4×6 Actual Size in Construction in USA

In U.S. construction, dimensional lumber such as 4×6 plays a pivotal role in structural frameworks, beam support, and heavy-duty applications. While the label “4×6” refers to a nominal measurement, the actual size of a 4×6 differs significantly due to standard industry processing methods. Every professional builder, contractor, and architect must be acutely aware of the true dimensions of this lumber to ensure accuracy in structural performance and material planning.

What Is the Actual Size of a 4×6 in Construction?

The actual size of a 4×6 lumber board in the United States is:

3.5 inches by 5.5 inches
(3 1/2″ x 5 1/2″)

This dimensional reduction occurs as a result of the surfacing process, where the lumber is milled, planed, and dried after its initial rough cut. Though it starts as a full 4 inches by 6 inches, the end product is consistently reduced by approximately 0.5 inches per dimension, resulting in the actual usable size.

Nominal vs. Actual Dimensions of 4×6 Lumber

Understanding the difference between nominal and actual dimensions is crucial for accurate construction.

This discrepancy is standard across all softwood dimensional lumber in the United States and is especially important when connecting lumber, framing walls, designing decks, or estimating load-bearing capacity.

Applications of Actual 4×6 Lumber in Building Construction

The actual 3.5″ x 5.5″ size of a 4×6 board makes it ideal for several key structural and architectural uses:

Structural Load-Bearing Beams

4×6 beams are commonly used as headers, girders, and roof supports due to their increased width and height. Their bulk allows them to span wider distances while carrying more weight than smaller dimensional lumber. Builders often employ actual size 4x6s in residential framing, especially above windows, garage doors, and open interior spaces.

Deck Posts and Support Columns

The stability and mass of actual 4×6 lumber make it a top choice for deck support posts and porch columns. They can be used vertically or horizontally to carry load from overhead framing and are often pressure-treated to resist decay and weather exposure.

Pergolas and Outdoor Structures

For pergolas, gazebos, carports, and wood-framed awnings, actual 4×6 lumber is favored for both aesthetic appeal and structural performance. The thickness adds visual heft, while the real dimensions offer solid joinery options for bolts, lag screws, and notched framing connections.

Interior Timber Accents

In custom homes, actual 4×6 boards are frequently featured as exposed beams, mantels, and ceiling trusses to enhance the rustic or industrial appearance. Their real dimension of 3.5″ x 5.5″ offers a commanding profile when used in open interior spaces.

Species Commonly Used for Actual Size 4×6 Lumber

Lumber species impact not just the appearance but also the strength and longevity of a 4×6 board. The actual size remains consistent, but characteristics vary:

Douglas Fir

• Excellent strength-to-weight ratio
• High load-bearing capacity
• Ideal for framing and beams

Southern Yellow Pine (Pressure-Treated)

• Frequently used for outdoor support posts
• Pressure treatment resists rot and pests
• Dense and heavy with reliable performance

Redwood and Cedar

• Preferred for decorative outdoor uses
• Natural resistance to decay and insects
• Lighter than fir or pine, but less structural

Load-Bearing Capacity of Actual 4×6 Beams

The actual 3.5″ x 5.5″ size allows a 4×6 to perform well in both vertical and horizontal applications. When installed properly, a single 4×6 beam can support hundreds of pounds of dead and live loads, depending on the span, orientation, and spacing.

• Vertical Compression (Post Use): Easily supports roof and deck weight over short heights
• Horizontal Span (Beam Use): Can support 8 to 12 feet spans depending on spacing, wood species, and load

Always refer to span tables and local building codes for precise engineering guidelines.

Weight of Actual Size 4×6 Lumber

The weight of a 4×6 board depends on its length, moisture content, and wood type, but we can estimate:

• Douglas Fir: ~5.3 pounds per foot
• Pressure-Treated Pine: ~6.5 pounds per foot
• Cedar: ~4.2 pounds per foot

For a 10-foot long 4×6, the total weight can range from 42 to 65 pounds.

Board Foot Calculation for 4×6 Actual Size

To calculate board feet, use the formula:

(Thickness in inches × Width in inches × Length in feet) / 12

For an actual 4×6 board (3.5″ x 5.5″), per foot:

(3.5 × 5.5 × 1) / 12 = 1.604 board feet per linear foot

So:

• 10 feet = 16.04 board feet
• 20 feet = 32.08 board feet
• Multiply by cost per board foot for budgeting

Moisture, Shrinkage, and Dimensional Stability

Lumber’s actual size is affected by its moisture content. Kiln-dried 4x6s retain their actual dimensions better than green or air-dried lumber. Most construction-grade lumber is dried to about 19% moisture content, but interior use wood may be dried further to 6–10%, improving stability and reducing warping.

Rough-Cut vs. Surfaced 4×6 Lumber

Not all 4x6s are created equal. You may encounter:

Rough-Cut 4×6

• True to size: Full 4″ x 6″
• Sawn but not planed
• Used for rustic projects or custom milling

S4S (Surfaced Four Sides) 4×6

• Planed smooth on all sides
• Actual size = 3.5″ x 5.5″
• Standard for framing and structural applications

Fastening and Joining Actual 4×6 Lumber

The 3.5″ thickness and 5.5″ width of actual 4×6 lumber allow for:

• Lag screws, bolts, and timber connectors
• Precise notching and lap joints
• Adequate surface area for load distribution and secure connections

In post-and-beam framing, the full depth of the 4×6 is used for notching into top plates, rim joists, and horizontal girders.

Where to Source Actual Size 4×6 Lumber

Home centers, lumberyards, and wholesale mills across the United States stock actual 4×6 dimensional lumber in various lengths:

• Standard lengths: 8 ft, 10 ft, 12 ft, 14 ft, and 16 ft
• Long lengths: 18 ft, 20 ft, and custom cuts for special orders

Always verify that you’re receiving S4S milled boards unless rough-cut is specified. Retail tags often read “4×6” but the actual dimensions are listed below in fine print.

Conclusion: Know the Real Size of 4×6 Lumber for Precision and Performance

The actual size of a 4×6 in U.S. construction is 3.5 inches by 5.5 inches, a critical specification that influences load capacity, design compatibility, and joinery planning. Using these dimensions ensures that all components align perfectly and deliver long-term performance whether used for beams, posts, decks, or architectural detailing. Proper understanding and implementation of actual 4×6 lumber dimensions contribute to safe, code-compliant, and structurally sound construction outcomes.