Lumber Measurements and Board Feet Explained

Nominal vs Actual Lumber Dimensions

One of the most confusing aspects of lumber is that a “2×4” is not actually 2 inches by 4 inches. The name refers to the nominal dimensions, which are the rough-cut size of the board immediately after sawing. After the lumber is dried (kiln-dried or air-dried) and planed smooth on all four sides, it shrinks to its actual dimensions. This is standardized by the American Lumber Standard Committee.

Common Nominal vs Actual Sizes

What Is a Board Foot?

A board foot (BF) is the standard unit for measuring lumber volume in North America. One board foot equals a piece of wood 1 foot long, 1 foot wide, and 1 inch thick — or 144 cubic inches. The formula is:

Board Feet = (Thickness × Width × Length) / 12

where thickness and width are in inches and length is in feet. Board feet are used to price hardwoods and specialty lumber.

Worked Examples

Example 1: Board feet in an 8-foot 2×6

• Step 1: Use nominal dimensions: 2" thick × 6" wide × 8 feet long.
• Step 2: BF = (2 × 6 × 8) / 12 = 96 / 12 = 8 board feet.
• Result: One 8-foot 2×6 contains 8 board feet.

Example 2: Board feet in a 10-foot 1×8

• Step 1: BF = (1 × 8 × 10) / 12 = 80 / 12 = 6.67 board feet.
• Result: One 10-foot 1×8 contains 6.67 board feet.

Example 3: Cost calculation

• You need 10 boards of 8-foot 2×4. Each board = (2 × 4 × 8) / 12 = 5.33 BF.
• Total = 10 × 5.33 = 53.3 board feet.
• At $6.50 per board foot: 53.3 × $6.50 = $346.45.

Metric Lumber Sizes

Countries outside North America use metric lumber sizes, which are actual dimensions in millimeters. Common metric structural sizes include 45 × 90 mm, 45 × 140 mm, and 45 × 190 mm. Unlike the nominal/actual confusion in North American lumber, metric sizes represent the actual dimensions.

• 1 board foot = 0.002360 cubic meters
• 1 cubic meter = 423.78 board feet
• 1 inch = 25.4 mm

Use our length converter for precise conversions between inches and millimeters.

Real-World Applications

Home Construction

Framing a standard interior wall uses 2×4 studs spaced 16 inches on center. A 10-foot wall requires roughly 8 studs (including top and bottom plates). Understanding actual dimensions is critical for calculating drywall dimensions, insulation fit, and electrical box placement.

Woodworking Projects

Hardwoods sold at specialty lumber yards are priced per board foot. When planning a furniture project, calculate total board feet needed, then add 20–30% for waste from cuts, defects, and kerf loss.

Deck Building

Deck boards are typically 5/4 × 6 (actual: 1" × 5-1/2") or 2 × 6. Joists are commonly 2 × 8 or 2 × 10 depending on span. Calculating material needs accurately prevents costly over-ordering or mid-project shortages.

Frequently Asked Questions

What is the actual size of a 2×4?

A 2×4 actually measures 1.5 inches by 3.5 inches (38 mm × 89 mm). The “2×4” name refers to the nominal (rough-cut) size before the lumber is dried and planed smooth.

What is a board foot?

A board foot is a unit of lumber volume equal to 144 cubic inches, equivalent to a board that is 1 foot long, 1 foot wide, and 1 inch thick. Calculate it as: (Thickness in inches × Width in inches × Length in feet) / 12.

Why are lumber sizes different from their names?

Lumber is named by its nominal size — the rough-cut dimensions before drying and planing. The drying and surfacing process removes about 1/2 inch from each dimension for boards under 2 inches nominal, and about 3/4 inch for larger dimensions.

How do I convert board feet to cubic meters?

One board foot equals 0.002360 cubic meters. Multiply board feet by 0.002360 to get cubic meters. Use our volume converter for other volume unit conversions.