Body mass index, or BMI, is calculated by dividing your weight by the square of your height. The result is a single number that places you into a weight category, from underweight to severe obesity. The math is straightforward, and you can do it by hand in under a minute.
The BMI Formula
The formula changes slightly depending on whether you’re using metric or imperial units, but the underlying math is the same.
Metric (kilograms and meters): Divide your weight in kilograms by your height in meters squared. If you’re 75 kg and 1.75 m tall, that’s 75 ÷ (1.75 × 1.75) = 24.5. If you only know your height in centimeters, divide your weight by your height in centimeters, divide by your height in centimeters again, then multiply by 10,000.
Imperial (pounds and inches): Divide your weight in pounds by your height in inches squared, then multiply by 703. So if you weigh 170 pounds and stand 5’10” (70 inches), that’s 170 ÷ (70 × 70) × 703 = 24.4.
The multiplication by 703 is just a conversion factor that bridges the gap between pounds/inches and the metric-based scale. Both formulas produce the same BMI value for the same person.
What Your BMI Number Means
For adults 20 and older, BMI falls into these categories:
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Class 1 obesity: 30 to 34.9
- Class 2 obesity: 35 to 39.9
- Class 3 (severe) obesity: 40 or higher
These thresholds haven’t changed in the most recent updates. What did change, as of October 2024, is how doctors code obesity in medical records. New diagnosis codes now distinguish between Class 1, 2, and 3 obesity more precisely, which means your medical chart may reflect a more specific classification than it used to. The BMI ranges themselves remain the same.
How BMI Works Differently for Kids
The formula for children and teens is identical, but the interpretation is completely different. Because kids are still growing, a raw BMI number doesn’t mean much on its own. Instead, it gets plotted on a growth chart that compares the child’s BMI to other children of the same age and sex. The result is a percentile rather than a fixed category.
A child at the 60th percentile, for example, has a BMI higher than 60% of kids the same age and sex. The CDC categories for children and teens break down like this:
- Underweight: below the 5th percentile
- Healthy weight: 5th to 84th percentile
- Overweight: 85th to 94th percentile
- Obesity: 95th percentile or above
- Severe obesity: 120% of the 95th percentile or a BMI of 35 or higher
This percentile system matters because a BMI of 23 could be perfectly healthy for a 16-year-old boy but overweight for an 8-year-old. The adult cutoffs (18.5, 25, 30) only apply once someone turns 20.
A Worked Example
Say you weigh 185 pounds and you’re 5 feet 8 inches tall. First, convert your height entirely to inches: 5 × 12 + 8 = 68 inches. Then square it: 68 × 68 = 4,624. Divide your weight by that number: 185 ÷ 4,624 = 0.04001. Finally, multiply by 703: 0.04001 × 703 = 28.1. That places you in the overweight category, just under the obesity threshold of 30.
In metric, the same person weighs about 83.9 kg and stands 1.73 m tall. The calculation is 83.9 ÷ (1.73 × 1.73) = 83.9 ÷ 2.993 = 28.0. Same result.
What BMI Doesn’t Tell You
BMI is a screening tool, not a diagnosis. It uses only two inputs, weight and height, so it can’t distinguish between muscle and fat, and it says nothing about where your body stores fat. Someone with a muscular build can land in the “overweight” range while carrying very little body fat. Conversely, someone with a normal BMI can still carry excess fat around internal organs.
That second scenario is more common than most people realize. Research from the University of Pittsburgh found that waist-to-height ratio outperforms BMI at predicting heart disease risk, and that much of this predictive advantage is concentrated among people with a BMI under 30. In other words, people who look fine by BMI standards may still carry dangerous visceral fat around their midsection. A waist-to-height ratio above 0.5 (your waist measurement is more than half your height) signals elevated cardiovascular risk even when weight, cholesterol, and blood pressure all appear normal.
To calculate it, measure your waist at the narrowest point (usually just above the navel) and divide by your height, both in the same unit. If your waist is 36 inches and you’re 70 inches tall, your ratio is 0.51, which puts you just above the threshold worth paying attention to.
BMI remains useful as a quick, population-level screening tool. Pairing it with a simple waist measurement gives you a much clearer picture of where you stand.