What is BMI?
BMI (Body Mass Index) is a number calculated from your height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s as a simple screening tool to categorize underweight, healthy weight, overweight, and obese people in populations.
The formula: BMI = weight (kg) ÷ height² (m²)
For imperial units: BMI = (weight in lbs × 703) ÷ height in inches²
Use our free BMI Calculator to calculate your number instantly.
BMI Categories (WHO Standards)
| BMI Range | Category | Health Risk | |---|---|---| | Below 18.5 | Underweight | Moderate risk (nutritional deficiency, osteoporosis) | | 18.5 – 24.9 | Normal weight | Low risk | | 25.0 – 29.9 | Overweight | Increased risk | | 30.0 – 34.9 | Obese Class I | High risk | | 35.0 – 39.9 | Obese Class II | Very high risk | | 40.0 and above | Obese Class III | Extremely high risk |
What BMI Does Not Tell You
BMI is a useful starting point, but it has well-documented limitations:
- It ignores body composition. A 200-pound bodybuilder and a 200-pound sedentary person will have the same BMI, but vastly different health profiles. Muscle is denser than fat.
- It does not account for age. Older adults naturally lose muscle mass. A BMI of 22 at age 70 may indicate more body fat than the same BMI at age 25.
- It does not show fat distribution. Where you carry fat matters more than total fat. Abdominal (visceral) fat is strongly linked to heart disease and diabetes, while hip/thigh fat is less harmful.
- It varies by ethnicity. Asian populations show elevated health risks at lower BMIs. The WHO recommends lower cutoffs for Asian adults: overweight begins at 23, obese at 27.5.
Better Metrics to Use Alongside BMI
Waist Circumference
Waist circumference directly measures abdominal fat. Risk increases at waist measurements above:
- Men: 94 cm (37 inches)
- Women: 80 cm (31.5 inches)
Waist-to-Height Ratio
A simple rule: your waist circumference should be less than half your height. A ratio below 0.5 is associated with lower cardiovascular risk in most populations.
Body Fat Percentage
More direct than BMI. Healthy ranges:
- Men: 6–24% (athletes: 6–13%, fitness: 14–17%, acceptable: 18–24%)
- Women: 16–30% (athletes: 16–20%, fitness: 21–24%, acceptable: 25–30%)
Use our free Body Fat Calculator to estimate yours using the US Navy method.
How to Use BMI Effectively
BMI is best used as a population-level screening tool, not an individual diagnostic. Here is how to use it wisely:
- Calculate your BMI using our free calculator
- Check your waist circumference as a secondary measure
- Consider your context — are you very muscular? Athletic? Post-pregnancy?
- Consult a doctor for a full health picture including blood pressure, cholesterol, and blood sugar
Frequently Asked Questions
What is a healthy BMI for women?
The healthy BMI range is the same for men and women: 18.5 to 24.9. However, women naturally have more body fat than men at the same BMI, so consider body fat percentage for a more complete picture.
What is a healthy BMI for men?
18.5 to 24.9 is the healthy range for men. Athletes and heavily muscular men may have BMIs in the overweight range (25–29) while having very low body fat percentages.
Can BMI be wrong?
BMI is a simple screening tool, not a medical diagnosis. It can misclassify fit, muscular people as overweight and sedentary people with low muscle mass as normal weight. It is most reliable when used alongside other measurements.