BMI Explained: What Your Body Mass Index Tells You, and What It Misses

If you have ever looked into weight loss, a doctor’s chart or a GLP-1 medication, you have run into BMI. It decides which category you land in, it shows up on insurance forms, and it is the number used to decide who qualifies for medicines like Wegovy and Zepbound. It is also one of the most criticized numbers in medicine.

This guide explains what BMI actually is, how to work yours out, what the categories mean, why it matters so much for GLP-1 treatment, and where it falls short. We also cover the simple measurements that fill in the gaps, so you can treat BMI as a useful starting point rather than a verdict.

Educational only. This is not medical advice. BMI is a screening tool, not a diagnosis. Talk with a licensed clinician about what your numbers mean for you, especially before starting or changing any weight-loss treatment.

The short answer

BMI is a ratio of your weight to your height. The formula is weight in kilograms divided by height in meters squared. For adults 20 and older, the Centers for Disease Control and Prevention (CDC) sorts BMI into four main categories: underweight (under 18.5), healthy weight (18.5 to under 25), overweight (25 to under 30) and obesity (30 or higher).

BMI is quick, free and reasonably good at spotting excess weight across large groups of people, which is why it is used everywhere from national health surveys to drug trials. Its weakness is the individual. It does not measure body fat directly, cannot tell muscle from fat and says nothing about whether fat sits around your belly or your hips. That is why the CDC, the American Medical Association (AMA) and an international expert commission all recommend reading BMI alongside other information, such as waist size, blood pressure, blood sugar and cholesterol.

What BMI is, and where it came from

BMI is older than most people think. According to a 2008 history by kidney specialist Garabed Eknoyan, the Belgian mathematician and statistician Adolphe Quetelet described the ratio in 1832, after concluding from his studies of human growth that in adults “the weight increases as the square of the height.” For more than a century it was known as the Quetelet Index. In 1972, the American physiologist Ancel Keys gave it the name body mass index, after studies of weight and heart disease showed it was the most practical simple index of relative body weight.

That origin matters. Quetelet was describing averages across populations, not judging individual health. BMI still works best in that role today.

How to calculate your BMI

The CDC gives three versions of the same formula:

• Metric: weight (kg) ÷ height (m)²
• Metric with centimeters: [weight (kg) ÷ height (cm) ÷ height (cm)] × 10,000
• US units: weight (lb) ÷ height (in)² × 703

Two quick examples:

• US units: someone who is 5 feet 4 inches (64 inches) and weighs 170 pounds has a BMI of 170 ÷ (64 × 64) × 703, which is about 29.2.
• Metric: someone who weighs 80 kg and is 1.70 m tall has a BMI of 80 ÷ (1.70 × 1.70), which is about 27.7.

The CDC’s own example is an adult who is 5 feet 8 inches tall and weighs 200 pounds, which works out to a BMI of 30.4. If you would rather skip the math, our free BMI calculator does it for you and shows the matching CDC category.

For accurate numbers, weigh yourself at a similar time of day in light clothing and use a measured height rather than the one on your driver’s license. Small errors in height have a big effect because height is squared.

What the adult BMI categories mean

Here are the CDC categories for adults 20 and older, with the CDC’s example weight ranges for someone who is 5 feet 9 inches tall:

BMI category	BMI range (kg/m²)	Example: 5 ft 9 in adult
Underweight	Less than 18.5	124 lb or less
Healthy weight	18.5 to less than 25	125 to 168 lb
Overweight	25 to less than 30	169 to 202 lb
Obesity, class 1	30 to less than 35	203 to 236 lb
Obesity, class 2	35 to less than 40	237 to 270 lb
Obesity, class 3 (severe obesity)	40 or greater	271 lb or more

The CDC uses these same adult categories for everyone 20 and older. Children and teens are different: their BMI is compared with others of the same age and sex using growth-chart percentiles, so the adult cutoffs do not apply to anyone under 20.

BMI thresholds at common heights

Because BMI depends on height, the weight at which you cross a threshold varies a lot from person to person. The table below shows the approximate weight, in pounds, at which several BMI cutoffs are reached. We calculated these with the CDC’s US-units formula and rounded them to the nearest pound.

