BMI Explained: What the Number Really Means (and Its Limits)
How Body Mass Index is calculated, what the categories mean, and the important limitations to keep in mind before reading too much into one number.
Body Mass Index, or BMI, is the number your doctor's chart, your smartwatch and countless websites use to sort people into weight categories. It is quick, free and surprisingly useful, but only if you understand what it does and does not tell you.
How BMI is calculated
BMI compares your weight to your height with a simple formula:
BMI = weight (kg) ÷ height (m)²
In imperial units: BMI = 703 × weight (lb) ÷ height (in)².
Example: someone 1.75 m tall weighing 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, right in the healthy range.
What the categories mean
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These thresholds are the same for adult men and women. They are drawn from large population studies linking BMI ranges to health outcomes, which is exactly why BMI is useful as a screening tool.
Where BMI falls short
BMI measures weight relative to height, not body composition. It cannot tell muscle from fat, so it can be misleading for some people:
- Athletes and very muscular people may register as "overweight" despite low body fat.
- Older adults who have lost muscle may have a "healthy" BMI while carrying more fat than ideal.
- It ignores fat distribution. Fat around the abdomen carries more health risk than fat elsewhere, something BMI cannot see.
Pair BMI with other measures, waist circumference, waist-to-height ratio, and how you actually feel and function. No single number captures health on its own.
How to use your BMI sensibly
- Treat it as a first check, a flag for a conversation, not a verdict.
- Track the trend over time rather than obsessing over one reading.
- Combine it with lifestyle factors: activity, diet, sleep and energy.
- Talk to a healthcare professional before making big changes, especially if your BMI is near the edges.
Children and teenagers use age- and sex-specific BMI percentiles, not the adult categories above. Do not apply adult thresholds to young people.
Check your BMI in seconds. Enter your height and weight in metric or imperial units and see your category instantly.
Open the BMI Calculator →The bottom line
BMI is a fast, useful starting point for thinking about weight and health, but it is a blunt instrument. Know the formula, understand the categories, and remember its limits. Use it alongside other signals rather than treating one number as the whole story.
Frequently asked questions
How is BMI calculated?
BMI equals your weight in kilograms divided by the square of your height in metres. In imperial units, it is 703 × weight in pounds ÷ height in inches squared.
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is classed as a healthy weight. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese.
Is BMI accurate for everyone?
No. BMI does not measure body fat directly, so it can overstate risk for muscular people and understate it for those with low muscle mass. It is a population-level screening tool, not an individual diagnosis.