Body mass index, or BMI, is a ratio of weight to height. It is quick to calculate and widely used as a screening tool, but it is only a starting point and says nothing about body composition, fitness, or overall health.
How BMI is calculated
In metric units, BMI equals weight in kilograms divided by height in meters squared. A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ 3.0625 ≈ 22.9.
In imperial units, multiply weight in pounds by 703 and divide by height in inches squared. Both methods give the same result.
Adult categories
For adults aged 20 and over, the CDC describes a BMI under 18.5 as underweight, 18.5 to 24.9 as a healthy weight range, 25.0 to 29.9 as overweight, and 30.0 or above as obesity. These cutoffs are population screening ranges, not diagnoses.
Why children are different
For ages 2 to 19, BMI is interpreted using age- and sex-specific percentiles because body fat changes as children grow. A standard adult calculator should not be used to classify a child. The CDC provides a dedicated child and teen calculator for that purpose.
What BMI misses
BMI cannot tell muscle from fat. A muscular athlete can land in the overweight range while having low body fat. It also does not show where fat is stored, and abdominal fat carries different risks than fat elsewhere. Age, sex, and ethnic background can all affect how well BMI reflects body fat.
Better to use it alongside other measures
Waist circumference, blood pressure, blood sugar, cholesterol, fitness level, and family history all add information BMI lacks. A healthcare professional can combine these into a more useful picture.
How to use a BMI result
Treat it as one data point. If your result surprises you, or you are concerned about your weight in either direction, speak with a doctor rather than relying on a single number. This site's calculators provide general information and are not medical advice.
An imperial example
A person who weighs 180 pounds and is 5 feet 10 inches (70 inches) tall has a BMI of 703 × 180 ÷ 70² = 126,540 ÷ 4,900 ≈ 25.8. That falls in the overweight range by the standard cutoffs, though the result alone says nothing about whether the weight is muscle, fat, or a mix.
Working backwards: the range for your height
Because BMI is a formula, you can reverse it. For someone 1.75 m tall, a BMI of 18.5 corresponds to about 56.7 kg, and a BMI of 24.9 corresponds to about 76.3 kg. That span is the healthy weight range defined by the adult categories for that height. It is a wide range, which is a reminder that BMI describes populations better than individuals.
Trends matter more than a single reading
A change over months or years can be more informative than one number. Unplanned weight loss or gain is worth discussing with a doctor regardless of the BMI category. In older adults, a slightly higher BMI is sometimes associated with better outcomes, and muscle loss with age can hide changes in body composition, so interpretation should be individualized.
Frequently asked questions
Is the formula different for men and women? Adult BMI uses the same formula and categories for both. Body fat percentages differ between the sexes at the same BMI.
Can BMI be accurate for athletes? Often not. Muscle is dense, so a very muscular person can have a high BMI with low body fat.
Should I use BMI to set goals? It is better to set goals around habits, fitness, and how you feel, and to ask a clinician what is appropriate for you.
Try the calculators
BMI Calculator · Body Fat Calculator · Ideal Weight Calculator
This guide is for general information only. It is not financial, medical, or legal advice.