A 27-inch waist can equate to anything from a size 4 to a size 6 in women's pants sizes. Sizing varies dramatically from manufacturer to manufacturer and can also depend on the style and fit of the clothing. In traditional jeans sizing, a 27-inch waist would be a size 27.
Many people do not know how to accurately take a waist measurement, which can lead to buying the wrong size. The waist measurement should be taken at the natural waist, which is usually the narrowest part on a woman. The tape measure should be loose but not hanging and should never be pulled too snug.