Find your Body Mass Index (BMI) and weight category using either US or metric units.
BMI is a simple screening measure based on height and weight - it doesn't account for muscle mass, bone density, or where fat is distributed on your body.
Body Mass Index is calculated from your height and weight. In US units, the formula is 703 x weight (lb) divided by height (inches) squared. In metric units, it's simply weight (kg) divided by height (meters) squared. Both formulas give the same result - this tool handles the conversion for you either way.
BMI sorts adults into four standard categories: underweight (below 18.5), normal weight (18.5 to 24.9), overweight (25 to 29.9), and obese (30 and above). These ranges come from the World Health Organization and are widely used by healthcare providers as a quick screening tool.
Someone who is 5'9" (69 inches) tall and weighs 160 lb has a BMI of 23.6, which falls in the "normal weight" range of 18.5 to 24.9.
What is BMI and how is it calculated?
BMI (Body Mass Index) is a number calculated from your height and weight that's used as a rough screening tool for weight categories. It's calculated as weight divided by height squared, with a conversion factor applied depending on whether you're using US or metric units.
What do the BMI categories mean?
Under 18.5 is classified as underweight, 18.5 to 24.9 as normal weight, 25 to 29.9 as overweight, and 30 or above as obese. These categories are population-level screening guidelines, not an individual diagnosis.
Is BMI accurate for everyone?
No - BMI doesn't distinguish between muscle and fat, so very muscular people (like athletes) often show a high BMI despite having low body fat. It also doesn't account for age, sex, bone structure, or where fat is distributed, all of which affect health risk.
Does BMI work the same for children?
No - children and teens use age- and sex-specific BMI percentile charts instead of the fixed adult categories used here, since growth patterns change significantly during childhood. This calculator is intended for adults.
Does BMI account for muscle mass?
No. Because muscle is denser than fat, two people at the same height and weight can have very different body compositions. Someone with a lot of muscle may show an "overweight" BMI while actually having low body fat.
What's a healthy BMI range?
For most adults, 18.5 to 24.9 is considered the "normal weight" range by the WHO and CDC. That said, a healthy range can vary by individual, ethnicity, and body composition - it's one data point, not the full picture of health.
Should I use BMI alone to judge my health?
No - most health professionals recommend using BMI alongside other measures like waist circumference, body composition, blood pressure, and other health markers, not as a standalone measure of health or fitness.
Does the formula differ between metric and US units?
The underlying idea is identical (weight relative to height squared) - only the conversion factor changes. The metric formula (kg / m squared) doesn't need an extra multiplier, while the US formula (lb / in squared) uses 703 as a conversion constant so the result lines up with the same BMI scale.