In more detail
BMI = weight (kg) / height (m)²; with pounds and inches, multiply lb / in² by 703. The WHO adult bands are under 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight and 30 or more obese, with obesity classes starting at 30, 35 and 40. Some Asian populations carry more health risk at a lower BMI, so lower action points (such as 23 and 27.5) are often used for them. Children and teenagers are assessed with age- and sex-specific percentiles, not these bands.
Why it matters
BMI is cheap and reproducible, which makes it useful for screening and for tracking trends across a population. It cannot tell muscle from fat or show where fat is stored, so a muscular athlete can read as overweight while an older person with little muscle can fall in the normal band. Combine it with the Waist-to-hip ratio or Body fat percentage, and ask a health professional for an individual assessment.
Example
70 kg, 1.75 m: 70 / 1.75² = 22.86 (normal)
154 lb, 69 in: 703 × 154 / 69² = 22.74
Normal band at 1.75 m: 18.5 × 1.75² to 24.9 × 1.75² = 56.7 to 76.3 kg
95 kg, 1.80 m rugby player: 95 / 1.80² = 29.32 ("overweight", despite little fat)