BMI: What the Number Means and What It Misses

BMI is a ratio of body weight to the square of height. It was designed as a quick population-level screening index, not a diagnosis for an individual. It is cheap to compute and needs only two measurements, which is why clinics, insurers and public-health surveys use it.

In metric units, with weight in kilograms and height in metres:

BMI = weight (kg) ÷ height (m)²

In imperial units, with weight in pounds and height in inches:

BMI = 703 × weight (lb) ÷ height (in)²

Worked examples: a person weighing 70 kg and 1.75 m tall has BMI 70 ÷ 3.0625 ≈ 22.9. Someone at 85 kg and 1.80 m has 85 ÷ 3.24 ≈ 26.2. In imperial units, 154 lb at 69 in gives 703 × 154 ÷ 4,761 ≈ 22.7, which agrees with the metric result for the same person within rounding.

For adults aged 20 and over, the World Health Organization categories are widely used:

At a height of 1.75 m, those boundaries correspond to about 56.7 kg (18.5), 76.6 kg (25) and 91.9 kg (30). Using the boundaries in reverse — weight = BMI × height² — is a quick way to see the range associated with a given height.

• Body composition. BMI cannot distinguish muscle from fat. A muscular athlete can be in the overweight category without excess body fat.
• Fat distribution. Fat stored around the abdomen is associated with higher health risk than the same amount stored elsewhere, and BMI does not capture this.
• Age. Older adults may lose muscle and height, which changes what a given BMI means.
• Sex and ethnicity. Average body composition differs between groups, and some health bodies recommend different action thresholds for some populations.
• Children and teenagers. For people under 20, BMI is interpreted against age- and sex-specific growth percentiles rather than the adult cut-offs, so an adult calculator is not appropriate for a child.
• Pregnancy. Weight gain is expected and BMI categories do not apply in the usual way.
• Health itself. Fitness, diet, blood pressure, blood sugar, family history and many other factors matter more for an individual than BMI alone.

Several simple measurements add information. Waist circumference and the waist-to-hip ratio (waist ÷ hip) reflect abdominal fat; commonly cited health-agency thresholds for increased risk are about 0.90 for men and 0.85 for women. Body-fat estimates (for example from skinfolds, the U.S. Navy circumference method, or bioimpedance devices) try to separate fat from lean mass, though they have their own margins of error. Blood pressure, blood tests and activity levels measured by a clinician give a far fuller picture.

• Treat it as a starting point for a conversation, not a verdict.
• Look at the trend over time, measured the same way, rather than a single reading.
• If your result is outside the normal range, check it against waist measurement, fitness and any symptoms, and talk to a clinician before changing diet or exercise significantly.
• If you are very muscular, pregnant, under 20 or over about 65, treat adult categories as especially rough.

A small error in height has a large effect because it is squared. Measuring height with shoes on, or rounding 1.74 m to 1.80 m, changes BMI noticeably. Weigh yourself at a consistent time of day. Enter height and weight in the units the form asks for; mixing centimetres with inches, or pounds with kilograms, is the most common cause of absurd results.

The adult categories above do not apply to children and teenagers, whose body shape changes as they grow; for them, BMI is interpreted against age- and sex-specific growth charts, usually as a percentile. In older adults, losing muscle can leave BMI unchanged while body composition worsens. Some health organisations also use lower thresholds for people of certain ancestries, because risk can appear at lower BMI values. If you are in any of these groups, ask a clinician which reference applies to you.

• Using adult cut-offs for children.
• Reading the category as a diagnosis.
• Entering height in centimetres where the form expects metres.
• Forgetting the 703 factor in imperial units.
• Comparing your BMI with someone else’s as if it measured the same thing.

The BMI Calculator accepts metric or imperial inputs and shows the category. The Waist–Hip Ratio and Body Fat tools offer a second view. All results are educational estimates; they do not replace a clinical assessment.