Is BMI Accurate? What It Can and Can't Tell You

The short answer

BMI is accurate at what it was meant to do and unreliable at what people often use it for. As a quick, cheap screen for weight-related health risk across large groups, it works well. The CDC calls it “a quick, safe, and reliable screening measure” and says it is moderately to strongly linked with measures that do capture body fat.

But BMI does not measure body fat. It can’t tell fat from muscle or bone, and it says nothing about where fat sits. The CDC calls BMI a screening measure, not a diagnostic one. Use the BMI calculator as a starting point, then add the measures in this guide before drawing any conclusions.

How BMI is worked out

BMI is your weight divided by the square of your height:

Metric: BMI = weight in kg ÷ (height in m)²
US units: BMI = 703 × weight in lb ÷ (height in inches)²

The 703 just converts pounds and inches into the metric formula.

Example: 5 ft 9 in (69 in) and 170 lb. 703 × 170 = 119,510, and 69 × 69 = 4,761, so BMI = 119,510 ÷ 4,761 = 25.1.

The adult categories

For adults 20 and over, the CDC uses these categories “regardless of age, sex, or race”:

BMI	Category
Under 18.5	Underweight
18.5 to under 25	Healthy weight
25 to under 30	Overweight
30 to under 35	Obesity, class 1
35 to under 40	Obesity, class 2
40 or more	Obesity, class 3 (severe)

So the person in the example, at 25.1, is just into “overweight.” At 5 ft 9 in, a BMI under 25 means about 125 to 169 lb. They are about a pound over that range, which is a good reminder that a category line is a line, not a cliff.

For children and teens aged 2 to 19, the adult categories don’t apply. The CDC Child and Teen BMI Calculator compares a child’s BMI with others of the same age and sex and reports a percentile.

What BMI can tell you

A screening flag. The CDC says BMI can help you and your health care provider judge chronic disease risk. Its guidance for practitioners describes BMI as an initial screen for overweight and obesity in children and teens; for adults, it is a screen, not a diagnosis.
A trend. Tracking BMI over time shows weight loss or gain at a glance.
Population health. BMI is how public health agencies compare weight across groups and years, which is what it does best.

What BMI can’t tell you

The CDC’s considerations for practitioners list examples of where the same BMI means different things:

Muscle. “Muscular individuals, or highly-trained athletes, may have a high BMI because of increased muscle mass.”
Age. On average, older adults have more body fat than younger adults at the same BMI.
Sex. On average, women have more total body fat than men at the same BMI.
Where fat sits. BMI gives no indication of how fat is distributed, and the CDC adds that it can’t show what types of fat you have.

Ethnicity and the 25 cut-off

A 2004 World Health Organization expert consultation found that a substantial share of Asian people have a high risk of type 2 diabetes and heart disease at BMIs below 25, the standard overweight line. The point where risk rises varied from 22 to 25 across different Asian populations. The WHO kept the international cut-offs but proposed extra public health action points at 23, 27.5, 32.5 and 37.5. The CDC’s practitioner guidance notes the debate about separate categories for some groups, but the US categories stay the same for everyone. If you are of Asian descent, ask your doctor which numbers to use for you.

Worked example: same BMI, different bodies

Two men are both 6 ft tall and 200 lb. Each has a BMI of 27.1, which the CDC table calls overweight.

Now add a tape measure. The body fat calculator estimates body fat with the US Navy method, from height, neck and waist:

	Man A	Man B
Height, weight	6 ft, 200 lb	6 ft, 200 lb
BMI	27.1	27.1
Neck	16.5 in	16 in
Waist at the navel	34 in	42 in
Navy body fat estimate	about 13.6%	about 28.4%

BMI can’t separate them. The tape suggests Man A carries more of his weight as muscle and Man B more as fat around the middle. Tape-based body fat comes from a population equation, so treat it as a rough estimate too, but a 15-point gap is hard to ignore.

Better ways to look at your health

The CDC’s guidance for practitioners says other measures, including skinfold thickness, bioelectrical impedance, underwater weighing and DXA scans, may be more accurate than BMI for body fat, though they can be expensive, intrusive or hard to find. It names waist circumference as a simple measure of fat distribution. In practice:

Start with BMI. It is free and takes seconds.
Measure your waist. Use a tape at the navel, at the end of a normal breath out. The body fat calculator turns it into an estimate.
Look at the numbers your doctor checks. The CDC says BMI should be considered alongside blood pressure, cholesterol and a physical exam.
Watch the trend, not one reading. A slow change in BMI or waist over months says more than any single day.

Common mistakes

Reading BMI as body fat. It is weight for height, nothing more.
Using adult categories for kids. Anyone under 20 needs the percentile method.
Worrying over a decimal. At 5 ft 9 in, a BMI of 24.9 and one of 25.1 are less than 1.5 lb apart. Only the category name changes.
Ignoring a “healthy” BMI with a growing waist. Older adults carry more fat at the same BMI on average, so a healthy number doesn’t rule out extra body fat.

If your BMI points to a weight goal, the calorie calculator sets a daily target, and the TDEE calculator shows what you burn now.

BMI is a screening tool, not a diagnosis. Talk to your doctor about what your BMI and other measurements mean for you, especially if you are pregnant, an athlete, an older adult, or under 20.