Nominal vs Actual Lumber Sizes

In the North American softwood standard, dimensions described as nominal belong to an industry naming convention; the standard separately lists minimum dressed dimensions. That distinction should not be generalized into one reduction rule for every lumber or timber product worldwide.

A supplier's product specification and the dimensions measured from the stock are more specific than the nominal name.

Rough lumber has been sawn but has not been fully surfaced to a smooth, uniform dressed size. Surfaced or dressed stock has been machined on one or more faces or edges, so its finished dimensions can differ from a rough or nominal description.

PS 20 lists S2S as surfaced on two sides and S4S as surfaced on four sides. If a supplier uses other shorthand such as S3S, confirm which faces and edges were surfaced rather than inferring the condition from the number alone.

Sawing establishes the initial piece, while drying and planing or other surfacing can change its cross-section. Product standards and established trade conventions may set named and minimum dressed sizes for particular categories.

Species, moisture condition, milling specification, product type, supplier, and region can all affect which convention applies. The practical question is not simply how much smaller a board should be, but which specification governs the stock in front of you.

Use actual dimensions when parts must fit an opening, when estimating finished width or depth, and when preparing cut lists, joinery, or material layouts.

Actual thickness and width also affect volume. The How to Calculate Board Feet guide explains how to organize those measurements for purchasing.

For direct arithmetic from measured dimensions, use the Board Foot Calculator.

Use the same units throughout the drawing or cut list and record enough precision for the decision being made. When consistency matters, sample more than one piece instead of assuming an entire batch matches the first board.

Suppose an illustrative board sold as nominal 1×8 measures 3/4 inch thick and 7 3/16 inches wide. Those measured values describe this example board; they are not a universal 1×8 standard.

A drawing that treats the label as an 8-inch finished shelf depth overstates the measured width by 13/16 inch: 8 − 7 3/16 = 13/16. Record 7 3/16 inches for this shelf layout, then check the other boards if variation would affect the opening or alignment.

Woodshop Math's 11-row lookup uses the minimum dressed dry inch dimensions in Table 3 of PS 20-20 Revision 1. The nominal 1-inch rows use the standard's Boards classification, and the nominal 2-inch rows use its Dimension classification.

The lookup is a North American softwood reference, not a universal manufacturing conversion or a substitute for a supplier specification. Its displayed metric values are converted from the referenced inch dimensions and rounded for convenience; they are not separate metric manufacturing standards.

The reference data was last verified on 2026-08-12. Use the calculator's custom measured override whenever the actual stock dimensions are available.

The Nominal to Actual Lumber Size Calculator shows the sourced reference, accepts measured custom dimensions, and calculates volume from the values actually selected or entered.

A nominal label is useful identification, but it should not silently become a fit-critical dimension.