Lumber Size Converter – Nominal vs Actual Dimensions
Anyone who has ever bought a 2×4 and tried to use it as a true 2-inch by 4-inch piece of wood quickly discovers the central puzzle of the lumber industry: nominal sizes and actual sizes are not the same thing. This converter resolves that confusion instantly, covering every standard softwood dimension size from 1×2 through 8×8, with full imperial and metric outputs and a board footage calculator for project planning.
Why Do Nominal and Actual Lumber Sizes Differ?
When a log is first cut at a sawmill, the resulting board roughly matches its nominal dimension. However, two subsequent processes shrink it significantly. First, kiln drying removes moisture from the wood, which causes the fibers to contract — a 2-inch-thick green board can lose around 0.3–0.5 inches of thickness during drying. Second, planing (surfacing) smooths all four faces (S4S — surfaced four sides), removing more material to produce a uniform, splinter-free surface. The result is that a nominal 2×4 ends up measuring exactly 1.5 in × 3.5 in when you buy it at the lumber yard.
The naming convention has persisted for over a century purely as a trade convenience. Every professional carpenter and contractor knows to design with actual dimensions; the confusion most often trips up first-time DIYers and students who take the printed label at face value.
Standard Softwood Lumber Dimensions (AWC PS 20)
The actual dimensions shown in this converter follow the American Softwood Lumber Standard (AWC PS 20), the nationally recognized standard for dimension lumber sold in the United States and Canada. A few key sizes to memorize:
1×4→ 0.75 in × 3.5 in (19 mm × 89 mm)2×4→ 1.5 in × 3.5 in (38 mm × 89 mm)2×6→ 1.5 in × 5.5 in (38 mm × 140 mm)2×8→ 1.5 in × 7.25 in (38 mm × 184 mm)4×4→ 3.5 in × 3.5 in (89 mm × 89 mm)6×6→ 5.5 in × 5.5 in (140 mm × 140 mm)
Larger timbers (4× and 6× series) retain more of their nominal dimension because they are typically sold in a rougher state and dried more slowly, resulting in less material removal during processing.
What Is Board Footage?
Board footage (bd ft) is the standard volumetric unit used in North America to price and order lumber. Unlike linear feet, board footage accounts for the cross-sectional size of the board. The formula is:
Board Feet = (nominal thickness in inches × nominal width in inches × length in feet) ÷ 12
Critically, the formula uses nominal dimensions — not actual. This is an industry convention: you pay for the nominal volume even though the board has been planed down. For example, five 2×6 boards each 10 feet long equals:
(2 × 6 × 10 × 5) ÷ 12 = 50 board feet
If your lumber yard charges $4.00 per board foot, your total cost would be $200.00. The board footage calculator in this tool computes this automatically when you enter a quantity and an optional price per board foot.
Metric Equivalents for International Projects
Outside North America, lumber is typically specified in millimeters. The metric equivalents of U.S. standard sizes can be confusing because the nearest metric round number (e.g., 40 mm) rarely matches the exact conversion of the actual inch dimension (38.1 mm). This converter displays both the mathematically exact metric equivalent (using the precise factor of 1 in = 25.4 mm) and accepts millimeter inputs in the Metric → Nominal mode to help you identify the closest standard U.S. size for imported lumber or when working from metric blueprints.
Finding the Nominal Size from a Measured Board
If you have a board with unknown provenance and want to identify its nominal designation, measure its actual width and thickness carefully with a tape measure or calipers, then use the Actual → Nominal mode. The tool searches the full standard lookup table and finds the nearest match using a Euclidean nearest-neighbor algorithm. If the board falls outside the ±0.125 in tolerance, it is likely custom-cut, rough-sawn (undressed), or hardwood — which follows different sizing conventions from softwood dimension lumber.
Practical Tips for Woodworkers and Contractors
Always design structural and finish work using actual dimensions. When calculating how many boards you need to cover a given width, use actual thickness; when pricing and ordering material, use board footage based on nominal dimensions. Stud spacing in standard construction (16 in or 24 in on center) is designed around the actual 1.5-inch thickness of nominal 2× lumber — this means a 4-foot wall panel requires exactly 3 stud bays. Understanding the nominal-to-actual relationship is fundamental for reading blueprints, estimating materials, and avoiding expensive over- or under-ordering.