Body mass index (BMI) is a simple screening measure calculated from height and weight — it doesn't directly measure body fat, and it was originally developed for population-level statistics rather than individual diagnosis. The World Health Organization's standard adult categories are: under 18.5 (underweight), 18.5–24.9 (healthy weight), 25–29.9 (overweight), and 30+ (obese) — the same ranges this calculator uses.
BMI's biggest known limitation is that it can't distinguish muscle from fat. Someone with a lot of muscle mass — an athlete, for example — can have a "high" BMI while carrying very little body fat, because muscle is denser than fat. Similarly, BMI doesn't account for where fat is distributed on the body, which matters for health risk in ways BMI alone can't capture. Because of this, healthcare providers typically treat BMI as one data point among several, not a standalone diagnosis.
Not reliably — since muscle weighs more than fat by volume, a muscular person can show a BMI in the "overweight" range despite having low body fat. This is one of the most well-documented limitations of the BMI measure.
Standard adult BMI categories aren't designed for pregnancy, since healthy weight gain during pregnancy is expected and tracked differently. Pregnant users should rely on guidance from their healthcare provider rather than standard BMI categories.
No — this tool uses the adult BMI formula and WHO adult categories. Children and teens require age- and sex-specific percentile charts (like those from the CDC) rather than the fixed adult thresholds used here.
BMI is a starting point for a conversation, not a conclusion — a doctor can consider your BMI alongside other factors (body composition, family history, activity level, and overall health) to give guidance that's actually specific to you.