In one sentence
Your weight divided by the square of your height, a number that says how heavy you are for your height and nothing about what the weight is made of.
Why it exists
BMI was built for populations, not people. It lets a study compare thousands of adults with one number, and the familiar bands (under 18.5, 18.5 to 24.9, 25 to 29.9, 30 and over) are the World Health Organization's population cut-points. Almost every laboratory reference range and every large study you will read about uses it, which is the only reason Vitality Tracker shows it.
Why it fails for you
It cannot tell fat from muscle. A muscular person reads "overweight"; an older adult who has lost muscle and gained fat can read "normal" at a body fat that is anything but. At the same BMI, body fat differs between people by ten points or more, which is why the DEXA reference curves VT uses for the body-fat percentile are indexed to age and sex and not to BMI (Kelly 2009).
How Vitality Tracker uses it
It is displayed, and it enters three calculations because their published formulas ask for it: the hepatic steatosis index on the labs dashboard, the metabolic age (whose BMI term is one of two shape terms), and the ideal weight range. VT never scores on it and never bands you by it.
How far to trust the number
Calculated, exactly, from weight and height; the limits are in what it means, not in the arithmetic.
Sources
- World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894, 2000
- Kelly TL, Wilson KE, Heymsfield SB. Dual energy X-ray absorptiometry body composition reference values from NHANES. PLoS One 2009;4:e7038. doi:10.1371/journal.pone.0007038.