BMI Explained: How to Calculate Yours and Read the Chart Correctly

You have probably seen “BMI” printed on a doctor’s chart or flashed inside a fitness app. It looks official, but almost nobody explains what it actually measures or how much weight to give it. This guide fixes that. You will get the exact formula, a real classification chart, a step-by-step example in both pounds and kilos, and an honest look at where BMI gets things wrong so you can read your own number without panic or false comfort.

What BMI actually measures

BMI stands for Body Mass Index. It is a single number that compares your weight to your height, and it was designed as a quick population screening tool, not a full health verdict. A high BMI can flag that someone is carrying more weight than is typical for their height, which statistically links to higher risk of type 2 diabetes, high blood pressure, and heart disease. That is all it does. It does not measure fat, fitness, or health directly.

The CDC is blunt about this. It calls BMI “a screening measure” and says it “should be considered with other factors when assessing an individual’s health.” Keep that framing in mind for the rest of this article.

The BMI formula

The math is simple. BMI is your weight divided by your height squared.

• Metric: BMI = weight (kg) ÷ [height (m)]²
• Imperial: BMI = 703 × weight (lb) ÷ [height (in)]²

The 703 in the imperial version is just the conversion factor that lets you use pounds and inches instead of kilograms and meters. The result is the same number either way.

A worked example, step by step

Say you are 5 feet 6 inches tall and weigh 150 pounds.

1. Convert height to inches: 5 ft 6 in = 66 inches.
2. Square it: 66 × 66 = 4,356.
3. Multiply weight by 703: 150 × 703 = 105,450.
4. Divide: 105,450 ÷ 4,356 = 24.2.

A BMI of 24.2 lands in the healthy range. In metric that same person is about 1.68 m and 68 kg: 68 ÷ (1.68 × 1.68) = 68 ÷ 2.82 = 24.1. Same answer, rounding aside.

If arithmetic is not your thing, the NIH and CDC both host free calculators, but doing it once by hand makes the categories below far easier to trust.

The adult BMI chart

For adults 20 and older, the CDC uses these standard categories.

Obesity itself is split into three classes, because the health picture at BMI 31 is very different from BMI 45.

Here is a detail most charts skip. These cutoffs were built largely from white European and American populations, and they do not fit every body the same way. For people of Asian descent, health risk tends to climb at a lower BMI. That is why the American Diabetes Association recommends screening Asian Americans for type 2 diabetes starting at a BMI of 23, not the usual 25. A 2009 meta-analysis of more than 300,000 Asian participants found the diabetes-risk threshold sat around a BMI of 23 to 24. If that describes you, treat 23 as the number to watch rather than 25.

Why the number matters at a population level

BMI stuck around because it tracks real problems at scale. Per NHANES data, roughly 42% of U.S. adults now have obesity, up from about 30% two decades ago. When public health researchers need one cheap, repeatable measure across millions of people, BMI does the job. The trouble starts when you shrink that lens down to one person, which is where its limits show.

Where BMI gets it wrong

BMI never asks what your weight is made of. Muscle is denser than fat, so it pushes the number up without adding any of the risk that fat around your organs does. A few clear cases:

• Athletes and lifters. A muscular rugby player or a regular at the weight rack can post a BMI of 27 or 28 while carrying very little fat. On paper that reads “overweight.” In reality it is muscle. If you strength train regularly, expect your BMI to run high and mean less.
• Older adults. Muscle mass drops with age, so someone can sit in the “healthy” band while carrying more fat than the number suggests.
• Fat location. Two people with identical BMIs can have very different risk if one carries weight around the hips and the other around the belly. Visceral fat around the organs is the dangerous kind, and BMI cannot see it.

In 2024 the Lancet Commission on obesity, backed by the European Association for the Study of Obesity, recommended that obesity should no longer be diagnosed by BMI alone. That is not fringe opinion anymore. It is where the science is heading.

Better measures to check alongside BMI

You do not need a lab. Two tape-measure numbers add most of what BMI is missing.

Waist-to-height ratio (WHtR). Measure your waist at the belly button, then divide by your height in the same units. The UK’s NICE guidance is easy to remember: keep your waist under half your height, so a ratio below 0.5. Their bands run like this.

Waist circumference. As a standalone flag, U.S. guidance treats a waist over 40 inches (102 cm) in men and over 35 inches (88 cm) in women as a sign of abdominal obesity and higher metabolic risk, even when BMI looks fine.

Together, BMI plus a waist number gives you a far more honest read than either alone.

How to act on your result

Your BMI is a starting point for a conversation, not a sentence. Here is how to respond to each zone without spiraling.

• Under 18.5. Being underweight carries its own risks, from weak bones to a run-down immune system. Focus on nutrient-dense food and a doctor’s input before assuming thinner is safer. A high-protein approach helps here too, for building rather than cutting.
• 18.5 to 24.9 (or up to 22.9 for Asian bodies). Maintenance mode. Keep moving, eat well, and check your waist number once in a while so muscle-versus-fat drift does not sneak past you.
• 25 and up (or 23 and up for Asian bodies). A prompt to look closer, not to crash diet. Small, durable habits win. Even a daily walk moves the needle, as we cover in can walking really help you lose weight. Pair that with a sensible eating pattern, whether that is portion control or 16:8 intermittent fasting, and refuel smart with the right post-workout food.

Whatever your number, the workout you actually enjoy is the one you repeat. Comfortable kit removes an excuse, so a pair of well-made women’s leggings that move with you is a small nudge worth having. Track your waist and how your clothes fit alongside the scale, and let BMI be one input among several.

FAQ

Is BMI accurate for women?

The formula and adult chart are the same for men and women, so yes, it works as a general screen. But women naturally carry more body fat than men at the same BMI, so a woman and a man with an identical number can have different body compositions. Use your waist measurement as a second data point.

What is a healthy BMI for someone of Asian descent?

Risk tends to rise earlier for Asian bodies, so many clinicians treat 23 as the start of the overweight range and 25 as the start of obesity, rather than the standard 25 and 30. The American Diabetes Association already screens Asian Americans for diabetes from a BMI of 23.

Can I have a high BMI and still be healthy?

Yes, especially if you are muscular. A lifter or athlete can read “overweight” on BMI while carrying low body fat and excellent fitness. That is exactly why BMI should never be your only measure. Check your waist-to-height ratio and how you feel and perform.

How often should I check my BMI?

For most adults, a few times a year is plenty. Weight fluctuates daily from water and food, so chasing the number too often just adds stress. Trends over months matter far more than any single reading.

Does BMI work for kids?

Not the same way. Children and teens use age- and sex-specific BMI percentiles rather than the fixed adult categories, because their bodies are still developing. The chart in this article is for adults 20 and older only.