What is BMI?
Body Mass Index (BMI) is a single number that describes how your weight relates to your height. It is calculated by dividing weight in kilograms by height in meters squared, or with the imperial equivalent used above. First described by Belgian statistician Adolphe Quetelet in the 1830s, BMI became the worldwide standard because it is quick, free, and needs nothing more than a scale and a tape measure.
The World Health Organization sorts adult results into four standard bands:
- Below 18.5 — Underweight
- 18.5–24.9 — Normal weight
- 25.0–29.9 — Overweight
- 30.0 and above — Obese
A BMI between 18.5 and 24.9 is associated with the lowest overall health risk in large population studies. The calculator above uses these WHO cutoffs and also shows the weight range that would place you in the normal band at your exact height.
Limitations of BMI
BMI measures weight, not composition — it cannot tell muscle from fat. People with high muscle mass often register as overweight despite carrying little excess fat, which is why athletes frequently score higher than their physique suggests. The measure also ignores where fat is stored: fat around the abdomen carries more risk than fat elsewhere, and BMI says nothing about distribution. Age matters too, since older adults tend to have a different body composition at the same score, and children and teenagers need age- and sex-specific percentile charts instead of the fixed adult cutoffs.
When to talk to a doctor
Treat BMI as a screening tool, never a diagnosis. A result outside the normal range does not automatically mean poor health, and a normal result does not guarantee it. If your BMI falls outside 18.5–24.9, or you are losing or gaining weight without trying, bring it up with a healthcare professional who can assess the fuller picture using waist circumference, blood pressure, blood work, and family history.