The Complete Guide to Understanding Lumber Sizes

Walking into a lumberyard or home improvement store can be confusing if you don't understand lumber sizing. That '2x4' you're picking up isn't actually 2 inches by 4 inches. This discrepancy between the name (nominal size) and the actual dimensions is a standard practice in the wood industry that dates back to the early 20th century. This guide will demystify lumber sizes, explaining why a 2x4 measures 1.5" x 3.5", how moisture content and planing affect dimensions, and the standards that govern dimensional lumber, hardwood, and softwood. You'll learn to read lumber labels, calculate board feet, and select the right material for framing, decking, and fine woodworking, ensuring your next project is built accurately and efficiently.

By Jordan Rivera|Updated Feb 20, 2026

Nominal vs. Actual Lumber Sizes: Why a 2x4 Isn't 2" by 4"

The most fundamental concept in lumber is the difference between its nominal size and its actual, dressed size. Nominal size refers to the rough, unfinished dimensions of the board when it is first cut from the log, historically representing its size in quarters of an inch (e.g., a '2x4' was roughly 2" by 4"). However, after the drying process (which shrinks the wood) and planing on all four sides (S4S) to create a smooth, uniform surface, the board loses about 1/2 inch in width and thickness. This results in the standardized actual dimensions we use today. For example, a nominal 2x4 has an actual size of 1.5 inches by 3.5 inches. A nominal 1x6 is actually 0.75" x 5.5". This system is consistent across North America for softwood dimensional lumber used in framing and construction. Understanding this is crucial for reading plans, making accurate cuts, and purchasing the correct volume of material. It prevents costly errors where a project designed for actual dimensions fails because nominal-sized lumber was used in calculations. Always verify the actual size on the retailer's specification sheet or by measuring a sample piece.

Nominal size is the historical name (e.g., 2x4), not the actual measurement.
Drying and planing reduce rough lumber to standardized actual dimensions.
The standard deduction is typically 1/2 inch from the nominal width and thickness.
Always use actual dimensions for project planning and material calculations.

Standard Dimensional Lumber Sizes and Common Uses

Dimensional lumber, primarily made from softwoods like pine, spruce, and fir, is categorized by its nominal dimensions. Here are the most common sizes and their typical applications in construction and DIY projects. For framing walls, floors, and roofs, the 2x4 (actual 1.5"x3.5") is the ubiquitous workhorse. 2x6 lumber (1.5"x5.5") is used for exterior walls to allow for deeper insulation. Larger members like 2x8, 2x10, and 2x12 are used for floor joists, ceiling rafters, and headers where greater load-bearing capacity and span are required. In the '1x' category, 1x4 and 1x6 boards are common for sheathing, shelving, and trim work. For posts and structural supports, nominal 4x4 (actual 3.5"x3.5") and 6x6 (5.5"x5.5") are standard. It's also essential to know standard lengths, which are commonly available in even-foot increments from 8 feet up to 20 feet. When selecting lumber, also check its grade (like Select, #2, or Construction grade), which indicates the number and size of knots and other imperfections, affecting both aesthetics and strength.

2x4s and 2x6s are standard for wall studs and framing.
Larger 2x boards (8,10,12) are used for joists and rafters.
1x boards are used for trim, shelving, and decorative projects.
4x4 and 6x6 are standard for deck posts and structural supports.
Always consider both dimensions and grade for your project's needs.

Hardwood Lumber Sizing: Board Feet and Quarter System

Hardwood lumber, used for furniture, cabinetry, and flooring, follows a different sizing system than softwood dimensional lumber. Hardwoods are often sold rough-cut and are measured in board feet, a volume measurement. One board foot is a piece of wood 12 inches wide by 12 inches long by 1 inch thick (or its volumetric equivalent). The thickness of rough hardwood is described using a 'quarter' system, referring to increments of 1/4 inch. For example, '4/4' (pronounced 'four-quarter') lumber is roughly 1 inch thick, '8/4' is roughly 2 inches thick, and so on. When planning a hardwood project, you must account for significant waste from planing the rough lumber down to your desired finished thickness. A 4/4 board typically yields about 3/4" of finished thickness after planing. Hardwoods are also sold in random widths and lengths, adding another layer of planning complexity. Understanding board foot calculations is essential for budgeting, as hardwoods are sold by price per board foot. To calculate board feet, use the formula: (Thickness(in) x Width(in) x Length(ft)) / 12.

