Practical guide
How to Calculate BMI
Use the BMI formula correctly and interpret standard BMI ranges.
Quick Answer
Step-by-Step Method
- Measure weight and height in consistent units — kilograms and metres, or pounds and inches.
- Square the height. For 1.75 m that is 3.0625; for 68 in that is 4,624.
- Divide weight by the squared height. If you are working in imperial units, multiply the result by 703.
- Compare the number against the adult BMI bands, and read it as a screening signal rather than a diagnosis.
Worked Example
Metric: a person weighing 70 kg at 1.75 m has BMI = 70 ÷ 3.0625 = 22.9, inside the normal range. Imperial: 160 lb at 68 in gives BMI = 703 × 160 ÷ 4,624 = 24.3, also normal but close to the overweight boundary at 25.
Detailed Explanation
BMI was designed by Adolphe Quetelet in the 1830s to describe populations, not individuals, and that origin explains most of its limitations. It measures total mass against height without knowing what the mass is made of. A muscular athlete and a sedentary person of the same height and weight produce the same BMI despite very different body composition, which is why sports scientists rarely use it alone.
The formula also treats height as if body mass scaled with the square of height, when real bodies scale closer to a power between 2 and 3. The practical consequence is that BMI reads slightly high for tall people and slightly low for short people. It says nothing at all about where fat sits, and abdominal fat carries more metabolic risk than the same mass on hips or limbs — a waist-to-height ratio captures that better.
Treat BMI as one cheap input among several. It is genuinely useful for tracking your own trend over months, because your height is fixed and your body composition changes slowly, so a moving BMI mostly reflects moving weight. Standard adult bands also do not apply to children, pregnant people, or the elderly, and several health bodies apply lower thresholds for people of South Asian descent.
Use the Calculator
Run your real values in the interactive tool and review assumptions before taking action.
Open related calculatorFAQ
Is BMI accurate for athletes?
Often not. Muscle is denser than fat, so athletes with low body fat frequently land in the overweight band. Body-fat percentage or waist measurements are more informative for trained populations.
Does BMI work the same for children?
No. Children and teenagers are assessed with age- and sex-specific percentile charts, because normal body composition changes throughout growth. Adult cut-offs do not apply under 20.
What is a healthy BMI range?
For most adults, 18.5 to 24.9 is the standard reference range. Some health authorities use a lower upper bound of 23 for people of South Asian descent, reflecting higher metabolic risk at the same BMI.