Height	BMI 18.5	BMI 25	BMI 27	BMI 30	BMI 35	BMI 40
5 ft 0 in	95 lb	128 lb	138 lb	154 lb	179 lb	205 lb
5 ft 3 in	104 lb	141 lb	152 lb	169 lb	198 lb	226 lb
5 ft 6 in	115 lb	155 lb	167 lb	186 lb	217 lb	248 lb
5 ft 9 in	125 lb	169 lb	183 lb	203 lb	237 lb	271 lb
6 ft 0 in	136 lb	184 lb	199 lb	221 lb	258 lb	295 lb
6 ft 3 in	148 lb	200 lb	216 lb	240 lb	280 lb	320 lb

The BMI 27 column is included because it is one of the cutoffs used for GLP-1 eligibility, which we cover below.

How common higher BMIs are in the US

Having a BMI in the overweight or obesity range is very common in the United States. According to the National Center for Health Statistics, measured heights and weights from the National Health and Nutrition Examination Survey (NHANES) showed that between August 2021 and August 2023:

• 40.3% of US adults had obesity (a BMI of 30 or higher), with no significant difference between men (39.2%) and women (41.3%).
• Obesity was most common in adults aged 40 to 59 (46.4%), compared with 35.5% of adults aged 20 to 39 and 38.9% of those 60 and older.
• 9.4% of adults had severe obesity (a BMI of 40 or higher), and it was more common in women (12.1%) than men (6.7%).

The same report reminds readers that BMI “has limitations”: body fat can vary by sex, age and ethnicity at the same BMI, and BMI does not show how fat is distributed.

Why BMI still matters

For all its critics, BMI has earned its place. The CDC describes it as a quick, low-cost and reliable screening measure, and notes that although it does not measure body fat directly, it is “moderately to strongly associated” with measures that do, such as a dual-energy X-ray absorptiometry (DEXA) scan.

It also tracks with health outcomes across large populations. The Prospective Studies Collaboration pooled 57 long-term studies of about 900,000 adults, mostly in Western Europe and North America, and published the results in The Lancet in 2009. Among their findings:

• Death rates were lowest at a BMI of about 22.5 to 25.
• Above that range, each 5-unit increase in BMI was linked on average to about 30% higher overall mortality, driven mainly by heart and blood vessel disease.
• At a BMI of 30 to 35, median survival was reduced by an estimated 2 to 4 years; at 40 to 45, by 8 to 10 years.

The CDC lists the health problems linked to obesity in adults, including high blood pressure, high cholesterol, type 2 diabetes, stroke, many types of cancer, sleep apnea, osteoarthritis, depression and anxiety. The NHLBI adds that the more body fat a person has, the more likely they are to develop heart disease, high blood pressure, type 2 diabetes, breathing problems and certain cancers.

Tracking your own BMI over time is also useful. The CDC points out that routine tracking can show weight gain or loss and help you and your clinician watch health risks. Because your height does not change much as an adult, a change in your BMI mostly reflects a change in your weight.

BMI and GLP-1 medications

BMI is central to who can get a GLP-1 weight-loss medicine. The US labels for Wegovy (semaglutide) and Zepbound (tirzepatide) say they are indicated, alongside a reduced-calorie diet and more physical activity, for adults with obesity, or adults with overweight plus at least one weight-related condition. Zepbound is also approved to treat moderate to severe obstructive sleep apnea in adults with obesity.

In practice, that has meant:

• a BMI of 30 or higher, or
• a BMI of 27 or higher with a condition such as high blood pressure, type 2 diabetes, high cholesterol or sleep apnea.

Those were the cutoffs used in the main clinical trials. Our guide to who qualifies for GLP-1 weight loss covers the details, including the adolescent indication for Wegovy and how diabetes-only GLP-1s differ.

A few practical points:

• Meeting a BMI cutoff is not the same as getting a prescription. A licensed clinician still weighs your history, other medicines and possible risks. See who should not take a GLP-1 for the main cautions.
• Insurers may use stricter rules. Many plans require prior authorization and documented BMI, and some set higher thresholds. Our guide to insurance coverage for GLP-1s explains what to expect.
• Your BMI will change during treatment. As weight comes off, your BMI falls, and some people move down one or more categories. A falling BMI on treatment does not mean you no longer need care; continuing, adjusting or stopping a medicine is a decision for you and your prescriber.

Where BMI falls short

BMI’s biggest problem is that it measures size, not composition. Here are the main blind spots.