Hardwoods are sold by volume (board feet), not by the piece.
The 'quarter' system (4/4, 8/4) denotes rough thickness in 1/4-inch increments.
You must account for material loss when planing rough hardwood to finished size.
Hardwood comes in random widths and lengths, requiring careful selection.

Moisture Content, Seasoning, and How They Affect Size

The moisture content (MC) of wood is a critical factor that directly impacts its dimensions and stability. Freshly cut 'green' lumber has a very high MC and will shrink significantly as it dries. Lumber is seasoned (dried) to a target MC suitable for its end use. Kiln-dried (KD) lumber, common in stores, is dried in a chamber to around 15-19% MC or lower, making it stable for interior use. Air-dried lumber dries naturally over a longer period. As wood loses moisture below its fiber saturation point (around 28-30% MC), it shrinks. Shrinkage is not uniform; it is greater tangentially (around the growth rings) than radially (across the rings). This anisotropic shrinkage can cause warping, cupping, or twisting if the wood is not dried properly or is used in an environment with different humidity. For outdoor projects, pressure-treated lumber is often still 'green' and wet when purchased and will shrink as it weathers. A best practice is to store lumber in the environment where it will be used for a week or two before cutting to let it acclimate, minimizing future movement and ensuring more precise, lasting results.

Wood shrinks as it dries below its fiber saturation point (~30% MC).
Kiln-dried lumber is stabilized for interior use; green lumber will shrink.
Shrinkage is uneven, potentially causing warping if not properly seasoned.
Acclimate lumber to your project's environment before cutting for best results.

Practical Tips for Buying and Measuring Lumber

Armed with knowledge of sizes and standards, here's how to apply it when purchasing lumber. First, always bring a tape measure and check the actual dimensions of the boards you select, as there can be slight variations. For dimensional lumber, inspect each piece for straightness (sight down the edge for bowing or twisting), large knots (especially on the edge, which weakens the board), and excessive warping or crowning. For hardwoods, examine the end grain for checking (cracks) and understand the grading (FAS, Select, etc.) which dictates clear face yield. When calculating materials for a project, always use actual dimensions, not nominal, in your formulas. If a plan calls for a '2x4', substitute 1.5" and 3.5" in your calculations. For decking, remember that a 5/4x6 board (nominal) is actually 1" thick by 5.5" wide. Finally, buy 10-15% more lumber than your calculated needs to account for defects, cutting errors, and waste, especially for complex projects or when using hardwood.

Always measure lumber yourself; don't assume the nominal size.
Inspect for straightness, large knots, and warping before purchasing.
Use actual dimensions, not nominal, in all project calculations.
Purchase extra material (10-15%) to account for waste and defects.

Key Takeaways

Lumber has a nominal name (e.g., 2x4) and smaller actual dimensions due to drying and planing.
Softwood dimensional lumber is standardized; a 2x4 is always 1.5" x 3.5".
Hardwood is sold by board foot volume and measured in a 'quarter' thickness system.
Wood moisture content affects size and stability; acclimate lumber before use.
Always use actual dimensions for project planning and inspect each piece before buying.

Frequently Asked Questions

Why is a 2x4 not actually 2 inches by 4 inches?

A 2x4 starts as a rough-cut piece of wood approximately 2" by 4". It then undergoes drying (which shrinks it) and is planed smooth on all four sides to ensure uniform dimensions. This milling process removes about 1/2 inch from the width and thickness, resulting in the standard finished size of 1.5" by 3.5".

What is the actual size of a 4x4 post?

A nominal 4x4 post, after drying and planing, has an actual size of 3.5 inches by 3.5 inches. This is the standard for pressure-treated pine, cedar, and other common softwoods used for deck posts, fence posts, and structural supports.

How do I calculate how much lumber I need for a project?

First, convert all nominal sizes in your plan to actual dimensions. Then, calculate the linear feet, square feet, or board feet required using these actual measurements. For framing, calculate the number of studs, plates, and joists. For decking, calculate square footage using the actual width of the boards. Always add 10-15% to your total for waste, cuts, and defects.

What does 'S4S' mean on a lumber tag?

S4S stands for 'Surfaced Four Sides.' It means the lumber has been planed smooth on all four faces (two faces and two edges), giving it uniform, ready-to-use actual dimensions. Almost all dimensional lumber and project boards sold at retail are S4S.

What's the difference between dimensional lumber and hardwood?

Dimensional lumber is typically softwood (like pine or fir), milled to standard nominal sizes (2x4, 1x6) and used primarily for structural framing and construction. Hardwood (like oak or maple) is often sold rough, measured in board feet by volume, and used for furniture, cabinets, and flooring where appearance and durability are key.