Waist

The waist is the part of the abdomen between the rib cage and hips. Normally, it is the narrowest part of the torso.

Waistline refers to the horizontal line where the waist is narrowest, or to the general appearance of the waist.

Because of this and because the waist is often synonymous with the stomach, one can become confused as to the exact location of the waist. Another confusing factor is that the waistline differs on different people. A study showed that self-reported measurements, as opposed to measurement done by a technician, underestimated waist circumference and this underestimation increased with increased body size. In the study, waist circumference measured at the level of the umbilicus was larger than that measured at the natural waist.

To locate the natural waistline, one need simply stand upright and then tilt over to the side, keeping the legs and hips straight. Where the torso creases is the natural waistline.^{[citation needed]}

Strictly, the waist circumference is measured at a level midway between the lowest palpable rib and the iliac crest. This is the definition used by the World Health Organization.

Practically speaking, the waist is usually measured at the smallest circumference of the natural waist, usually just above the belly button. Where the waist is convex rather than concave, as in pregnancy and obesity, the waist may be measured at a vertical level 1 inch above the navel. Some laypersons use the level at the navel, but this can lead to underestimates.

When using an automated body scanner that produces a point cloud, neither the bones nor the navel can be located from the scan itself. A commonly used definition is the circumference at 4 cm above small of the back (SOB), though this does not appear to be the best fit for the strict definition, and 2 cm above SOB appears better (in a sample of 106 women of different ages, heights, weights, and body shapes).

Variables such as posture significantly influence the measurement of the waist, and therefore any measurements for a group need to maintain a constant posture between the subjects.