BMI and Body Composition
How Do I Calculate BMI? Formula, Chart and Examples
Quick answer
For adults age 20 and older, BMI equals weight in kilograms divided by height in meters squared. In U.S. units, BMI = 703 x weight in pounds / height in inches squared. CDC categories are under 18.5 underweight, 18.5 to under 25 healthy weight, 25 to under 30 overweight, and 30 or higher obesity. BMI is a screening measure, not a diagnosis.
BMI formula in metric units
Metric formula
- BMI = weight (kg) / height (m)^2
Example: if you weigh 75 kg and are 1.75 m tall, BMI = 75 / (1.75 x 1.75) = 24.5.
BMI formula in pounds and inches
U.S. formula
- BMI = 703 x weight (lb) / height (in)^2
Example: if you weigh 165 lb and are 69 inches tall, BMI = 703 x 165 / (69 x 69) = about 24.4.
Adult BMI chart
| BMI | CDC category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to <25 | Healthy weight |
| 25.0 to <30 | Overweight |
| 30.0 to <35 | Class 1 obesity |
| 35.0 to <40 | Class 2 obesity |
| 40.0 or higher | Class 3 obesity (severe obesity) |
These adult categories apply to people age 20 and older. Children and teenagers use BMI-for-age percentiles because growth changes the interpretation.
For a deeper critique of what BMI misses, read Is BMI Accurate?.
What does BMI actually measure?
BMI is a weight-for-height index. It does not directly measure body fat, muscle mass, bone mass or fat distribution. Its strength is simplicity: it can help screen weight-related risk at a population level and can be one data point in an individual assessment. Its weakness is that the same BMI can represent different body compositions.
When can BMI be misleading?
- People with unusually high muscularity may have a higher BMI without the same body-fat level as a less muscular person.
- Older adults can lose muscle while BMI changes little.
- Two people with the same BMI can store fat differently, including different amounts around the abdomen.
- Health risk can vary within the same BMI category because blood pressure, glucose, lipids, fitness, smoking, sleep, family history and other factors matter.
What should you check alongside BMI?
For a fuller picture, consider waist measurement, weight trend, blood pressure, relevant laboratory markers, physical fitness, strength, health history and, when useful, a body-composition estimate. None of these should be interpreted in isolation either.
Go beyond the number with How Much Should I Weigh for My Height? and How Can I Measure Body Fat Percentage at Home?.
How often should you calculate BMI?
BMI changes only when weight or height changes. For adults, height is usually stable, so recalculating every day adds no value. If you are intentionally changing weight, reviewing BMI occasionally can provide context, while your weekly weight trend and health markers may be more actionable.
BMI is a screening tool, not a verdict
CDC explicitly describes adult BMI as a screening measure that should be considered with other factors when assessing health. Use the category to ask better questions, not to label your health by one number. If your BMI is far outside the reference range or you have health concerns, a clinician can interpret it with your history and other measurements.
Frequently Asked Questions
References
- CDC: Adult BMI Categories. - Official adult category ranges and screening caveat.
- CDC: Adult BMI Calculator. - Adult BMI calculation context.
- CDC: Child and Teen BMI Categories. - Why people under 20 use age- and sex-specific percentiles.