BMI Calculator
Find your Body Mass Index (BMI) based on your height and weight, and see how it compares to standard weight categories.
Your BMI will appear here
Enter your weight and height, then click calculate to see your BMI and category.
CalcClearly Health & Fitness Team
Content reviewed against CDC and WHO guidelines
This page is researched and maintained by CalcClearly's editorial team, and every health claim on it is checked against current CDC and WHO publications. It's meant as general education, not a substitute for advice from a doctor or registered dietitian.
Health Risks by BMI Category
BMI itself isn't a diagnosis, but each category is linked to a different pattern of health risk at the population level. These associations come from CDC and WHO research and describe general trends, not a prediction about any one individual.
- Underweight (below 18.5)
- Linked to a weakened immune system, nutrient deficiencies, osteoporosis, fertility problems, and in more severe cases, higher overall mortality risk. Being underweight isn't automatically unhealthy, but it's worth a conversation with a doctor if it's unintentional or came on suddenly.
- Overweight (25.0–29.9)
- Associated with a moderately increased risk of type 2 diabetes, high blood pressure, and heart disease compared to the healthy weight range, though the size of that risk varies a lot depending on other factors like fitness level, waist circumference, and family history.
- Obesity (30.0 and above)
- Associated with a further increased risk of type 2 diabetes, heart disease and stroke, certain cancers, sleep apnea, osteoarthritis, gallbladder disease, and fatty liver disease. Risk generally rises with each WHO obesity class, and Class III in particular is linked to a meaningfully higher risk of serious complications.
Sources: CDC, About Adult BMI and WHO, Obesity and Overweight.
Why BMI Works Differently for Children
A 10-year-old and a 40-year-old with the same BMI number aren't in the same situation. Kids are still growing, and the amount of body fat that's normal for a given height changes constantly through childhood and puberty, so a fixed cutoff like 25 or 30 doesn't mean the same thing at every age.
That's why the CDC uses BMI-for-age percentile charts for children and teens instead, plotting a child's BMI against thousands of other kids of the same age and sex to see where they fall on the curve, rather than comparing them to a flat adult scale. A percentile in the 85th to 94th range, for instance, is considered the overweight range for a child, which is a completely different calculation than the adult 25.0-29.9 range shown here.
This calculator uses the fixed adult categories and is built for people 20 and older. For ages 2-19, use the Kids BMI Calculator, which uses CDC growth chart percentiles instead.
Quick Answer
BMI is calculated by dividing your weight by the square of your height (kg/m², or lb/in² × 703). A BMI between 18.5 and 24.9 is considered a healthy weight for most adults.
How It Works: Formula & Variables
BMI = weight (kg) / height (m)²
BMI = weight (lb) / height (in)² × 703
- Underweight
- BMI below 18.5
- Healthy weight
- BMI 18.5 – 24.9
- Overweight
- BMI 25.0 – 29.9
- Obesity Class I
- BMI 30.0 – 34.9
- Obesity Class II
- BMI 35.0 – 39.9
- Obesity Class III
- BMI 40.0 and above
The World Health Organization splits obesity into these three classes since health risk tends to climb with each one. Source: WHO, Obesity and Overweight.
Worked Examples
Example 1: Imperial units
A person weighing 160 lb at 5'9" (69 inches): BMI = (160 / 69²) × 703 = 23.6, which falls in the healthy weight range.
Example 2: Metric units
A person weighing 73 kg at 175 cm (1.75 m): BMI = 73 / 1.75² = 23.8, also in the healthy weight range.
Key Concepts
BMI categories: BMI groups people into underweight, healthy weight, overweight, and obesity categories based on weight relative to height.
A screening tool, not a diagnosis: BMI is useful for spotting general trends across populations, but it doesn't directly measure body fat or account for muscle mass.
Limitations: Athletes, older adults, and people with different body compositions may have a BMI that doesn't reflect their actual health status.
Common Mistakes
Treating BMI as a complete health picture: BMI doesn't account for factors like waist circumference, muscle mass, or overall fitness.
Mixing up units: Make sure to enter height and weight consistently in either imperial or metric units.
Applying adult categories to children: BMI for children and teens should be assessed using age- and sex-specific growth charts, not the adult categories shown here.
Frequently Asked Questions
BMI (Body Mass Index) is a measure that uses your height and weight to estimate whether your weight is in a healthy range for your height. It's calculated by dividing your weight by the square of your height.
For most adults, a BMI between 18.5 and 24.9 is considered a healthy weight. Below 18.5 is considered underweight, 25.0–29.9 is overweight, and 30.0 or above is classified as obesity, which the WHO further splits into Class I, II, and III.
BMI is a useful general screening tool, but it doesn't account for muscle mass, bone density, body composition, or distribution of fat. Athletes and very muscular people, for example, may have a high BMI without excess body fat.
For imperial units, BMI = (weight in pounds / (height in inches)²) × 703. For metric units, BMI = weight in kilograms / (height in meters)².
No. Children and teens are still growing, so a fixed cutoff like 25 or 30 doesn't mean the same thing at age 10 as it does at age 40. The CDC instead plots a child's BMI against age- and sex-specific percentile growth charts. This calculator uses the fixed adult categories and is intended for adults 20 and older; use the Kids BMI Calculator for ages 2-19.
The World Health Organization splits obesity (BMI 30 and above) into three classes: Class I (30.0–34.9), Class II (35.0–39.9), and Class III (40.0 and above, sometimes called severe or morbid obesity). Health risks generally increase with each class.
BMI Prime is your BMI divided by the upper healthy-weight cutoff (25), so a value of 1.0 sits exactly at the healthy/overweight line, values below 1.0 are in the healthy range, and values above 1.0 indicate overweight or obesity. The Ponderal Index is a similar height-to-weight ratio that uses height cubed instead of squared, which makes it less sensitive to height than standard BMI, especially useful for very tall or very short people.
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