How to Calculate Lumber
Lumber is sold in nominal sizes that differ from actual dimensions, and hardwoods are priced by the board foot. Understanding these conventions helps you order accurately and avoid costly mistakes.
Board Feet Formula
A board foot is a unit of volume equal to a 1" thick, 12" wide, 12" long piece of wood. Hardwood lumber is priced per board foot:
Board Feet=Tin×Win×Lft12
Linear Feet for Framing
Dimensional framing lumber (2x4, 2x6, etc.) is sold by the linear foot. For wall framing, studs are spaced 16" on center (OC):
Studs=Wall Lengthin16+1
Add 1 for the starting stud, plus extras for corners, intersections, and door/window framing (typically 10-15% additional).
Stud Spacing
16" OC is standard for load-bearing walls and most applications. 24" OC is acceptable for non-load-bearing interior walls and some advanced framing techniques that reduce material use.Nominal vs. Actual Dimensions
Lumber is sold by its nominal size, but the actual dimensions are smaller after drying and planing. This matters for every measurement:
| Nominal | Actual | Common Use |
|---|---|---|
| 2 × 4 | 1.5" × 3.5" | Wall framing, general |
| 2 × 6 | 1.5" × 5.5" | Exterior walls, floor joists |
| 2 × 8 | 1.5" × 7.25" | Floor joists, headers |
| 2 × 10 | 1.5" × 9.25" | Floor joists, beams |
| 2 × 12 | 1.5" × 11.25" | Ridge boards, rafters |
| 4 × 4 | 3.5" × 3.5" | Posts, columns |
Lumber Grades
For framing, #2 grade is the most common and cost-effective choice. Use pressure-treated (PT) lumber for any wood in contact with the ground or within 6" of soil. Select or premium grades are for finish carpentry.