# Nominal vs. Actual Lumber Sizes A chart of real lumber dimensions from the national standard, dry versus green sizes, lengths, and board feet. Guide page: (published 2026-10-05, updated 2026-10-05). This Markdown version has the guide's key points, outline, FAQs and sources; the full article, with its worked examples and diagrams, is on the page. ## Key takeaways - A lumber size like 2×4 is a name, not a measurement: under the national standard a dry 2×4 is 1-1/2 by 3-1/2 inches. - For dry dimension lumber, nominal widths up to 6 in lose 1/2 in and widths of 8 in and up lose 3/4 in; 1-in boards are 3/4 in thick. - Dry means 19% moisture content or less; the grade stamp says S-DRY, KD, or S-GRN, and green lumber is dressed slightly larger. - Standard lengths come in 1 or 2 ft steps, and lumber up to 2 in thick is trimmed to at least the nominal length and usually no more than 3 in over. - Board feet are counted on the nominal size, so a 2×6 that is 10 ft long is 10 board feet even though it holds less wood. ## In this guide 1. [Why a 2×4 isn't 2 by 4 inches](https://www.howmanyyardsdoineed.com/guides/nominal-vs-actual-lumber-sizes/#why-a-2x4-is-not-2-by-4) 2. [Actual size chart](https://www.howmanyyardsdoineed.com/guides/nominal-vs-actual-lumber-sizes/#size-chart) 3. [Dry and green sizes](https://www.howmanyyardsdoineed.com/guides/nominal-vs-actual-lumber-sizes/#dry-and-green) 4. [Lengths](https://www.howmanyyardsdoineed.com/guides/nominal-vs-actual-lumber-sizes/#lengths) 5. [Reading the grade stamp](https://www.howmanyyardsdoineed.com/guides/nominal-vs-actual-lumber-sizes/#grade-stamp) 6. [Board feet use the nominal size](https://www.howmanyyardsdoineed.com/guides/nominal-vs-actual-lumber-sizes/#board-feet) 7. [Planning with actual sizes](https://www.howmanyyardsdoineed.com/guides/nominal-vs-actual-lumber-sizes/#planning-with-actual-sizes) ## Frequently asked questions ### What are the actual dimensions of a 2x4? Under the American Softwood Lumber Standard (PS 20), a dry 2×4 is dressed to 1-1/2 by 3-1/2 inches. Green (unseasoned) 2×4s are dressed slightly larger, to 1-9/16 by 3-9/16 inches. ### How big is a 4x4 post really? A dry 4×4 is 3-1/2 by 3-1/2 inches under PS 20. A 6×6 is a timber, dressed 1/2 inch under nominal, so it is 5-1/2 by 5-1/2 inches. ### Why is lumber smaller than its name? The nominal size names a size category; the standard says no one should infer that nominal sizes are dressed sizes. Lumber is sawn oversize, then surfaced (dressed) smooth, and the standard's tables set the minimum finished sizes. ### Is an 8-foot board actually 8 feet long? At least. PS 20 says lumber 2 inches or less in nominal thickness is trimmed to no less than the nominal length and no more than 3 inches over it, so an 8 ft board is between 8 ft and 8 ft 3 in. Very long or wide 2-in lumber may run up to 12 inches over. ### Do board feet use nominal or actual dimensions? Nominal. PS 20 defines board measure as nominal thickness in inches times nominal width in feet times length in feet, so a 10 ft 2×6 is 10 board feet, although its actual wood volume is about 69% of that. ## Related calculators and guides ### Calculators - [Board Foot Calculator](https://www.howmanyyardsdoineed.com/board-foot-calculator/) — Board feet per piece and in total from thickness, width, length, and quantity, with optional cost at your price per board foot. - [Deck Calculator](https://www.howmanyyardsdoineed.com/deck-calculator/) — Decking board count from deck size, actual board width, gap, and board length, with the cutting assumptions shown. - [Stair Calculator](https://www.howmanyyardsdoineed.com/stair-calculator/) — Number of risers and treads, exact riser height, total run, stair angle, and stringer length from floor-to-floor rise. ### Guides - [Deck board spacing and layout: gaps, direction, and joints](https://www.howmanyyardsdoineed.com/guides/deck-board-spacing-and-layout/) — Side and end gaps, board direction, joint placement, and last-board planning from Trex and American Wood Council guides. - [How to Lay Out Stairs: Rise, Run, and Stringers](https://www.howmanyyardsdoineed.com/guides/stair-layout-basics/) — From total rise to equal risers, tread depth, total run, and a marked-out stringer, with a worked example. ## Sources - [Voluntary Product Standard PS 20-25, American Softwood Lumber Standard: nominal and dressed sizes](https://www.nist.gov/system/files/documents/2024/12/11/PS%2020-25%20Final.pdf) — National Institute of Standards and Technology. Used for: Dry dressed: 1 → 3/4 in; 2 → 1-1/2; 4 → 3-1/2; 6 → 5-1/2; 8 → 7-1/4; 10 → 9-1/4; 12 → 11-1/4. Green: 2 → 1-9/16; 4 → 3-9/16; 6 → 5-5/8; 8 → 7-1/2. Timbers 5–6 in: 1/2 in off. Dry ≤ 19% moisture. Lengths in 1 or 2 ft multiples, trimmed 0 to 3 in over nominal. Checked 2026-10-05. - [Voluntary Product Standard PS 20-25, American Softwood Lumber Standard: definition of board measure](https://www.nist.gov/system/files/documents/2024/12/11/PS%2020-25%20Final.pdf) — National Institute of Standards and Technology. Used for: Board feet = nominal thickness (in) × nominal width (ft) × length (ft). Checked 2026-10-05. - [Voluntary Product Standard PS 20-25, American Softwood Lumber Standard](https://www.nist.gov/system/files/documents/2024/12/11/PS%2020-25%20Final.pdf) — National Institute of Standards and Technology. Used for: Nominal 6 in width = 5-1/2 in minimum dressed (dry). Checked 2026-10-03. **Estimate only.** Results are planning estimates computed from the numbers you enter. They are not engineering, structural, or code advice. Confirm quantities, product yields, and installation requirements with your supplier, the product label, and your local building authority before you order or build.