What Is the Radius of the Edges of 2X4 Lumber?

Radius edge boards give projects a more finished look.

Straight Edge Thickness

The actual size of a board is smaller than its listed size. According to Voluntary Product Standard, published by the National Institute for Standards and Technology (NIST), the actual thickness of a finished straight edge 2X board is 1.5 inches (38 millimeters), rather than 2 inches.

Edge Geometry

The cross section of a straight edge board has 90-degree angles at the corners. A radius edge board is made by rounding these sharp corners. After rounding, each edge has a straight center portion with a smooth quarter-circle at each end.

Radius Edge Thickness

Lumberyards specify a radius edge board, which was made from a 2X straight edge board, as 5/4X because the straight center portion of the edge measures 1.25 (five-fourths) inches.

Radius of Edge Quarter-Circle

The difference between the 1.5-inch straight edge thickness and the 1.25-inch radius edge center thickness is due to the two corner quarter-circles. Each has a radius of 0.125 inch (one-eighth inch or 3.2 millimeters).

The Drip Cap

Board lumber is commonly available with either straight or radius edges.
According to Voluntary Product Standard, published by the National Institute for Standards and Technology (NIST), the actual thickness of a finished straight edge 2X board is 1.5 inches (38 millimeters), rather than 2 inches.
The cross section of a straight edge board has 90-degree angles at the corners.