Lumber Dimensions Explained: Nominal vs Actual Size Chart

Why Nominal and Actual Sizes Differ

When a sawmill cuts a log, the rough-sawn board IS the nominal size. A fresh-cut 2x4 really is 2" x 4". But then two things happen:

1. Drying: The lumber is kiln-dried to reduce moisture content from ~50% to ~12-19%. Wood shrinks as it dries — losing about 1/4" to 3/8" in each dimension.

2. Surfacing (planing): The rough, splintery surfaces are run through a planer to create smooth, consistent boards. This removes another 1/8" to 1/4" from each face.

The combined loss from drying and surfacing is about 1/2" from each dimension. So a 2x4 (which started at 2" x 4") ends up at 1-1/2" x 3-1/2" after processing.

This is the industry standard in North America. The nominal size tells you what the board started as; the actual (dressed) size tells you what you'll measure with a tape. When buying lumber, always plan your project using actual dimensions.

Standard Dimensional Lumber Sizes

2x material (framing lumber, construction):

2x2: actual 1-1/2" x 1-1/2"

2x3: actual 1-1/2" x 2-1/2"

2x4: actual 1-1/2" x 3-1/2"

2x6: actual 1-1/2" x 5-1/2"

2x8: actual 1-1/2" x 7-1/4"

2x10: actual 1-1/2" x 9-1/4"

2x12: actual 1-1/2" x 11-1/4"

1x material (boards, trim, shelving):

1x2: actual 3/4" x 1-1/2"

1x3: actual 3/4" x 2-1/2"

1x4: actual 3/4" x 3-1/2"

1x6: actual 3/4" x 5-1/2"

1x8: actual 3/4" x 7-1/4"

1x10: actual 3/4" x 9-1/4"

1x12: actual 3/4" x 11-1/4"

4x material (posts, beams):

4x4: actual 3-1/2" x 3-1/2"

4x6: actual 3-1/2" x 5-1/2"

6x6: actual 5-1/2" x 5-1/2"

8x8: actual 7-1/2" x 7-1/2"

Standard Lengths

Dimensional lumber comes in 2-foot increments: 8, 10, 12, 14, 16, 18, 20 feet. 8-foot boards are the most common and cheapest per foot. Longer boards (16-20 feet) are available but may need to be special-ordered and cost more per foot due to increased waste in the sawing process.

Lengths are nominal too — but they're actually accurate (or slightly over). An 8-foot board really is at least 96" long, often 96-1/4" to allow for square trimming.

Hardwood Lumber: The Quarter System

Hardwood lumber uses a completely different sizing system called "quarter" notation, based on rough-sawn thickness in quarters of an inch:

4/4 ("four-quarter"): 1" rough sawn, surfaces to ~13/16" or 3/4"

5/4 ("five-quarter"): 1-1/4" rough sawn, surfaces to ~1-1/16"

6/4 ("six-quarter"): 1-1/2" rough sawn, surfaces to ~1-5/16"

8/4 ("eight-quarter"): 2" rough sawn, surfaces to ~1-3/4"

10/4 through 16/4: Available for thick slabs, turning blanks, and specialty applications.

Hardwood width is not standardized — boards are sold in random widths (RW) and random lengths (RL). You buy by the board foot (144 cubic inches = 1 BF), not by the piece.

Sheet Goods

Plywood: Sold as 4'x8' sheets. Common thicknesses: 1/4", 3/8", 1/2", 5/8", 3/4". These are nominal — actual thickness is typically 1/32" less (e.g., "3/4" plywood" measures about 23/32"). This matters when cutting dadoes and rabbets to fit plywood panels.

MDF and particle board: Typically true to stated thickness. 3/4" MDF is actually 3/4".

OSB (oriented strand board): Common thicknesses: 7/16", 1/2", 5/8", 3/4". Used for sheathing and subflooring.