How it works
BMI = weight (kg) ÷ height (m)²
The index squares your height in meters and divides it into your weight in kilograms. Worked example: at 70 kg and 1.75 m the math is 70 ÷ 3.0625, which rounds to a BMI of 22.9. The WHO reads the number in four bands — below 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and above obese — and the calculator also inverts the formula against your height to show the weight span matching the normal band. Pounds and inches are converted before any math happens, so both unit choices land on the same BMI. Height and weight are the only variables, which is exactly why the index says nothing about muscle, bone density, or where fat sits.
Sources
- WHO BMI classification World Health Organization — BMI = weight(kg)/height(m)²; underweight <18.5, normal 18.5–24.9, overweight 25–29.9, obese ≥30.
- BMI limitations for athletes BMI does not distinguish lean mass from fat, so muscular athletes may be misclassified; body-composition measures are more informative for them.
- Origin of the index Adolphe Quetelet (19th c.) devised the weight-over-height-squared index; it was later termed Body Mass Index.
FAQ
How is BMI worked out from my height and weight?
Weight in kilograms divided by height in meters squared. Type 70 kg and 1.75 m into the tool and it computes 70 ÷ 3.0625 and returns 22.9, then labels the result with its WHO category. Enter pounds and inches instead and the conversion happens automatically.
What BMI range does the WHO call normal?
18.5 to 24.9. The calculator flags anything under 18.5 as underweight and anything from 25 upward as overweight or obese, and it also translates the normal band into actual kilograms or pounds for your height — the more practical number to remember.
Should runners trust their BMI?
Only as a first-pass screen. The formula has no idea whether a kilogram is quadriceps or fat, so strength-trained runners often score high while carrying little fat, and detrained light runners can score normal while being unfit. Body composition, bloodwork, and performance tell you far more.
Which units does the calculator accept?
Weight takes kilograms or pounds; height takes centimeters or inches — and you can mix them, entering pounds with centimeters if that is what you know. Everything is converted to metric before the division, so the BMI comes out identical whichever way you type it.
Is there an ideal BMI for distance running?
No single number. Durable, fast runners sit all across the normal band, and forcing BMI down through under-fueling costs bone density, hormones, and training quality long before it buys any speed. Aim for a weight you can fuel properly and repeat hard training weeks at.
Can I use the healthy weight range to set a target?
Use it as a wide bracket, not a bullseye. The range shown simply maps BMI 18.5–25 onto your height; where inside it you run best depends on your build and training history. Big moves in either direction deserve a conversation with a professional first.
BMI is a screening statistic built from height and weight alone: body fat, fitness, and health are outside what it can see, and muscular or very lean bodies often land in the wrong category. Treat the label as context and bring weight questions to a clinician — this page is not medical advice.