BMI, or body mass index, is calculated using just two measurements: your height and your weight. The formula divides your weight by the square of your height, producing a single number that places you into a weight category. It takes seconds to calculate, requires no special equipment, and is the most widely used screening tool for weight-related health risk in the world.
The BMI Formula
If you’re using metric units, the calculation is straightforward: divide your weight in kilograms by your height in meters squared. Someone who weighs 70 kg and stands 1.75 m tall would calculate 70 ÷ (1.75 × 1.75), which gives a BMI of 22.9.
If your height is in centimeters rather than meters, the CDC recommends an alternate version: divide your weight in kilograms by your height in centimeters squared, then multiply by 10,000. The result is identical.
For pounds and inches, the formula adds one extra step. Divide your weight in pounds by your height in inches squared, then multiply by 703. That conversion factor (703) bridges the gap between imperial and metric units. So a person weighing 160 pounds at 5 feet 8 inches (68 inches) would calculate 160 ÷ (68 × 68) × 703, landing at a BMI of about 24.3.
Getting Accurate Height and Weight
The formula is only as reliable as the numbers you feed it, and small errors in height especially can shift your result. The CDC outlines a specific protocol: remove shoes, hats, and any bulky clothing. Stand on a hard, flat floor (not carpet) against a flat wall with no molding. Your feet should be flat and together, legs straight, arms at your sides, and shoulders level. Look straight ahead rather than up or down.
For the actual measurement, a flat rigid object like a hardcover book is placed on top of the head at a right angle to the wall. The person reading the measurement should have their eyes level with the book to avoid parallax error. Height should be recorded to the nearest 1/8 inch or 0.1 centimeter.
For weight, use a digital scale on a hard floor. Spring-loaded bathroom scales are less accurate. Stand with both feet centered on the scale and record the weight to the nearest decimal. These details matter more than most people realize. Being off by even an inch on height can change your BMI category.
What the Numbers Mean for Adults
Once you have your BMI, it falls into one of several categories. The World Health Organization defines overweight as a BMI of 25 or above and obesity as 30 or above. The full breakdown most clinicians use looks like this:
- Underweight: below 18.5
- Normal weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Obesity class I: 30 to 34.9
- Obesity class II: 35 to 39.9
- Obesity class III: 40 and above
These thresholds were established based on the points where health risks, particularly for heart disease and type 2 diabetes, begin to rise in large population studies. They are not diagnostic on their own. A BMI of 26 does not mean you are unhealthy, and a BMI of 23 does not guarantee you are. The number is a starting point, not a verdict.
How BMI Works Differently for Children
For anyone between ages 2 and 20, the raw BMI number is not compared against a fixed scale. Instead, it’s plotted on age-and-sex-specific growth charts to determine a percentile. A 10-year-old boy and a 16-year-old girl with the same BMI could fall into entirely different categories because body fat naturally changes throughout childhood and adolescence, and differs between boys and girls.
The categories for children and teens use percentile cutoffs:
- Underweight: below the 5th percentile
- Healthy weight: 5th to less than the 85th percentile
- Overweight: 85th to less than the 95th percentile
- Obesity: 95th percentile or above
- Severe obesity: 120% of the 95th percentile or above, or a BMI of 35 or higher
This percentile system means a child’s BMI is always interpreted relative to other children of the same age and sex. The CDC also publishes extended growth charts for tracking children whose BMI falls above the 97th percentile, where the standard charts lose precision.
Why BMI Can Be Misleading
BMI’s biggest weakness is that it cannot tell the difference between muscle and fat. It measures total body mass relative to height, nothing more. This creates real problems at both ends of the fitness spectrum. Bodybuilders with very low body fat (around 6%) can register a BMI above 30, which would technically classify them as obese. Meanwhile, sedentary older adults who have lost muscle mass over the years may show a “normal” BMI while carrying a disproportionate amount of body fat.
Research in exercise science has consistently confirmed this limitation: BMI overestimates body fat in trained individuals with high muscle mass and underestimates it in untrained or elderly individuals with low lean mass. It also ignores where fat is stored on the body, which matters enormously for health. Fat around the organs in the midsection (visceral fat) carries far greater metabolic risk than fat stored under the skin on the hips or thighs, and BMI has no way to capture that distinction.
Age, sex, and ethnicity also affect what a given BMI number actually means in terms of body fat percentage. At the same BMI, women typically have more body fat than men, older adults have more than younger adults, and significant differences exist across ethnic groups.
Adjusted Thresholds for Asian Populations
The standard BMI cutoffs were developed primarily from data on white European populations and don’t apply equally everywhere. A WHO expert consultation found that Asian populations generally carry a higher percentage of body fat than white populations at the same BMI. This means health risks like type 2 diabetes and cardiovascular disease begin rising at lower BMI values in many Asian groups.
The risk cutoff point varies across Asian populations, ranging from a BMI of 22 to 25 depending on the specific population studied. The consultation proposed additional public health action points at 23, 27.5, 32.5, and 37.5 for Asian populations, giving individual countries a framework to set thresholds appropriate for their people. In practice, many health systems in East and South Asia now use a BMI of 23 rather than 25 as the overweight threshold.
Waist Circumference as a Companion Measure
Because BMI cannot detect where fat is stored, many clinicians now measure waist circumference alongside it. A joint statement from the International Atherosclerosis Society and the International Chair on Cardiometabolic Risk recommended that both measurements become a routine part of clinical visits, calling them a standard “vital sign.” The combination identifies high-risk patients far better than either measure alone.
Waist circumference is measured at the top of the hip bone (iliac crest) or at the midpoint between the lowest rib and the hip bone. The key is consistency: whichever location your provider uses, they should use the same spot each time. A single universal waist threshold turns out to be insufficient, so researchers have proposed BMI-specific cutoffs for white adults. For women, an elevated waist starts at 80 cm in the normal-weight BMI range and rises to 115 cm in the highest obesity categories. For men, it starts at 90 cm for normal weight and rises to 125 cm for the highest categories.
If your BMI puts you in one risk category but your waist circumference is high for that category, your actual metabolic risk is likely higher than BMI alone suggests. This is particularly relevant for people who carry weight around their midsection but may have relatively lean arms and legs.