About Body Mass Index (BMI)
BMI is a screening tool used to identify possible weight problems in adults. It divides your weight (in kg) by the square of your height (in meters). While not a perfect measure of health as it does not account for muscle mass, it is a useful starting point.
BMI Categories
- Underweight: BMI less than 18.5
- Normal weight: BMI 18.5 to 24.9
- Overweight: BMI 25 to 29.9
- Obesity: BMI 30 or greater
The Formula
Imperial: BMI = 703 × weight(lb) / height(in)²
For example, a person weighing 70 kg at 1.75 m tall has a BMI of 70 / (1.75 × 1.75) = 22.9, which falls in the "Normal Weight" range. The imperial formula is simply a unit-converted version of the same calculation.
What BMI Measures — and What It Doesn't
BMI cannot distinguish between muscle and fat. A muscular athlete and a sedentary person of the same height and weight will show an identical BMI despite very different body compositions. It also doesn't account for age, sex, bone density, or where fat is stored. That's why BMI is best treated as a quick population-level screening tool and a starting point for conversation — not a personal diagnosis.
Common Mistakes
- Forgetting to square the height — a frequent arithmetic slip that produces a nonsensical result.
- Using centimeters without converting to meters — always divide cm by 100 first in the metric formula.
- Treating BMI as a complete health verdict — pair it with body fat percentage or waist circumference for a fuller picture.