Lumber Size Guide
The most common source of confusion in lumber is sizing. A 2x4 is not 2 inches by 4 inches. This guide gives you every nominal-to-actual conversion, board foot formulas, coverage calculations, and standard length you will ever need.
The Basics
When lumber is first cut from a log at the sawmill, it is rough-sawn to the nominal dimension. A board labeled 2x4 starts its life as a piece of wood that genuinely measures 2 inches by 4 inches. But that rough-sawn board is wet, rough on all four faces, and not dimensionally stable.
To make the lumber usable, it goes through two processes: drying (either air-dried or kiln-dried) and surfacing (planing smooth on all four sides, abbreviated S4S). Drying causes the wood to shrink, and planing removes material from each face to create smooth, uniform surfaces.
The result is a finished board that is smaller than its nominal designation. A 2x4 ends up at 1-1/2 inches by 3-1/2 inches. This standard has been codified by the American Lumber Standard Committee (ALSC) and applies to virtually all surfaced softwood lumber sold in the United States.
Important note for reclaimed lumber: Older reclaimed lumber was often cut to true nominal dimensions or close to them. A reclaimed 2x4 from a 1940s barn may actually measure a full 2 inches by 4 inches. This is one of the reasons builders prize reclaimed lumber for renovation work on older structures where modern dimensions do not match existing framing.
Dimension Tables
All dimensions below are for kiln-dried, surfaced (S4S) lumber per American Lumber Standard Committee specifications.
Used for framing, structural applications, decking, and general construction. The workhorse of the lumber industry.
| Nominal Size | Actual Size (Fractional) | Actual Size (Decimal) |
|---|---|---|
| 2 x 2 | 1-1/2" x 1-1/2" | 1.5" x 1.5" |
| 2 x 3 | 1-1/2" x 2-1/2" | 1.5" x 2.5" |
| 2 x 4 | 1-1/2" x 3-1/2" | 1.5" x 3.5" |
| 2 x 6 | 1-1/2" x 5-1/2" | 1.5" x 5.5" |
| 2 x 8 | 1-1/2" x 7-1/4" | 1.5" x 7.25" |
| 2 x 10 | 1-1/2" x 9-1/4" | 1.5" x 9.25" |
| 2 x 12 | 1-1/2" x 11-1/4" | 1.5" x 11.25" |
| 2 x 14 | 1-1/2" x 13-1/4" | 1.5" x 13.25" |
| 2 x 16 | 1-1/2" x 15-1/4" | 1.5" x 15.25" |
Used for shelving, trim, paneling, siding, craft projects, and non-structural applications. Thinner and lighter than dimensional lumber.
| Nominal Size | Actual Size (Fractional) | Actual Size (Decimal) |
|---|---|---|
| 1 x 2 | 3/4" x 1-1/2" | 0.75" x 1.5" |
| 1 x 3 | 3/4" x 2-1/2" | 0.75" x 2.5" |
| 1 x 4 | 3/4" x 3-1/2" | 0.75" x 3.5" |
| 1 x 5 | 3/4" x 4-1/2" | 0.75" x 4.5" |
| 1 x 6 | 3/4" x 5-1/2" | 0.75" x 5.5" |
| 1 x 8 | 3/4" x 7-1/4" | 0.75" x 7.25" |
| 1 x 10 | 3/4" x 9-1/4" | 0.75" x 9.25" |
| 1 x 12 | 3/4" x 11-1/4" | 0.75" x 11.25" |
Used for posts, beams, pergolas, timber framing, and heavy structural applications. Sizes 5 inches and larger in both dimensions are typically sold as full-sawn (rough) with actual dimensions matching or very close to nominal.
| Nominal Size | Actual Size (Fractional) | Actual Size (Decimal) |
|---|---|---|
| 4 x 4 | 3-1/2" x 3-1/2" | 3.5" x 3.5" |
| 4 x 6 | 3-1/2" x 5-1/2" | 3.5" x 5.5" |
| 4 x 8 | 3-1/2" x 7-1/4" | 3.5" x 7.25" |
| 4 x 10 | 3-1/2" x 9-1/4" | 3.5" x 9.25" |
| 4 x 12 | 3-1/2" x 11-1/4" | 3.5" x 11.25" |
| 6 x 6 | 5-1/2" x 5-1/2" | 5.5" x 5.5" |
| 6 x 8 | 5-1/2" x 7-1/2" | 5.5" x 7.5" |
| 6 x 10 | 5-1/2" x 9-1/2" | 5.5" x 9.5" |
| 6 x 12 | 5-1/2" x 11-1/2" | 5.5" x 11.5" |
| 8 x 8 | 7-1/2" x 7-1/2" | 7.5" x 7.5" |
| 8 x 10 | 7-1/2" x 9-1/2" | 7.5" x 9.5" |
| 8 x 12 | 7-1/2" x 11-1/2" | 7.5" x 11.5" |
Calculating Quantity
A board foot is the standard unit of measurement for lumber volume in the United States. One board foot equals a piece of wood 12 inches long, 12 inches wide, and 1 inch thick — or 144 cubic inches of wood.
