SW Woodwork — Beginner and small-shop woodworking | swwoodwork.com
A 2x4 measures 38 x 89 mm and a 1x4 measures 19 x 76 mm — the names are nominal, the cut list is not.
Lumber is named by its rough-sawn dimension and sold after planing, so the name and the measurement never match. A nominal 2x4 is actually 1.5 x 3.5 inches (38 x 89 mm) once the board is planed to size, and a nominal 1x4 comes out at 0.75 x 3.25 inches (19 x 76 mm).
The pattern behind the numbers is simple. One-by stock planes to 19 mm thick and two-by stock to 38 mm thick, and widths shrink in the same pass. Because planing varies, the working habit is to measure the actual board in hand and write that number down — never the name on the rack.
On narrow screens, swipe or scroll the plate sideways.
Sheet goods behave the opposite way: the stated size is the real size. Panels are sold at 1220 x 2440 mm (4 x 8 ft), and plywood comes in thicknesses of 6, 9, 12, 15, 18 and 25 mm, quoted in inches across the common range as 1/4, 3/8, 1/2, 5/8 and 3/4. An 18 mm panel is the standard shelf material.
Weight is the hidden dimension of sheet goods. One 1220 x 2440 x 18 mm sheet of MDF weighs about 35-40 kg because MDF density runs 700-800 kg per cubic metre, so moving and cutting MDF is a two-person plan, while plywood of the same size is noticeably easier to handle.
A cut list written in finished millimetres connects everything else in the build. The 19 mm thickness picks the 32 mm pocket screw from the first tools list, the 18 mm panel sets the 600 mm span limit described on the main page, and the width of each solid board tells you how much it can move — the subject of the moisture and movement page.
Further reading