To measure your BMI, divide your weight by your height squared. The result places you in a category from underweight to obese, giving you a rough snapshot of whether your weight falls in a typical range for your height. You can do this with a calculator in under a minute.
The BMI Formula
BMI stands for body mass index. The math is straightforward, but the formula differs slightly depending on whether you’re using metric or imperial units.
If you measure in kilograms and meters: divide your weight in kilograms by your height in meters squared. So a person who weighs 70 kg and stands 1.75 m tall would calculate 70 ÷ (1.75 × 1.75) = 22.9.
If you measure in pounds and inches: divide your weight in pounds by your height in inches squared, then multiply by 703. A person who weighs 160 pounds and stands 5’8″ (68 inches) would calculate 160 ÷ (68 × 68) × 703 = 24.3.
If you’d rather skip the math, the CDC and most health organization websites offer free BMI calculators where you plug in your height and weight and get your number instantly.
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 were set based on population-level data linking BMI ranges to health risks like heart disease, type 2 diabetes, and high blood pressure. They’re useful as a screening tool, but they don’t tell the full story for every individual.
Getting an Accurate Measurement
Small details in how you weigh and measure yourself can shift your BMI by a point or more, so consistency matters. Weigh yourself in light indoor clothing, without shoes. Use the bathroom first if possible. Clinical guidelines recommend weighing at the same time of day each time, ideally in the morning, since your weight can fluctuate by several pounds over the course of a day due to food, water, and activity.
For height, stand against a wall with your heels, back, and head touching it. Place a flat object like a book on top of your head parallel to the floor, mark the wall, and measure from the floor to the mark. If you haven’t been measured recently, it’s worth checking. Adults can lose height gradually with age, which would change the BMI calculation.
If you’re tracking BMI over time, use the same scale in the same spot on a hard, flat surface. Scales on carpet or uneven flooring give inconsistent readings.
BMI Works Differently for Children
The formula is identical for kids, but the result is interpreted completely differently. Because children are still growing, a raw BMI number doesn’t map to the same fixed categories adults use. Instead, a child’s BMI is compared to other children of the same age and sex using growth chart percentiles.
For children and teens ages 2 through 19, the categories are:
- Underweight: below the 5th percentile
- Healthy weight: 5th to 84th percentile
- Overweight: 85th to 94th percentile
- Obesity: 95th percentile or above
A BMI of 22 might be perfectly healthy for a 16-year-old boy but could signal a different category for a 10-year-old girl. The CDC provides an online child and teen BMI calculator that does the percentile lookup for you.
Where BMI Falls Short
BMI cannot distinguish between fat, muscle, and bone mass. It only measures total weight relative to height. This creates blind spots in both directions.
A muscular athlete might register as overweight or even obese by BMI despite having low body fat. Conversely, an older adult who has lost muscle mass over the years could have a “healthy” BMI while carrying a higher proportion of body fat than is ideal. People with larger or smaller bone structures face similar distortions.
Ethnicity also matters. The standard cutoffs underestimate health risk in people of Asian descent. WHO and NIH guidelines use lower thresholds for Asian populations: overweight starts at a BMI of 23, and obesity at 25, compared to 25 and 30 for the general population. If you’re of Asian or South Asian background, those adjusted numbers give a more accurate picture of your risk.
Waist Circumference as a Complement
Because BMI misses where your body stores fat, waist circumference fills an important gap. Fat concentrated around the abdomen poses greater health risks than fat stored in the hips or thighs, and your waist measurement captures that directly.
The NIH thresholds for increased health risk are a waist circumference of 40 inches (102 cm) or more for men and 35 inches (88 cm) or more for women. To measure, wrap a tape measure around your bare abdomen just above your hip bones, keep it snug but not compressing the skin, and read it after a normal exhale.
Combining your BMI with your waist measurement gives a more complete picture than either number alone. Someone with a BMI of 26 and a waist well below the threshold faces a different risk profile than someone with the same BMI but a waist measurement above it.