How to Calculate BMI: Formula, WHO Chart and Its Limits

You heard about BMI at the doctor's office, in an app or in an article, and you want to know what your number actually means. Learning how to calculate BMI is easy, but interpreting it is a lot less straightforward. In this guide, you'll find the BMI formula (metric and imperial), step-by-step examples, the official World Health Organization (WHO) chart and, most importantly, the limits of this index. You'll also learn which measurements to add for a more accurate picture of your health and your progress.

In short

• Formula: BMI = weight (kg) ÷ height (m)². Example: 70 kg (154 lb) at 1.75 m (5 ft 9 in) gives a BMI of 22.9.
• WHO cutoffs for adults: underweight below 18.5; healthy weight from 18.5 to 24.9; overweight from 25 to 29.9; obesity at 30 and above.
• Main limit: BMI can't tell muscle from fat and says nothing about where your fat is stored.
• Pair it with: waist circumference, waist-to-height ratio and, if possible, body fat percentage.
• What it's for: a screening tool for populations, not an individual diagnosis.

What is BMI (body mass index)?

Body mass index, or BMI, is a ratio between your weight and your height. It was devised in the 19th century by the Belgian statistician Adolphe Quetelet, which is why it's sometimes called the Quetelet index. The WHO has used it for decades as its reference tool to classify body size in adults.

Its appeal is practical: all you need is a scale and a measuring tape, it's free, and it makes it possible to compare entire populations. Large epidemiological studies have shown that a high BMI is associated, on average, with a higher risk of type 2 diabetes, high blood pressure and cardiovascular disease.

The key words are "on average." BMI describes groups well and individuals far less well. We'll come back to that.

How to calculate BMI: the formula step by step

The formula fits on one line:

BMI = weight in kilograms ÷ (height in meters × height in meters)

The result is expressed in kg/m². To avoid mistakes, follow three steps:

1. Convert your height to meters (172 cm becomes 1.72 m).
2. Multiply that height by itself (1.72 × 1.72 = 2.96).
3. Divide your weight by the result (65 kg ÷ 2.96 = 22.0).

The BMI formula in pounds and inches

If you think in pounds and feet, the imperial version gives the same result:

BMI = 703 × weight in pounds ÷ (height in inches × height in inches)

For example, at 143 lb and 5 ft 8 in (68 inches): 68 × 68 = 4,624, then 143 × 703 ÷ 4,624 = 21.7.

Worked examples

The last example already shows a limitation: someone who is 1.90 m tall and weighs 95 kg could be a very muscular rugby player or a sedentary person carrying belly fat. Same number, completely different bodies.

The most common calculation mistakes

• Using your height in centimeters instead of meters (the result becomes absurd).
• Multiplying weight by height instead of dividing.
• Forgetting the 703 factor when you use pounds and inches.
• Weighing yourself at an unusual time, for example in the evening after a big meal. For a number you can compare over time, weigh yourself in the morning, on an empty stomach, after using the bathroom.

What is a healthy BMI? The WHO BMI chart

Here is the WHO classification for adults aged 18 and over. It's the same for women and men.

The WHO also splits underweight into several grades (severe thinness corresponds to a BMI below 16). These cutoffs are conventions. Going from 24.9 to 25.1 doesn't change your health overnight. They help flag situations where it's worth talking to a health professional, not to put a label on you.

Is there an ideal BMI?

You'll often read that the "ideal BMI" is around 22. In reality, the WHO talks about a range, 18.5 to 24.9, not a perfect number. Within that range, other factors matter much more: how active you are, what you eat, your waist size, your sleep and your medical history.

Is BMI accurate? Why it has limits

BMI is useful, but it measures only one thing: weight relative to height. It knows nothing about what that weight is made of.

It can't tell muscle from fat

Muscle is denser than fat. Someone who has been lifting weights for several years can have a BMI of 27 or 28 with a low body fat percentage. On the flip side, a sedentary person can have a "normal" BMI while carrying little muscle and a lot of fat: the scientific literature sometimes calls this "normal weight obesity."

That's also why BMI is poor at tracking body recomposition: if you lose fat and gain muscle at the same time, your BMI can stay flat while your shape changes noticeably.

It says nothing about where fat is stored

Fat stored around your abdominal organs (visceral fat) is more strongly linked to cardiometabolic risk than fat stored on the hips or thighs. Two people with the same BMI can therefore have very different risk profiles. Your waist size and waist-to-height ratio give you information that BMI simply can't.

It varies with age, sex and ethnicity

• Sex: at the same BMI, women carry on average more body fat than men, which is physiologically normal.
• Age: over the years, people tend to lose muscle and often gain fat without their weight changing. In older adults, a very low BMI is a warning sign of undernutrition, and many clinicians consider a slightly higher BMI acceptable after 65 or 70.
• Ethnicity: back in 2004, the WHO pointed out that some Asian populations face a higher metabolic risk at the same BMI, and suggested lower action thresholds for them.
• Children and teens: adult cutoffs don't apply. BMI is read on growth charts by age and sex, as percentiles.
• Pregnancy: your BMI at the start of pregnancy is used to set recommended weight gain, but follow-up belongs to your midwife or doctor.

Which measurements should you add to BMI?

To track your health or your body transformation, BMI works best when you cross-check it with other indicators. Each one adds a piece of the puzzle.

To dig deeper into each one:

• Body fat percentage tells you how much of your weight is fat. Our guide to the ideal body fat percentage for men and women covers the reference ranges, and our comparison of ways to measure body fat looks at smart scales, skinfold calipers and DEXA.
• Body measurements (waist, hips, chest, arms, thighs) show where your body is changing. Our step-by-step guide on how to take body measurements accurately shows you exactly where to place the tape.
• Progress photos reveal what numbers can't: a waist getting narrower, shoulders taking shape.

How can you use BMI to track your progress?

BMI is more useful as a trend than as a one-off snapshot. A single value says little; a curve over several months says a lot more, especially when you read it alongside your waist measurement.

A few simple principles:

• Measure under the same conditions: same scale, same time of day, ideally in the morning on an empty stomach.
• Watch the trend, not today's number: your weight can swing by 0.5 to 2 kg (1 to 4 lb) from one day to the next because of water, salt or digestion, which moves your BMI even though your body hasn't really changed.
• Don't get discouraged if your BMI stalls: when you start lifting or get back into exercise, your body composition can shift long before your BMI moves.

In Marava, your BMI is calculated automatically from the weight you enter and shows up on your dashboard next to your body fat percentage and your trend line. Seeing these numbers next to your latest progress photos helps you keep perspective: a stable BMI can hide a body that's changing.

When should you see a health professional?

BMI is a screening tool. It's worth talking to your doctor in the following situations:

• a BMI below 18.5, or a low BMI combined with unintended weight loss as you get older;
• a BMI of 30 or more, especially if it comes with a large waist size;
• rapid, unexplained weight loss or weight gain;
• pregnancy, plans to become pregnant or a chronic condition;
• a difficult relationship with food or with your body image.

If you recognize yourself in patterns of restriction, bingeing or constantly controlling your weight, a doctor, a psychologist or a registered dietitian can support you. There is no shame in asking for help.

Conclusion: a useful benchmark, not a verdict

Calculating your BMI takes a few seconds: weight divided by height squared, then a quick look at the WHO chart. It's a good starting point, simple and universal. But it says nothing about your muscle mass or how your fat is distributed.

For a more accurate picture, pair it with your waist size, your body fat percentage and regular photos. And if your scale seems stuck while your clothes fit better, read our article on losing inches but not weight: it's often an excellent sign.