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Guide to BMI
What is BMI?
Body Mass Index (BMI) is the most popular method for assessing body mass relative to height. Developed by Belgian statistician Adolphe Quetelet in the 19th century, it became a standard tool in medicine and health promotion. BMI is calculated by dividing body weight in kilograms by height in meters squared. It doesn't directly measure body fat but correlates well with health risks.
BMI categories
- Underweight - BMI below 18.5, may indicate malnutrition or health issues
- Normal weight - BMI 18.5-24.9, optimal range for health
- Overweight - BMI 25-29.9, increased risk of some diseases
- Obesity - BMI 30 or higher, significantly elevated health risk
BMI limitations
BMI doesn't distinguish between muscle mass and body fat, so athletes may have high BMI despite low body fat. Fat distribution also matters - visceral fat (around organs) is more dangerous than subcutaneous fat. BMI doesn't account for sex, age, or body build either. Additional measurements are used for more accurate assessment.
Health consequences
Both underweight and obesity increase the risk of disease and premature death. Obesity increases risk of type 2 diabetes, heart disease, hypertension, certain cancers, and joint problems. Underweight can lead to osteoporosis, weakened immune system, and fertility issues. Maintaining BMI in the normal range is one of the simplest ways to care for health.
Two people with identical body shape get BMIs of 21.5 and 25.5
BMI divides weight by height squared, but bodies are three-dimensional and scale roughly with height cubed. That mismatch means BMI drifts upward with height even when body composition is unchanged. Scale a 1.60 m person of 55 kg up to 1.90 m without altering their proportions at all and their BMI moves from 21.5 to 25.5 — from mid-normal to overweight, on geometry alone.
How it works
- Divides weight in kilograms by height in metres squared, the standard definition.
- Places the result against the conventional categories, which are the same numbers for everyone.
- Shows the weight range each category implies for your height, which is more useful than the single figure.
BMI = weight (kg) ÷ height (m)² under 18.5 underweight 18.5 – 24.9 normal 25.0 – 29.9 overweight 30.0 and up obese bodies scale as height³, the formula divides by height² — the mismatch is the whole problem
Worked example
One body shape, scaled to four heights without changing its proportions.
- 1.60 m, 55.0 kg → BMI 21.5
- 1.70 m, 66.0 kg → BMI 22.8
- 1.80 m, 78.3 kg → BMI 24.2
- 1.90 m, 92.1 kg → BMI 25.5
Every one of these is the same body, photographically identical apart from scale. BMI still rises 18.75% across the range — exactly the height ratio — because it divides by a square while the body grows as a cube.
Reading the result
- The categories were derived from population averages, not from individual diagnosis, and Quetelet devised the index in the 1830s to describe groups rather than to assess a person. Treating a population statistic as a personal verdict is the underlying error.
- It cannot distinguish muscle from fat, because it only sees total mass. A trained athlete routinely lands in the overweight band with very little body fat, and the formula has no mechanism to notice.
- The height bias runs both ways. To reach the same BMI of 25, someone 1.60 m needs 64.0 kg and someone 1.90 m needs 90.3 kg — but the taller person hits that threshold at a lower relative body fat, so the cutoff is effectively stricter for them.
- Waist circumference and waist-to-height ratio predict health outcomes better and take one measurement. A waist under half your height is a more useful target than any BMI figure.
Common questions
- Is BMI useless then?
- Not useless, but it is a screening tool for populations rather than a measurement of an individual. It is cheap, needs no equipment and tracks reasonably across large groups. As a statement about one specific person it carries far less information than it appears to.
- Why does being tall push my BMI up?
- Because mass grows with the cube of height while the formula only divides by the square. For a fixed body shape BMI is proportional to height, so a 1.90 m frame reads 18.75% higher than an identically proportioned 1.60 m one.