Lumber is cut to nominal size when green and wet, then dried and planed smooth — which removes material. The nominal name stuck even though the actual size shrank. Always use actual dimensions when planning cuts and spacing.
A 2×4 is not 2 inches by 4 inches, and that single fact is behind a large share of DIY mistakes. Here are the sizes lumber actually is.
How it works
Lumber is named for its rough size before it is dried and planed smooth. That milling removes about 1/4 inch from each face, so a nominal 2×4 arrives at your house measuring 1 1/2 × 3 1/2 inches. The nominal name stuck; the actual dimensions are what you have to build with.
Example
A 2×4 is really 1 1/2 × 3 1/2 inches. A 1×6 is really 3/4 × 5 1/2. A '3/4-inch' sheet of plywood is usually about 23/32.
Tips & common mistakes
Always design with the actual dimensions, never the nominal ones. A wall of five '6-inch' boards is 27 1/2 inches of board, not 30 — and that 2 1/2-inch difference will wreck your spacing.
Plywood is the sneakiest one. Sheet goods are often a hair under their stated thickness, and in joinery that hair is the difference between a tight joint and a sloppy one.
Measure the actual boards you bought rather than trusting any chart, including this one. Mills vary, and moisture makes lumber move after it leaves the yard.
Hardwood is sold by quarters of an inch of rough thickness — 4/4 is one inch rough, which finishes to about 13/16. It is a completely different naming system from construction lumber.
Frequently asked questions
What are the actual dimensions of a 2x4?
1 1/2 inches by 3 1/2 inches. The '2×4' refers to its rough size before drying and planing.
Why is lumber smaller than its name?
It is named at its rough-sawn size. Drying and planing it smooth removes roughly a quarter inch from each face, but the original name stayed.
How wide is a 1x6 board really?
5 1/2 inches, and 3/4 of an inch thick. This is the single most common reason accent-wall spacing comes out wrong.