Nominal and actual sizes: lumber and plywood

Almost nothing in a lumberyard measures what its label says. A 2x4 is not 2 by 4 inches, and a sheet of 3/4-inch plywood is usually not 3/4 inch thick. The names are nominal sizes, a trade description. The numbers that matter for a design are the actual sizes you measure.

The difference is small on one part and large across an assembly. It is also the most common reason a dado that should be snug is sloppy, or a cabinet ends up a fraction of an inch off.

Softwood dimensional lumber

Construction lumber is named by its rough green size and sold dried and surfaced. In North America the finished sizes are standardised:

Nominal	Actual (dry, surfaced)
1x (boards)	3/4” thick
2x	1-1/2” thick
4x	3-1/2” thick
2 wide	1-1/2”
3 wide	2-1/2”
4 wide	3-1/2”
6 wide	5-1/2”
8 wide	7-1/4”
10 wide	9-1/4”
12 wide	11-1/4”

So a 2x4 is 1-1/2 × 3-1/2 inches, a 1x6 is 3/4 × 5-1/2 inches, and a 2x10 is 1-1/2 × 9-1/4 inches. Lengths are usually true or slightly long. Edges are eased, and construction lumber is often not dry enough for fine furniture without time to acclimate.

Hardwood

Hardwood is named by its rough thickness in quarters of an inch: 4/4, 5/4, 8/4 and so on. Widths and lengths are random. A 4/4 board is about 1 inch rough and typically about 13/16 inch once surfaced; you will usually mill it to 3/4 inch. The full table, and how the dealer charges for it, is in board feet.

Plywood and sheet goods

Sheet goods vary the most, and the label is the least reliable:

Sold as	Commonly measures
3/4” softwood or construction plywood	23/32” (0.719”)
3/4” hardwood plywood	about 0.70” to 0.72”, varies by mill
18 mm Baltic birch	0.709”
1/2” plywood	15/32” (0.469”)
12 mm Baltic birch	0.472”
1/4” hardwood plywood	often 7/32” or 5.2 mm (about 0.20”)
6 mm Baltic birch	0.236”
3/4” MDF	usually close to 0.75”, but check

Two sheets with the same label from different mills, or even from different bundles, can differ by a few hundredths of an inch. Thickness can also vary across a single sheet.

Why a thirty-second matters

• Joints. A 3/4-inch dado for a shelf that is really 23/32 inch leaves a 1/32-inch gap. It will glue, but the shelf can rock during assembly and the joint shows. Size dadoes, grooves and rabbets from the stock you have; see dadoes and rabbets in plywood.
• Accumulation. A case with five vertical partitions of “3/4-inch” plywood that are each 1/32 inch thin comes out 5/32 inch narrower than drawn. Doors, drawers and face frames sized from the drawing will not fit.
• Hardware. Drawer slides and hinges have tight tolerances; a drawer opening 1/16 inch narrower than planned can make side-mount slides bind.

Measure, then design

1. Buy the sheet or board before you finalise the dimensions that depend on its thickness.
2. Measure thickness with calipers at several points, away from the very edge where the sheet may be crushed or swollen. Use the average, and note the range.
3. Mark the measurement on the stock so it does not get mixed up with another sheet.
4. Put the measured thickness into your drawing or model and let the dependent sizes follow it.

In GrainDraft, give each part the thickness you measured, not the label. Joints such as dadoes and rabbets are sized from the modelled stock, so a shelf modelled at 0.709 inch gets a housing that fits 0.709 inch. Where you named a thickness and linked other sizes to it with formulas, changing that one value updates the parts that depend on it, and the Dimensions preview shows what will change before you apply it.

A quick reference

• Dimensional lumber: subtract 1/4” from nominal thickness under 2x, 1/2” from 2x and 4x; widths up to 6 are 1/2” less, 8 and wider are 3/4” less.
• Hardwood: know the surfaced thickness your yard delivers.
• Sheet goods: never trust the label. Measure every sheet.

Related: wood movement, sizing a mortise and tenon.