Nominal vs Actual Lumber Sizes

The full chart

Nominal	Actual (in)	Board feet per 8 ft	Actual BF per 8 ft
1 × 2	¾ × 1½	1.33	0.75
1 × 3	¾ × 2½	2.00	1.25
1 × 4	¾ × 3½	2.67	1.75
1 × 6	¾ × 5½	4.00	2.75
1 × 8	¾ × 7¼	5.33	3.63
1 × 12	¾ × 11¼	8.00	5.63
2 × 4	1½ × 3½	5.33	3.50
2 × 6	1½ × 5½	8.00	5.50
2 × 8	1½ × 7¼	10.67	7.25
2 × 10	1½ × 9¼	13.33	9.25
2 × 12	1½ × 11¼	16.00	11.25
4 × 4	3½ × 3½	10.67	8.17
6 × 6	5½ × 5½	24.00	20.17

Actual board feet are shown for reference only — lumber is bought and sold on nominal dimensions.

Why the sizes shrink

Sawmills cut a 2 × 4 at two inches by four inches (rough green). It is then dried and planed on all four faces. Surfacing removes about ⅛ inch per face on 1-inch stock and roughly ¼ inch per face on 2-inch stock, which lands dimensional softwood at ¾ × 3½ and 1½ × 3½.

Hardwood follows a different convention: it is sold by the board foot and surfaced to 13/16 inch for 4/4 stock, but priced on the full nominal inch.

Where the difference actually matters

Framing layout: studs, joists, and rafters are spaced on centers using actual thickness, not nominal.
Built-ins and jigs: a “2-inch” dado will not fit a 2 × 4. Cut to 1½ inches.
Decking and stairs: tread depth and riser math use actual dimensions.
Pricing: board feet always use nominal, so comparing a price per board foot with a price per actual cubic foot is apples to oranges.

Why the convention survived

Lumber used to be sold rough, straight off the saw, and the nominal size was the size in the log. Planing was a service added later, and rather than reprice every size the trade kept the old name and standardized the surfaced dimension. That is why a 2 × 4 is not undersized: the name is a name, and the dimensions behind it were fixed by the American Softwood Lumber Standard, which is where the surfacing allowances in the chart above come from.

One inch of thickness loses ⅛ inch per face, landing at ¾. Two inches loses ¼ inch per face, landing at 1½. Widths lose between ½ and ¾ inch depending on size. None of it is damage, and none of it is negotiable at the yard.

Hardwood does not follow the same rules

Hardwood is sold by the board foot on the full nominal thickness and surfaced much less aggressively. A 4/4 board surfaces to 13/16 inch, not ¾, and an 8/4 board surfaces to 1¾. It is still billed as one inch and two inches respectively.

That is why a woodworker who moves from construction lumber to hardwood gets caught out: the 1½ inch assumption that works for a 2 × 4 is wrong for a piece of 6/4, which surfaces to 1 5⁄16 inches — 3/16 under the nominal inch and a half. Measure the stock in front of you, not the label on the rack.

Where the missing half inch costs real money

Insulation. A 2 × 6 cavity is 5½ inches deep, which is why R-21 batts are made to that size. Compute the R-value from six inches and you are about 8 percent optimistic.
Concrete forms. A 2 × 6 form gives 5½ inches of slab, not 6. Order for the nominal depth and the pour comes up short by roughly 8 percent.
Joist hangers and spans. Span tables and hanger sizes are built around actual thickness; a 1½ inch joist in a hanger sized for 2 inches will not seat.
Stairs. Tread depth is cut from 11¼ inch stock, not 12. Riser and tread math that assumes a full foot produces a stair that does not meet the nose properly.
Built-ins. A shelf opening framed with 2 × stock is 1½ inches narrower than the outside dimension, which is the difference between a cabinet that slides in and one that does not.

Rough-sawn is the exception that trips people

Rough-sawn 2 × 4 really is close to 2 inches by 4 inches. It is cheaper, heavier, and it will not fit anything dimensioned for surfaced lumber. Mixing the two in one assembly leaves a ½ inch bump that shows up as a wavy wall or a floor that rocks. If a project mixes them deliberately — a rough-sawn beam over surfaced studs, say — the transition has to be detailed rather than discovered.

Engineered lumber sidesteps the whole convention: an LVL beam or an I-joist is specified and sold at its actual dimensions, with no nominal alias. Read the spec sheet, not the name.

Frequently asked questions

Why is a 2 × 4 not 2 inches by 4 inches?

It is cut at nominal size, then dried and planed. Surfacing removes about ½ inch from each dimension at 2-inch thickness.

Do board feet use nominal or actual sizes?

Nominal. That is why a 2 × 4 × 8 is 5.33 board feet even though the finished piece contains about 3.5 board feet of wood.

Are all 2 × 4s the same?

Dimensional softwood in North America is standardized at 1½ × 3½ inches. Rough-sawn or imported lumber can differ — measure before cutting joinery.

How much is planed off a 1-inch board?

About 1/8 inch per face, so a nominal 1-inch board surfaces to 3/4 inch.

Why is hardwood surfaced to 13/16 instead of 3/4?

Hardwood is sold on the full nominal thickness and surfaced less aggressively, so 4/4 stock comes out at 13/16 inch rather than 3/4.

Does a 2x6 wall cavity take R-21 insulation?

Yes. The cavity is 5 1/2 inches deep, which is exactly what R-21 batts are made for. Computing from a nominal 6 inches overstates the R-value by about 8 percent.

Is rough-sawn lumber the same size as nominal?

Close to it, which is why it will not fit assemblies dimensioned for surfaced stock. Measure before framing if you mix the two.

Note: Quantities and prices are estimates for planning. Confirm against the bag yield or quote you are actually buying from.

Citing this page? FigureMill, “Nominal vs. actual lumber sizes” (last reviewed October 30, 2026). Nominal-to-actual dimensions for softwood framing lumber under PS 20-20, plus the different surfacing convention hardwood uses (4/4 rough to 13/16 in. surfaced). https://figuremill.com/materials/nominal-vs-actual-lumber-sizes

Last reviewed: October 30, 2026 · Sources: ALS/USDA surfaced-size standards; NHLA grading rules

Nominal vs. actual lumber sizes

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