Lumber Sizes — Nominal and Actual Dimensions
A 2x4 is not two inches by four inches. This is every standard dressed size with what it actually measures, what it weighs in each species, and why the two numbers differ.
Why the nominal size is not the actual sizethe one thing to understand first
The nominal figure — the “2” and the “4” in 2x4 — describes the rough-sawn board as it comes off the mill, green and still wet. What reaches the yard has since been dried and planed on all four faces, and both operations take material off. A 2x4 ends up at 1-1/2 by 3-1/2 inches, which is a quarter of an inch off each face in both directions.
Nothing is being hidden: nominal sizes are the commercial shorthand for the section class, and they persist because they are shorter to say and because they are the sizes the span tables and grade rules are written against. The number that matters when you are calculating anything is the actual one, and that is what every page below tabulates first.
Dimension does not vary by species. A 2x4 is 1-1/2 by 3-1/2 inches in oak exactly as in spruce — the species changes the weight, not the size.
Every sizenominal against dressed, in inches and millimetres
| Nominal | Actual (in) | Actual (mm) | Group |
|---|---|---|---|
| 1x2 | 0.75 × 1.5 | 19.05 × 38.1 | Board |
| 1x3 | 0.75 × 2.5 | 19.05 × 63.5 | Board |
| 1x4 | 0.75 × 3.5 | 19.05 × 88.9 | Board |
| 1x6 | 0.75 × 5.5 | 19.05 × 139.7 | Board |
| 1x8 | 0.75 × 7.25 | 19.05 × 184.15 | Board |
| 1x10 | 0.75 × 9.25 | 19.05 × 234.95 | Board |
| 1x12 | 0.75 × 11.25 | 19.05 × 285.75 | Board |
| 2x2 | 1.5 × 1.5 | 38.1 × 38.1 | Dimensional |
| 2x3 | 1.5 × 2.5 | 38.1 × 63.5 | Dimensional |
| 2x4 | 1.5 × 3.5 | 38.1 × 88.9 | Dimensional |
| 2x6 | 1.5 × 5.5 | 38.1 × 139.7 | Dimensional |
| 2x8 | 1.5 × 7.25 | 38.1 × 184.15 | Dimensional |
| 2x10 | 1.5 × 9.25 | 38.1 × 234.95 | Dimensional |
| 2x12 | 1.5 × 11.25 | 38.1 × 285.75 | Dimensional |
| 4x4 | 3.5 × 3.5 | 88.9 × 88.9 | Post |
| 4x6 | 3.5 × 5.5 | 88.9 × 139.7 | Beam |
| 6x6 | 5.5 × 5.5 | 139.7 × 139.7 | Timber |
| 8x8 | 7.25 × 7.25 | 184.15 × 184.15 | Timber |
Sizes are for kiln-dried stock surfaced on four sides. Material sold unseasoned is dressed slightly larger because it will shrink into these figures — each size page gives that figure where it applies.
Board7 nominal sizes
Dimensional7 nominal sizes
Post1 nominal sizes
- 4x4 — 3.5 × 3.5 in · Posts
Beam1 nominal sizes
- 4x6 — 3.5 × 5.5 in · Beams
Timber2 nominal sizes
Weighthow the figures on each page are produced
This database holds a reference density for every wood species in it, so the weight of a given size and length is arithmetic rather than a quoted figure. Multiply the actual volume by the density:
weight = (actual thickness × actual width × length) ÷ 1728 × density in lb/ft³, where lb/ft³ = density in g/cm³ × 62.428
That is why the weight tables run down every species rather than naming one. The same 2x4x8 is about 2.91 lb in balsa and 18.21 lb in ipe — a six-fold spread that a single quoted figure would hide.
Frequently asked questionsabout nominal and actual sizes
Is a 2x4 actually 2 inches by 4 inches?
No. A 2x4 measures 1-1/2 by 3-1/2 inches. The nominal figure is the rough green board; planing and drying remove a quarter of an inch from each face in both directions.
Do all species have the same actual size?
Yes, for a given nominal size and moisture condition. Dressed sizes are standardised commercial dimensions, so a 2x6 is 1-1/2 by 5-1/2 inches whether it is hemlock, Douglas fir or oak. Only the weight and the strength change with species.
Why does board feet use a different size from weight?
Board feet is a pricing convention and is calculated from the nominal section. Weight is physics and uses the actual section. For a 2x4x8 the two numbers are 5.33 board feet and 0.2917 cubic feet — both correct, measuring different things.
Is green lumber a different size?
Slightly larger, for the sizes where unseasoned structural material is sold. A green 2x4 is dressed to 1-9/16 by 3-9/16 inches and shrinks to the dry size as it seasons. Mixing green and dry stock in the same assembly produces a step once the green material dries — each size page flags where this applies.