It cannot tell muscle from fat

The CDC says plainly that BMI “does not distinguish between fat, muscle, and bone mass.” A very muscular person can land in the overweight or even obesity range while carrying little body fat. In a short PBS “Docs on Call” segment from April 2026, physician Dr. David Tennant gives the example of looking up the average BMI of an NFL team’s starting linebackers and finding it in the obesity range, even though “they’re just incredibly muscular.” He also says that “for most people, most of the time,” BMI is a reasonable, inexpensive estimate.

The reverse problem is less obvious but more common: someone with little muscle can have a “healthy” BMI and still carry a lot of body fat.

It misses a lot of people with high body fat

A 2010 meta-analysis in the International Journal of Obesity by Okorodudu and colleagues looked at 25 studies with almost 32,000 people whose body fat was measured directly. Standard BMI cutoffs for obesity had a pooled sensitivity of 50% and a specificity of 90%. In plain terms, BMI rarely labeled people as having obesity when they did not, but it missed about half of those who actually had high body fat. When the cutoff was 30, sensitivity was even lower, at about 42%.

It ignores where fat is stored

Fat around the belly and organs (visceral fat) is more closely tied to type 2 diabetes and heart disease than fat on the hips and thighs. BMI cannot see the difference. The NHLBI notes that if most of your fat is around your waist rather than your hips, you are at higher risk for heart disease and type 2 diabetes, and that a waist of more than 35 inches for women or 40 inches for men increases risk.

It works differently across ethnic groups

The standard cutoffs were built mostly on data from white European and North American populations. Research since then shows that risk starts at different BMIs in different groups:

• A 2004 World Health Organization expert consultation concluded that a substantial share of Asian people have a high risk of type 2 diabetes and heart disease at BMIs below 25. It kept the international cutoffs but suggested extra public health action points at 23, 27.5, 32.5 and 37.5.
• A 2021 study of about 1.47 million people in England by Caleyachetty and colleagues found that the type 2 diabetes risk seen at a BMI of 30 in white adults was reached at a BMI of about 23.9 in South Asian adults, 26.9 in Chinese adults, 26.6 in Arab adults and 28.1 in Black adults.
• The American Diabetes Association’s 2026 Standards of Care recommend considering diabetes testing in adults of Asian ancestry at a BMI of 23 or higher (instead of 25) when they have at least one other risk factor.
• England’s NICE guideline uses lower overweight and obesity thresholds (23 and 27.5) for people with South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds.

If you belong to one of these groups, a “healthy” BMI on the standard chart may not tell the whole story. It is worth asking your clinician whether a different threshold applies to you.

It is less reliable at older ages

People tend to lose muscle and gain fat as they age, so the same BMI can mean more body fat at 70 than at 30. NICE advises clinicians to interpret BMI with caution in people 65 and over, taking into account other illnesses, physical function and “the possible protective effect of having a slightly higher BMI when older.”

What the experts now recommend

Major medical groups have not abandoned BMI. They have moved it into a supporting role.

• The CDC calls BMI a screening measure and says it should be considered alongside medical history, health behaviors, physical exam findings such as blood pressure and muscle mass, and lab results such as glucose and cholesterol.
• The American Medical Association adopted a policy in June 2023 recognizing BMI’s “significant limitations” in clinical settings and suggesting it be used together with other measures such as visceral fat, body composition, waist circumference and genetic or metabolic factors. The policy notes that BMI correlates well with fat mass across the population “but loses predictability when applied on the individual level,” and says it should not be the only reason to deny insurance coverage.
• A Lancet Diabetes & Endocrinology Commission of 58 experts, published in January 2025, recommended using BMI only for population-level risk, research and screening, not as an individual measure of health. It advised confirming excess body fat with a direct measurement or at least one body measurement such as waist circumference, waist-to-hip ratio or waist-to-height ratio. In people with a BMI above 40, it said excess fat can reasonably be assumed. The Commission also proposed separating “clinical obesity,” where excess fat is already harming organs or daily function, from “preclinical obesity,” where function is preserved but future risk is higher.
• The American Diabetes Association’s 2026 Standards say screening for overweight and obesity should use BMI each year, with an extra measure of body fat used to confirm excess adiposity where available and feasible.

