Shrinkage is the reduction in a clay body's size that happens in two separate stages: once as water leaves the clay during drying, and again as the body vitrifies during firing. Total shrinkage from wet clay to finished, fired piece is typically in the range of ten to fifteen percent depending on the specific body, which is why potters generally throw or build a piece noticeably larger than its intended finished size.
Drying shrinkage happens as water sitting between clay particles evaporates and the particles draw closer together; drying too quickly, or unevenly across a single piece, is a common cause of cracking at this stage. Firing shrinkage happens separately as the body vitrifies and its particles fuse, closing pore space that drying alone does not remove.
Shrinkage matters beyond just sizing a piece correctly: a clay body's shrinkage rate has to be reasonably compatible with any glaze, slip, or additional clay applied to it, since a significant mismatch in how much two joined or layered materials shrink is a common source of cracking, separate from the thermal-expansion mismatch that causes crazing after firing is complete.
Because a given clay body shrinks by a fairly consistent, testable percentage, potters can plan for it directly with a shrinkage ruler: a ruler pre-stretched by that body's known shrinkage rate, so a measurement taken on a piece while it is still wet already shows what the finished, fired size will be, without needing to calculate the percentage by hand each time. Manufacturers publish a shrinkage percentage for each clay body alongside its recommended firing range for exactly this reason.
Drying shrinkage and firing shrinkage are normally measured and reported as separate figures, since each responds to different variables (particle size and water content for drying; flux content and peak temperature for firing) and a body can behave well at one stage while remaining troublesome at the other. As a rough sense of scale, dry-to-fired shrinkage (the firing stage measured on its own, after drying is already complete, not total wet-to-fired shrinkage) runs around 3 to 4 percent or less for earthenware, roughly 5 to 6 percent for stoneware, 7 to 8 percent for whiteware bodies, and can exceed 10 percent for a fully vitrified porcelain. These figures track flux content rather than temperature directly: more flux-derived glass forms and pulls particles together as a body approaches full vitrification, regardless of the specific temperature at which that happens for a given body.