To calculate BMI, divide your weight in kilograms by your height in meters squared. If you use pounds and inches, divide your weight by your height in inches squared, then multiply by 703. The result places you in a weight category ranging from underweight (below 18.5) to obesity (30 or higher).
The Formula in Metric and Imperial Units
BMI uses the same core math regardless of which units you start with. You’re comparing your weight to your height squared, which accounts for the fact that taller people naturally weigh more.
Metric: BMI = weight (kg) ÷ height (m) × height (m)
If you know your height in centimeters rather than meters, divide your weight in kilograms by your height in centimeters, divide by your height in centimeters again, then multiply by 10,000.
Imperial: BMI = weight (lb) ÷ height (in) × height (in) × 703
The 703 is a conversion factor that bridges pounds and inches to the metric-based scale. Without it, the number would be meaninglessly small.
A Quick Worked Example
Say you weigh 170 pounds and stand 5 feet 8 inches tall. First, convert your height entirely to inches: 5 × 12 + 8 = 68 inches. Square that: 68 × 68 = 4,624. Divide your weight by that number: 170 ÷ 4,624 = 0.0368. Multiply by 703: 0.0368 × 703 = 25.8. Your BMI is 25.8, which falls just inside the overweight range.
In metric, a person who weighs 72 kg and is 1.75 m tall would calculate it as: 1.75 × 1.75 = 3.0625, then 72 ÷ 3.0625 = 23.5. That falls in the healthy weight range.
What the Categories Mean
The CDC defines these BMI ranges for adults 20 and older:
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25.0 to 29.9
- Class 1 obesity: 30.0 to 34.9
- Class 2 obesity: 35.0 to 39.9
- Class 3 (severe) obesity: 40.0 or higher
These thresholds aren’t arbitrary. A large study tracking cardiovascular outcomes found that middle-aged adults in the overweight range had a 21% to 32% higher lifetime risk of cardiovascular disease compared to those at a healthy weight. For people with a BMI of 30 to 39.9, that risk jumped to 67% to 85% higher. At a BMI of 40 or above, men faced roughly triple the risk. Higher BMI also correlated with developing heart disease earlier in life, meaning more total years spent managing it.
BMI Works Differently for Children
The formula itself is identical for kids, but the result is interpreted differently. Because children’s body composition shifts dramatically as they grow, a raw BMI number doesn’t mean the same thing for a 7-year-old as it does for an adult. Instead, a child’s BMI is plotted on age-and-sex-specific growth charts to produce a percentile. A 10-year-old boy with a BMI of 21 might be at the 90th percentile (overweight for his age), while the same number for a 17-year-old boy could be perfectly average. If you’re calculating BMI for a child, you’ll need the CDC’s growth charts or an online calculator that factors in age and sex.
Where BMI Falls Short
BMI cannot tell the difference between muscle and fat. A bodybuilder with 6% body fat can easily register a BMI over 30, placing them in the “obese” category on paper. The formula also underestimates body fat in people with low muscle mass, which is common in older adults and people with chronic illness. It tells you nothing about where your fat is stored, and fat concentrated around the abdomen carries significantly more metabolic risk than fat stored in the hips or thighs.
Waist-to-hip ratio captures that abdominal fat distribution much better than BMI does. Measuring your waist at its narrowest point and your hips at their widest, then dividing waist by hip, gives a more targeted picture of the fat that actually drives health risk. Using both measurements together is more informative than relying on either one alone.
Adjusted Thresholds for Asian Populations
People of Asian descent tend to carry more visceral fat (the metabolically dangerous kind around the organs) at lower BMI levels. A WHO expert panel reviewed data from Asian countries and recommended lower cutoff points: a BMI of 23 to 27.5 for overweight and 27.5 or higher for obesity, compared to the standard 25 and 30. If you’re of Asian descent, a BMI of 24 may already signal elevated health risk even though it falls in the “healthy” range on the standard scale.
BMI Ranges Shift for Older Adults
The standard categories were built around data from younger and middle-aged adults. For people over 65, the picture looks different. Research on older adults found that those with a BMI below 25 and those above 35 both showed greater risks of declining physical function, balance problems, falls, and loss of muscle strength. A BMI between 25 and 35 appeared optimal for older adults, with the best outcomes clustering around 27 to 28 for men and 31 to 32 for women. This “obesity paradox,” where carrying some extra weight seems protective in later life, likely reflects the fact that older adults with low BMI often have less muscle reserve to draw on during illness or injury.
Pairing BMI With Other Measurements
BMI is a useful starting point because it requires nothing more than a scale and a tape measure. But it’s one data point, not a diagnosis. A waist circumference over 40 inches in men or 35 inches in women signals elevated risk from abdominal fat regardless of what BMI says. Blood pressure, blood sugar, and cholesterol levels fill in the metabolic picture that BMI can only hint at. Think of BMI as the first filter: if the number looks concerning, it’s worth investigating further with measurements that can distinguish between types and locations of body fat.