As with hardwood and softwood sizes, plywood sizes can be somewhat confusing. Plywood sheets usually sold as four-feet wide, but are occasionally found in two and five-foot widths. Similarly, a plywood sheet's length is commonly eight-feet, but some suppliers carry four and twelve-foot lengths. Metric sizes are also available.
While the length and width of a sheet is pretty straight forward, the thickness dimensions are anything but. The most common sizes of plywood sold in the United States are 3/4", 1/2" and 1/4". However, that's not exactly correct. A 3/4" sheet of plywood is really 23/32" (and is now typically labeled as such). Similarly, 1/2" should be correctly labeled as 15/32" and 1/4" labeled as 7/32". However, everyone still refers to them as 3/4", 1/2" and 1/4".
Although it doesn't seem like much, this 1/32" differential between the actual size and the common size can become an issue when working with plywood. For instance, if a woodworker is building a bookshelf where a 3/4" plywood shelf is placed into a dado cut into the shelf standards, there will be a very noticeable 1/32" gap, and the shelf will feel a bit sloppy in the dado. To combat this, the dado should be cut at 23/32" to ensure a proper fit.