BMI, Waist and Body-Fat Numbers Explained

A smart scale or a health check can hand you three kinds of number: BMI, a waist measure and a body-fat percentage. They answer different questions, and the bands that go with them come from different places. This guide sets out what the CDC and the National Heart, Lung, and Blood Institute (NHLBI) say about the first two, how to measure them yourself and why body-fat percentages are the hardest to interpret. It is general information for adults, not medical advice. Our guide to choosing a smart scale covers the buying checks.

BMI: what it is and what it cannot tell you

The CDC defines body mass index as weight in kilograms divided by height in meters squared. In US units it lists weight in pounds divided by height in inches squared, times 703. For a person who weighs 150 lb and is 5 feet 7 inches (67 inches) tall, that is 150 divided by 4,489, times 703, or 23.5. The CDC calls BMI a quick, safe and reliable screening measure of weight relative to height, and says it is not a direct measure of body fat: it cannot distinguish fat from muscle and bone, and it does not show where on the body a person carries fat. People with a lot of muscle may fall into the overweight or obesity category on BMI alone.

For adults 20 and older the CDC gives these categories:

• underweight: BMI below 18.5
• healthy weight: 18.5 to just under 25
• overweight: 25 to just under 30
• obesity: 30 or higher, in class 1 (30 to under 35), class 2 (35 to under 40) and class 3 (40 or higher)

The CDC says BMI should be considered with other factors when assessing an individual's health, and that it can help a person and their health care provider judge chronic disease risk.

One set of cutoffs, or several?

The CDC says to interpret adult BMI with the standard categories regardless of age, sex or race. Other countries differ. NICE, the guidance body for England, advises lower BMI thresholds for adults with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, with overweight from 23 to 27.4 and obesity from 27.5, and advises caution in adults with high muscle mass. That is another country's guideline, not CDC advice. If your background is in one of those groups, ask your clinician which thresholds they use for you.

Children, teens and pregnancy

Adult categories do not apply under 20. The CDC reads BMI for ages 2 through 19 against sex-specific BMI-for-age percentiles, says BMI is a screening measure not intended to diagnose disease or illness, and advises discussing results with a health care provider. In pregnancy, the CDC says recommended weight gain depends on your pre-pregnancy BMI and advises working with your health care provider on your goals, so do not use adult categories to judge weight during pregnancy.

Waist circumference: measure it yourself

The NHLBI says that if most of your fat is around your waist rather than at your hips, you are at higher risk for heart disease and type 2 diabetes. It says a waist circumference of more than 35 inches for women or more than 40 inches for men increases your risk (about 89 and 102 cm, our conversion). Its method:

• Stand and place a tape measure around your middle, just above your hipbones.
• Measure just after you breathe out.
• Read the tape without pulling it tight into the skin.

The last line is our own advice, not the NHLBI's. NICE describes a slightly different landmark, midway between the bottom of the ribs and the top of the hips, so use the landmark that goes with the threshold you are comparing with. A body measuring tape on Amazon (opens in a new tab) is a search for a flexible tape.

Waist-to-height ratio, a measure used in England

For adults with a BMI below 35, NICE advises measuring waist-to-height ratio as well as BMI, with waist and height in the same units. As an example of our own, a 34 inch waist and a 67 inch height give 34 divided by 67, or 0.51. NICE bands the result as 0.4 to 0.49 (no increased health risks), 0.5 to 0.59 (increased risks) and 0.6 or more (further increased risks), and tells people to try to keep the waist under half their height. The CDC and NHLBI pages we read do not use this ratio, so treat it as a second opinion from another country's guidance and ask your clinician which measure they prefer.

Body-fat percentage: the hardest number to interpret

We did not find a body-fat percentage target on the CDC or NHLBI pages we read. Smart scales still label readings with bands. Garmin's manual, for example, shows ranges for adults that it attributes to the American Council on Exercise, with separate columns for women and men, and states that the data is not intended to diagnose any medical condition. A band on your scale comes from its maker's choice of source, so check which source the manual names, and do not treat the label as a health verdict. A scale's "obese" label is not the same as the CDC's obesity category, which is defined by BMI.

The reading itself is uncertain. A 2026 review of twelve studies found limits of agreement of typically 15 to 20 percentage points between impedance devices and a laboratory model, and a French hospital study found three smart scales read fat mass too low by medians of 2.2 to 4.4 kg (about 5 to 10 lb) compared with a DEXA scan. Our guide to what bioimpedance scales can and cannot measure goes through both. The sensible use of the number is as a trend from the same scale under the same conditions.

Other labels such as visceral fat, muscle score and metabolic age are the maker's own constructs. The CDC and NHLBI pages we read set no bands for them, so read the manual's definition.

Which number answers which question

• Is my weight in a usual range for my height? BMI, with the CDC's categories.
• Am I carrying weight around the middle? Waist circumference, which you can measure with a tape.
• Is my weight changing over weeks? Your scale's weight, averaged over weeks; see how to weigh yourself for useful trends.
• Is the fat or muscle estimate moving? Only as that scale's own trend, with the uncertainty above.

Cautions

The National Institute of Mental Health says some people become fixated or obsessed with weight loss, body weight or shape, and that this may be a sign of an eating disorder. If measuring, weighing or comparing numbers feeds that worry, stop and talk to a doctor. The American Heart Association says electronic body fat scales likely interfere with an ICD or pacemaker, and Withings says not to use its Body Smart with a pacemaker or other internal medical device, Garmin says the same of its Index S2, so check the maker's wording for any scale that uses a current. A body-composition scale, such as those in a search for body composition scales on Amazon (opens in a new tab), adds estimates but not answers about your health; if a number worries you, take it to your doctor.

The measures above are screening aids, not diagnoses. A doctor or nurse can put them together with your history, and the label and manual of your own scale come first for how to use it.