Simple measures that add to BMI

You do not need a lab to get a fuller picture. Here are the most practical options:

Measure	How it works	What it adds	Main drawback
Waist circumference	Tape measure around your middle	Estimates belly fat; NHLBI risk lines are over 35 in (women) and over 40 in (men)	Hard to measure the same way every time
Waist-to-height ratio	Waist size divided by height, in the same units	Belly fat adjusted for height; NICE target is under 0.5	Needs a careful waist measurement
Waist-to-hip ratio	Waist size divided by hip size	Shows fat distribution	Hip measurement is also hard to standardize
Body-fat scans (such as DEXA)	Imaging that separates fat, muscle and bone	Direct measure of body composition	More expensive and less widely available
Health markers	Blood pressure, blood sugar, cholesterol and triglycerides	Shows whether weight is already affecting your health	Needs a clinic visit or lab test

How to measure your waist

The NHLBI’s method is simple: stand up, place a tape measure around your middle just above your hip bones, and measure just after you breathe out.

The waist-to-height rule of thumb

In 2022, England’s National Institute for Health and Care Excellence (NICE) began encouraging adults with a BMI under 35 to measure their own waist-to-height ratio. Its advice is to keep your waist to less than half your height. NICE classes a ratio of 0.4 to 0.49 as healthy, 0.5 to 0.59 as increased risk, and 0.6 or more as further increased risk, and says these cutoffs can be used for both sexes, all ethnicities and people with high muscle mass.

For example, someone who is 66 inches tall would aim for a waist under 33 inches. A popular YouTube short demonstrates the same idea with a piece of string cut to half your height: if it does not reach around your waist, your ratio is above 0.5. It is a fun, quick check, though it is a screening idea rather than a diagnosis, and a clinician can tell you what your numbers mean together.

What people get wrong about BMI online

On X, BMI comes up in almost every GLP-1 conversation, and a few patterns stand out. Some people describe being turned down for a prescription because they were just under a BMI cutoff, even though they felt their body composition told a different story. Others, including some clinicians, push back on prescriptions given to people with BMIs they consider too low. And many posts point out that “the scale looks fine” while waist size, triglycerides or blood sugar are already creeping up.

All three views contain some truth. BMI cutoffs are blunt, which is exactly why guidelines pair them with other measures and clinical judgment. A borderline BMI is a reason for a fuller assessment, not a reason to look for workarounds or unregulated products.

Using BMI wisely

Here is a practical way to think about your own number:

1. Use it as a starting point. Work out your BMI with our BMI calculator and note your category.
2. Add your waist. Measure your waist and work out your waist-to-height ratio. If both BMI and waist are high, the signal is stronger.
3. Check your health markers. Blood pressure, blood sugar (or A1C), cholesterol and triglycerides tell you whether weight is already affecting your health.
4. Track trends, not single readings. Your BMI and waist over months say more than one day’s weigh-in.
5. Focus on what changes risk. The NHLBI notes that losing just 3% to 5% of your weight can lower triglycerides, blood sugar and type 2 diabetes risk, and that health professionals often suggest aiming for 5% to 10% over about six months. Our guides to how fast you can safely lose weight and what a calorie deficit is explain how to approach that.
6. Protect your muscle. Because BMI cannot see muscle, it will not tell you if you are losing muscle along with fat. Strength training and enough protein help, especially during rapid weight loss.
7. Talk to a clinician. Bring your BMI, waist measurement and any recent lab results to your next visit, and ask what they mean together.

Questions to ask your clinician

• Based on my BMI, waist size and lab results, is my weight affecting my health right now?
• Does a different BMI threshold apply to me because of my ancestry, age or muscle mass?
• Would a body-composition test or other measurement be useful for me?
• Am I a candidate for any weight-management treatment, including a GLP-1 medicine, and what would that involve?
• Which numbers should we track over time, and how often?

The bottom line

BMI is a simple ratio of weight to height. It is useful for screening, tracking trends and comparing groups, and it is the gatekeeper number for GLP-1 weight-loss medicines. But it cannot see muscle, belly fat, age or ethnicity, so it can both overstate and understate an individual’s risk.

The best approach is to treat BMI as one clue among several. Pair it with your waist measurement and your blood pressure, blood sugar and cholesterol, and talk through the full picture with a clinician. For more on the fundamentals of weight and metabolism, visit our Fat Loss Basics hub, try the calorie (TDEE) calculator, or start with the GLP-1 Guide if you are exploring medication.