And for wood carving purposes, we’ll just be looking at straight, out-canel gouges, where the sharpened bevel is on the bottom of the blade. Bent gouges are specialty tools that allow you to approach the work at a steeper angle, as with bowl carving and other projects that involve carving out a cavity. And In-canel gouges are more used for specialty tasks, such as window sash work.
Wood carving tools sizing classifications differ depending on if the manufacture is (or was) in Europe or in Britain. The British included a skewed size in the lower sizes, and the Europeans didn’t, so that can make it difficult to compare apples to apples. But fortunately, the most popular modern manufacturer, Pfeil (A Swiss company, pronounced “File”), has become the standard for carving gouges, due to their quality and affordability, so their sizing system is most commonly used. The carving gouge sweep sizes lie on a spectrum from #1 (a flat carving chisel, with no curve), up to a U-shaped #11 sweep (the largest curve). And the largest sweeps are #12 through #16, but they aren’t curved, but v-shaped (e.g. “V-parting tool” or “V-gouge”). V-gouges are used to outline the elements of a carving, preparatory to giving the carving depth with the less-curved gouges.
In the case of Pheil wood carving gouges, the first number printed on the handle is the sweep, or curvature, and the second number is the width of the blade in millimeters. Below are a couple examples of caving gouge sizes. In the first photo you’ll see a Pfeil Swiss made carving gouge, with the marking “5/12”:
The number 5 refers to the curvature, and the number 12 refers to the width (in millimeters). So all the sizes on the above chart have a #5 sweep. Below is one more example, to provide a point of reference:
The gouge is stamped with 8/13. The number 8 would be the sweep, or curvature. The number 13 would be the width (in millimeters). So again, all the sizes in this chart have the same sweep (or curvature) of 8.
In the video below, Bill Anderson talks about the basics of wood carving tools for beginners. He specifically discusses sizes, and shows some introductory carving techniques. You can also read the accompany article here