In one sentence
The distance around your middle, and around your hips, measured with a tape; the first is the best home measure of where your fat sits.
Why it matters
Weight and body fat say how much; waist says where. Fat around the middle is the depot most closely tied to the metabolic measures in a blood panel, and a tape captures it better than a scale does. In a review of 31 studies covering more than 300,000 adults, the waist-to-height ratio picked out people with raised blood pressure, blood fats and glucose better than BMI did (Ashwell 2012). In Vitality Tracker the waist is the third input to your metabolic age and 40 percent of the body-composition area of the Vitality Score; without it, neither is complete.
What moves it
Fat around the middle, slowly; and the measurement itself, quickly. Where the tape sits, how tight it is, whether you have just eaten, whether you breathed in: each is worth a centimeter or more. Two sites are in common use, and they differ by a few centimeters on many bodies: the top of the hip bone (the iliac crest, the site the US survey behind VT's metabolic age used; Fryar 2021) and the midpoint between the lowest rib and the hip bone (the World Health Organization's site; WHO 2011). At the metabolic age's sensitivity of about a third of a year per centimeter, a change of site is worth about a year, which is why VT asks you to keep one site and says which.
How to measure it well
How to measure your waist has the routine: in the morning, unclothed, tape level at the top of the hip bone, snug without pressing, at the end of a normal breath out, twice, averaged. Hip at the widest point of the buttocks. Every two to four weeks, dated.
How Vitality Tracker uses it
With your height it gives the waist-to-height ratio, and with the hip the waist-to-hip ratio. The waist is dated, and VT prompts for a fresh one when the stored value is about a month old, because a stale waist is the metabolic age's main source of error. Entries from a connected tape and typed entries are kept as separate series.
How far to trust the number
Measured, and only as good as the technique; a careful tape beats any scale's estimate of central fat.
Sources
- Ashwell M, Gunn P, Gibson S. Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors: systematic review and meta-analysis. Obes Rev 2012;13:275–86. doi:10.1111/j.1467-789X.2011.00952.x
- Fryar CD, Carroll MD, Gu Q, Afful J, Ogden CL. Anthropometric reference data for children and adults: United States, 2015–2018. NCHS Vital Health Stat 3(46), 2021
- World Health Organization. Waist circumference and waist–hip ratio: report of a WHO expert consultation, Geneva, 8–11 December 2008. WHO, 2011
- Vitality Tracker, Metabolic age method (S-AnthropoAge), October 2026.