BMI Calculator: What It Is, How to Use It, and What Your Number Means
What Is BMI?
Body Mass Index (BMI) is a simple numerical value calculated from your height and weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet as a quick way to assess whether a person's weight falls within a healthy range. Today, it's used worldwide by doctors, researchers, and health organizations as a screening tool for weight-related health risks.
The formula is straightforward: divide your weight in kilograms by the square of your height in meters. A person weighing 70 kg who stands 1.75 m tall has a BMI of 22.9, which falls in the "normal" range. You can calculate yours instantly with our BMI Calculator.
BMI Categories Explained
The World Health Organization defines four main BMI categories for adults. Underweight is a BMI below 18.5, which may indicate malnutrition, eating disorders, or other health issues. Normal weight ranges from 18.5 to 24.9, which is generally associated with the lowest health risks. Overweight covers 25.0 to 29.9, where the risk of conditions like type 2 diabetes and heart disease begins to increase. Obese is 30.0 and above, further divided into Class I (30-34.9), Class II (35-39.9), and Class III (40+).
What your BMI number actually means
Your BMI is a statistical screening tool, not a diagnosis. A "normal" BMI doesn't automatically mean you're healthy, and a "high" BMI doesn't necessarily mean you're unhealthy. It's one data point among many that healthcare professionals use to assess overall health. Think of it as a starting point for a conversation with your doctor, not a final verdict.
How to Calculate BMI
Metric formula
BMI = weight (kg) ÷ height (m)². For example: 80 kg ÷ (1.80 m × 1.80 m) = 80 ÷ 3.24 = 24.7 BMI.
Imperial formula
BMI = (weight in lbs × 703) ÷ (height in inches)². For example: (176 lbs × 703) ÷ (70.9 in × 70.9 in) = 123,728 ÷ 5,026.8 = 24.6 BMI.
Skip the math and use our free BMI Calculator — just enter your height and weight to get an instant result.
Limitations of BMI
BMI is useful as a quick screening tool for large populations, but it has significant limitations at the individual level. It doesn't distinguish between muscle and fat — a bodybuilder and a sedentary person of the same height and weight will have identical BMIs despite very different body compositions. It also doesn't account for age, sex, ethnicity, or where your body stores fat (abdominal fat carries higher health risks than fat stored elsewhere).
For a more complete picture, consider measuring your body fat percentage alongside your BMI. Waist-to-height ratio is another useful metric that specifically accounts for abdominal fat.
BMI for Children and Teens
BMI is calculated the same way for children, but interpreted differently. Instead of fixed categories, children's BMI is compared to others of the same age and sex using percentile charts. A child at the 85th percentile has a higher BMI than 85% of children their age. The 5th to 85th percentile is considered healthy weight, 85th to 95th is overweight, and above the 95th is obese.
Beyond BMI: Other Health Metrics
If you're tracking your health, BMI is a good starting point but shouldn't be your only metric. Track your daily calorie intake to manage weight effectively. Measure your waist circumference — anything above 102 cm (40 in) for men or 88 cm (35 in) for women indicates higher risk regardless of BMI. And consider regular blood pressure and blood sugar checks, which are better predictors of cardiovascular health than weight alone.