BMI is calculated by dividing your weight by your height squared. The result is a single number that estimates where you fall on a spectrum from underweight to obese. The math is straightforward, and you can do it yourself with a calculator in about 30 seconds.
The Formula
If you’re using metric units, the equation is:
BMI = weight in kilograms ÷ (height in meters × height in meters)
So a person who weighs 70 kg and stands 1.75 m tall would calculate: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9.
If you’re using pounds and inches, the formula adds a conversion factor:
BMI = (weight in pounds ÷ (height in inches × height in inches)) × 703
For someone who weighs 160 pounds and is 5’8″ (68 inches): (160 ÷ (68 × 68)) × 703 = (160 ÷ 4,624) × 703 = 24.3.
The number 703 simply converts imperial units to match the metric scale. Both formulas produce the same result for the same person.
What the Number Means
The World Health Organization groups BMI into standard categories:
- Below 18.5: underweight
- 18.5 to 24.9: normal weight
- 25.0 to 29.9: overweight
- 30.0 and above: obesity
Large mortality studies have found that the lowest death rates fall within a fairly narrow window. A major study published in the New England Journal of Medicine, tracking hundreds of thousands of U.S. adults, found the lowest all-cause mortality at a BMI of 23.5 to 24.9 in men and 22.0 to 23.4 in women, among healthy people who had never smoked. That puts the “sweet spot” near the upper end of normal weight for men and squarely in the middle for women.
How BMI Works for Children
The formula itself is identical for kids, but the result is interpreted differently. Because children’s body composition changes rapidly as they grow, a raw BMI number doesn’t mean much on its own. Instead, the CDC compares a child’s BMI to other children of the same age and sex using percentile charts, which cover ages 2 through 19.
- Below the 5th percentile: underweight
- 5th to below 85th percentile: healthy weight
- 85th to below 95th percentile: overweight
- 95th percentile or above: obesity
A 10-year-old boy with a BMI of 22 and a 40-year-old man with a BMI of 22 get very different interpretations. The boy would be well above the 85th percentile for his age group. The man would be in the normal range.
Why BMI Doesn’t Tell the Whole Story
BMI was never designed for individual health assessments. It was invented in 1832 by Adolphe Quetelet, a Belgian statistician who wanted to define the “average man” for population-level research. The formula treats all weight the same, whether it comes from muscle, fat, bone, or water. A lean, muscular person and a sedentary person of the same height and weight will get identical BMI scores, even though their health profiles may look completely different.
This limitation matters in practice. BMI correlates well with body fat across large populations, but it loses predictive accuracy when applied to any single individual. It also doesn’t tell you where fat is stored, which turns out to be critical. Fat carried around the organs in your midsection (visceral fat) poses significantly greater cardiovascular and metabolic risk than fat stored in your hips or thighs.
BMI Thresholds Vary by Ethnicity
The standard cutoff points were developed primarily from data on non-Hispanic white populations, and they don’t apply equally to everyone. In 2004, the WHO proposed lower thresholds for many Asian populations, placing the overweight cutoff at 23 rather than 25 and the obesity cutoff at 27.5 rather than 30. This shift reflects the finding that people of South Asian, East Asian, and Southeast Asian descent tend to develop metabolic problems like type 2 diabetes at lower BMI levels.
The American Diabetes Association adopted this evidence in 2015, recommending that screening for prediabetes and diabetes in Asian Americans begin at a BMI of 23 or above. Research has also identified varying thresholds for other populations. One study found that the BMI cutoff equivalent to 30 in white populations corresponded to about 23.9 in South Asian populations, 26.9 in Chinese populations, 26.6 in Arab populations, and 28.1 in Black populations. In short, a single set of cutoffs doesn’t fit everyone.
Better Measures to Use Alongside BMI
In 2023, the American Medical Association adopted a policy acknowledging BMI’s limitations and recommending that it not be used as a sole measure of health. The AMA encouraged physicians to combine BMI with other indicators, including waist circumference, body composition measurements, and metabolic factors like blood sugar and cholesterol.
One of the simplest alternatives is the waist-to-height ratio: divide your waist circumference by your height, both in the same units. A ratio above 0.5 generally signals increased health risk. Systematic reviews comparing the two measures have found that waist-to-height ratio outperforms BMI in predicting cardiovascular events, particularly in people with diabetes. In one randomized controlled trial, waist-to-height ratio was better at predicting major cardiovascular events like heart attacks and strokes. BMI held an edge in only one area: predicting hypertension.
The practical takeaway is that BMI remains a useful screening tool because it’s fast and easy to calculate. But it works best as a starting point, not a final answer. Pairing it with a simple waist measurement gives you a much more complete picture of where you stand.