BMI Explained: What the Number Can and Can't Tell You

BMI is your weight divided by your height squared. In metric units that's kilograms divided by meters squared, so 70 kg at 175 cm works out to 70 ÷ 1.75² = 22.9. In pounds and inches the formula is 703 times your weight divided by your height squared: 180 lb at 5'9" (69 inches) is 703 × 180 ÷ 69² = 26.6. The BMI calculator on this site does the arithmetic for you.

The World Health Organization's adult categories are below 18.5 for underweight, 18.5 to 24.9 for normal weight, 25 to 29.9 for overweight, and 30 or above for obesity. So that 26.6 lands in the overweight range and the 22.9 in the normal range.

The formula comes from Adolphe Quetelet, a Belgian mathematician who was interested in describing averages across whole populations in the 1830s and 1840s. He wasn't trying to diagnose anyone. The name "body mass index" only arrived in 1972, when the physiologist Ancel Keys used it in a paper comparing ways to estimate body fatness in large groups. It caught on because it was cheap and needed nothing more than a scale and a tape measure.

That history matters. BMI was built to describe groups of people, and for that job it works reasonably well. It's a weaker tool when you point it at one individual.

BMI can't tell muscle from fat. A muscular person can land in the overweight range with very little body fat, and an older person who has lost muscle can land in the normal range while carrying more fat than the number suggests. It also says nothing about where the fat is stored, and fat around the abdomen is linked to more health risk than fat on the hips or thighs.

Age, sex and ethnicity matter too. Health risks can start rising at lower BMIs for people of South and East Asian descent, which is why some health bodies suggest lower cutoffs for those groups. For children and teenagers the adult categories don't apply at all. Pediatricians compare a child's BMI with growth charts for their age and sex.

Waist measurements cover the biggest blind spot. The waist-to-height ratio compares your waist with your height, with 0.5 (a waist half your height) as the common reference point, and the waist-to-hip ratio looks at fat distribution from another angle. Body fat estimates get closer to what BMI is trying to stand in for, though home methods are rough.

None of these replace measurements a clinician can take, such as blood pressure, blood sugar and cholesterol. Those say more about your health than any ratio of weight to height does.

Treat BMI as a first screen. If it falls outside the normal range, that's a reason for a conversation, not a verdict. If it falls inside the normal range, that doesn't prove everything is fine either. The useful question is what a doctor, who can see your whole picture, makes of it. If you're looking at weight targets, the ideal weight calculator gives formula-based ranges, and they carry the same limits.