The formula, in both unit systems
Body mass index is a ratio of weight to height squared. In metric units it is weight in kilograms divided by height in metres squared. A person of 78 kg and 1.75 m calculates 78 ÷ (1.75 × 1.75) = 25.5.
In imperial units the same relationship is weight in pounds divided by height in inches squared, multiplied by 703. A person of 172 lb and 69 inches calculates (172 ÷ 4,761) × 703 = 25.4. The two methods agree; only the units differ.
The mistakes that skew the result
Most incorrect BMI values come from measurement error rather than arithmetic. Height is frequently rounded up, weight is often taken from memory, and mixing units mid-calculation produces numbers that are wildly out of range.
- Measure height without shoes, heels together, looking straight ahead.
- Weigh in the morning, after the bathroom, before eating, on the same scale.
- Square the height, do not double it — this is the single most common error.
- Never mix pounds with metres or kilograms with inches.
- Round only at the end, not at each step.
How the standard categories are defined
Public health bodies including the CDC and NHS group adult BMI as below 18.5 (underweight), 18.5 to 24.9 (healthy range), 25.0 to 29.9 (overweight) and 30.0 or above (obesity). These are population screening bands, not a diagnosis.
Children and teenagers are assessed against age- and sex-specific percentile charts instead, so an adult BMI category should never be applied to a child.
What the number cannot tell you
BMI does not distinguish muscle from fat, does not describe where body fat sits, and does not account for bone structure or differences between populations. A muscular athlete and a sedentary adult can share a BMI while having very different health profiles.
Waist circumference, waist-to-height ratio, blood pressure, blood lipids and fasting glucose all describe risk that BMI misses. Treat BMI as one screening input among several, and discuss results with a qualified healthcare professional.
Frequently asked questions
Is the BMI formula different for men and women?
No. The adult calculation is identical. Interpretation differs, because body composition at a given BMI varies between individuals.
Does BMI work for athletes?
Poorly. Higher muscle mass raises weight without raising body fat, so athletes are often classified as overweight by BMI while carrying low body fat.
How often should I recalculate BMI?
Every few months is plenty. It changes slowly, and daily recalculation mostly captures normal weight fluctuation.
Keep reading
- BMI vs Body Fat Percentage — What each measurement captures and where each falls short.
- What Is a Healthy Weight for Your Height? — How healthy weight ranges are derived, and why they are ranges.
- Why Body Weight Changes From Day to Day — The everyday reasons the scale moves without fat changing.
Educational only — not medical, nutritional, or fitness advice. Consult a qualified healthcare professional before changing diet, exercise, supplement, or medication routines.