fat distribution), body fat percentage (via DEXA scan or calipers), waist circumference alone (indicates visceral fat risk), and bioelectrical impedance analysis. For most people, BMI combined with waist measurement provides a good initial health assessment.

How do I calculate BMI for children and teens?

BMI for ages 2-19 uses the same formula but is interpreted with CDC age- and sex-specific percentile charts. A BMI at the 85th-94th percentile is considered overweight, and at or above the 95th percentile is considered obese. Always use a pediatric BMI calculator rather than adult categories.

Does BMI differ by ethnicity?

Yes. Research shows that health risks may appear at lower BMI thresholds for some ethnic groups. For example, Asian populations may face higher metabolic risk at BMIs above 23, and the WHO recommends lower BMI cutoffs for certain Asian populations. Standard WHO categories may not apply universally.

How accurate is BMI for measuring body fat?

BMI correlates with body fat at the population level but can be inaccurate for individuals. It typically overestimates fat in muscular individuals and underestimates fat in older adults who have lost muscle. For individual assessment, body fat percentage measurements are more accurate than BMI alone.