Board foot calculations use nominal dimensions, not actual dimensions. This is an industry convention that dates back to when lumber was sold rough-sawn at full nominal size. Even though a modern surfaced 2x4 is smaller than 2" x 4", it is still priced and measured as if it were full-size.
Common Conversions
Quick reference for common sizes at standard lengths. Board feet calculated using nominal dimensions.
| Size | 8 ft | 10 ft | 12 ft | 16 ft |
|---|---|---|---|---|
| 1 x 4 | 2.67 | 3.33 | 4.00 | 5.33 |
| 1 x 6 | 4.00 | 5.00 | 6.00 | 8.00 |
| 1 x 8 | 5.33 | 6.67 | 8.00 | 10.67 |
| 1 x 10 | 6.67 | 8.33 | 10.00 | 13.33 |
| 1 x 12 | 8.00 | 10.00 | 12.00 | 16.00 |
| 2 x 4 | 5.33 | 6.67 | 8.00 | 10.67 |
| 2 x 6 | 8.00 | 10.00 | 12.00 | 16.00 |
| 2 x 8 | 10.67 | 13.33 | 16.00 | 21.33 |
| 2 x 10 | 13.33 | 16.67 | 20.00 | 26.67 |
| 2 x 12 | 16.00 | 20.00 | 24.00 | 32.00 |
| 4 x 4 | 10.67 | 13.33 | 16.00 | 21.33 |
| 6 x 6 | 24.00 | 30.00 | 36.00 | 48.00 |
| 8 x 8 | 42.67 | 53.33 | 64.00 | 85.33 |
Standard Lengths
Lumber is typically available in even-numbered foot lengths. Actual available lengths may vary by species, grade, and whether stock is reclaimed or new. Contact us for specific availability.
| Length | Metric Equivalent | Common Uses |
|---|---|---|
| 8 ft | 2.44 m | Interior framing, short spans, craft projects |
| 10 ft | 3.05 m | Wall studs (tall walls), short joists, fence rails |
| 12 ft | 3.66 m | Standard wall plates, headers, deck joists |
| 14 ft | 4.27 m | Floor joists, longer spans, rafters |
| 16 ft | 4.88 m | Floor joists, rafters, ridge beams, long spans |
| 18 ft | 5.49 m | Rafters, ridge boards, commercial framing |
| 20 ft | 6.10 m | Long-span beams, commercial applications |
| 24 ft | 7.32 m | Truss construction, large commercial spans (special order) |
Coverage Calculations
When you are covering a surface — flooring, siding, paneling, decking — you need to know how many linear feet of a given board width will cover your square footage. The table to the right gives you the coverage factor for common board sizes.
Coverage is based on actual face width, not nominal width. A 1x6 has an actual face width of 5-1/2 inches, so that is the dimension used for coverage calculations. For tongue-and-groove (T&G) boards, the exposed face is even narrower due to the tongue overlap — typically reducing face coverage by about 1/2 inch.
Always add waste factor. For straight runs on rectangular surfaces, add 5-10% for waste (cuts, defects, end-matching). For diagonal installations, angles, or irregular areas, add 15-20%. For reclaimed lumber with character-grade defects, add up to 25% to account for pieces you may cull during installation.
Square feet of coverage per linear foot of board, and linear feet needed to cover 100 square feet (before waste factor).
| Board Size | Sq Ft / LF | LF / 100 Sq Ft |
|---|---|---|
| 1 x 4 (3.5" face) | 0.292 | 343 |
| 1 x 6 (5.5" face) | 0.458 | 218 |
| 1 x 8 (7.25" face) | 0.604 | 166 |
| 1 x 10 (9.25" face) | 0.771 | 130 |
| 1 x 12 (11.25" face) | 0.938 | 107 |
| 5/4 x 6 Decking (5.5" face) | 0.458 | 218 |
| 2 x 6 T&G (5" face) | 0.417 | 240 |
| 2 x 8 T&G (6.75" face) | 0.563 | 178 |
Send us your project dimensions and we will put together a materials list with board foot calculations, a waste estimate, and a price quote — free of charge.