BMI Calculator (Adults)

Adult BMI with WHO categories

For adults aged 20 and up, Body Mass Index comes from the old Quetelet formula: BMI = weight / height², weight in kg and height in m. Take someone at 70 kg and 1.72 m: that works out to a BMI near 23.66, which lands in the Normal range. The WHO splits the scale this way: below 18.5 is underweight, 18.5–24.9 normal, 25.0–29.9 overweight, then obesity class I at 30.0–34.9, class II at 35.0–39.9, and class III (severe) once you hit 40. Keep in mind it's a screening number meant for populations, so it says nothing about body composition or where fat sits, and the thresholds aren't universal (Asian populations often shift the cut-points down to 23 and 27.5). If you're an athlete or you're older, a waist measurement, body-fat percentage or FFMI will tell you a lot more.

Applications

You'll see it in routine primary-care checks, in occupational-health and insurance assessments, and in public-health surveillance. It also sets eligibility thresholds for bariatric surgery and some medications, and people use it to keep tabs on their own weight-loss or maintenance plans.

FAQ

Is BMI valid for everyone? No. In muscular people it reads too high (a strength athlete at BMI 28 usually isn't "overweight"), and in sedentary folks who weigh normally but carry a lot of fat it reads too low, the so-called "skinny-fat" case.

Does it apply to children? No. Below age 20 you use percentile-based growth charts (WHO/CDC z-scores), not the fixed cut-points.

Should I worry at BMI 24.5? That's near the top of normal, nothing alarming. What your number does over time, your waist measurement and your lipid profile tell you far more than one reading.