The density of plywood is roughly 34.08 pounds per cubic foot. Because plywood is a flat material, it is generally measured in mass per area rather than mass per volume.
Thicker plywood is heavier per each square foot. Quarter-inch plywood is 0.71 pounds per square foot, 3/8-inch plywood is 1.06 pounds per square foot, 1/2-inch plywood is 1.42 pounds per square foot, 5/8-inch plywood is 1.77 pounds per square foot and 3/4-inch plywood is 2.13 pounds per square foot.
Plywood's density puts it roughly in the middle of the range of densities of woods. Balsa, one of the most lightweight woods, is sometimes as light as 7 pounds per cubic foot. On the other hand, the hardwoods ebony and lignum vitae can be as dense as 83 pounds per cubic foot.