QI = [ Weight (lb) x 703 ] ÷ [ Height (in) ]2
The calculated result for adults means:
< 18.5 underweight
18.5 - 24.9 Normal
25.0 - 29.9 Overweight
≥ 30.0 Obese
The index is the most common and quickest way to measure obesity but is not entirely accurate because it doesn't distinguish between fat and muscle. Two people of the same height and weight but with differing lean body mass (one is more muscular) will have the same BMI but intuitively, you know one is probably healthier than the other just by visual alone. There have been many criticisms over the years of its inaccuracy but I doubt its use will change any time soon. It's too ingrained in today's society and more accurate ways to measure body fat are more involved (hyrdostatic weighing) and expensive (DXA). Besides, people know when they need to lose or gain weight specially with reminders from friends and family.