What BMI Tells You, and What It Quietly Misses

This guide is general information about a screening number. It is not medical advice, and it cannot tell you whether your weight is right for your body. Your doctor can.

Body mass index has been around for well over a century, and it has stuck for a plain reason: all you need is a scale and a tape measure. It turns height and weight into one number that can be compared across large groups of people. That is useful for public health. For one individual it is a starting point at best, and it is worth knowing exactly what it does and does not measure.

The formula

BMI is weight divided by height squared.

• Metric: BMI = weight in kilograms ÷ (height in metres)²
• Imperial: BMI = 703 × weight in pounds ÷ (height in inches)²

The 703 simply converts pounds and inches into the metric units the formula was designed for. Both versions give the same answer for the same person.

A worked example

Take someone who weighs 70 kg and is 1.75 m tall. Height squared is 1.75 × 1.75 = 3.0625. Then 70 ÷ 3.0625 = 22.9.

In imperial units: a person who is 165 lb and 68 in tall has height squared of 68 × 68 = 4,624. Then 703 × 165 = 115,995, and 115,995 ÷ 4,624 = 25.1.

That second number sits just over a category line, which brings up the first thing to understand about BMI: the boundaries are conventions. At 68 inches, the top of the “normal” range works out to roughly 164 lb, so a person at 165 lb is classed differently from one at 164 lb. Nothing meaningful about their health changes across that one pound.

The adult categories

The World Health Organization uses these cut-offs for adults, and the CDC uses the same ranges:

For the 1.75 m person above, those boundaries correspond to about 56.7 kg at a BMI of 18.5, about 76.6 kg at 25, and about 91.9 kg at 30. A range of roughly 20 kg is classed as “normal” for that height, which is a reminder that the category is a band, not a target.

What BMI actually measures

BMI measures how heavy you are for your height. It does not measure body fat, and it cannot say where on the body weight is carried or what the weight is made of. Across big populations, higher BMI tends to go with higher rates of several health problems, which is why it is used for screening. But a screening tool is meant to prompt a closer look, not to deliver a verdict.

Where BMI misleads

• Muscle. Muscle is denser than fat. Someone who is 1.80 m and 95 kg has a BMI of 29.3, which is in the overweight band, yet a person with a lot of muscle may carry very little excess fat. Strength athletes are the classic example.
• Low muscle mass. The reverse also happens. A person can have a BMI in the normal range but carry relatively little muscle and proportionally more fat. BMI cannot tell these cases apart.
• Age. Body composition shifts with age, and older adults often lose muscle. Health guidance for older adults sometimes treats weight and BMI differently from younger adults, so this is a good thing to discuss with a clinician.
• Ethnicity and body shape. The same BMI can correspond to different levels of body fat and health risk in different populations. The WHO has recognised that some Asian populations may face increased risk at lower BMI values than the standard cut-offs, and some countries and health bodies use lower thresholds. Check what applies in your country, ideally with your doctor.
• Sex. On average, women carry more body fat than men at the same BMI.
• Height extremes. The squared-height formula is a simplification, and it tends to behave less well for very tall and very short adults.
• Pregnancy. The adult categories are not meant for use during pregnancy.

Children and teenagers are different

The adult cut-offs should not be applied to children or teenagers. Their bodies are growing, and what is typical changes with age and differs between boys and girls. For people aged 2 to 19, the CDC compares a child’s BMI with a reference group of the same age and sex and reports a percentile. A paediatrician usually interprets this alongside the child’s own growth curve over time, because the trend matters more than a single reading.

What else to look at

Where fat is stored matters. Fat around the abdomen is generally associated with greater health risk than fat stored on the hips and thighs, which is why many health organisations suggest adding a simple waist measurement to BMI.

• Waist circumference. Measured with a tape around the middle, at the level of the navel or midway between the lowest rib and the top of the hip bone, after breathing out normally. Commonly cited thresholds for raised risk are about 94 cm for men and 80 cm for women, with higher risk above about 102 cm and 88 cm. Different organisations use slightly different cut-offs, particularly for different ethnic groups, so treat these as rough guides.
• Waist-to-hip ratio. Waist measurement divided by hip measurement. For example, 85 cm ÷ 100 cm = 0.85. The WHO has cited ratios of about 0.90 for men and 0.85 for women as the point where abdominal obesity is considered present. You can work it out with the waist-to-hip ratio calculator.
• Body fat percentage. More direct, but the methods vary in accuracy. Home scales and tape-measure formulas are estimates with real error bars; clinical methods are better but not always accessible.
• Everything else. Blood pressure, blood sugar, cholesterol, fitness level, sleep, family history, and how you feel day to day tell you more about your health than any single body measurement.

Measuring accurately

1. Weigh yourself at a consistent time, ideally in the morning, in light clothing.
2. Measure height without shoes, standing straight against a wall. Many people are a centimetre or two off from the height they remember, and since height is squared, that error flows into the result.
3. Use the right units. Mixing centimetres with pounds is the most common source of nonsense results.
4. Look at the trend over months rather than a single reading.

When to talk to a doctor

A BMI calculator is a reasonable first check, but speak to a doctor or other qualified clinician if:

• your BMI is below 18.5 or at 30 or above, or you are unsure what your result means for you;
• your weight has changed noticeably without you trying;
• you have a medical condition, are pregnant, or take medication that affects weight;
• you are concerned about a child’s growth;
• you find yourself preoccupied with your weight or food in a way that feels unhealthy.

A professional can look at your whole picture, which a formula with two inputs never can.

The bottom line

Use BMI as one rough signal, not a score. Combine it with a waist measurement, pay attention to trends, and let a clinician interpret the result for your age, sex, ethnicity, and medical history. If you want to see where you land, the BMI calculator does the arithmetic in either unit system, and the ideal weight calculator and TDEE calculator are natural next steps if you are planning changes with professional guidance.