Lumber is priced by the board foot - a unit that confuses everyone the first time they encounter it. One board foot equals 144 cubic inches of wood, or equivalently a piece 1 inch thick by 12 inches wide by 12 inches long. The catch is that nominal lumber dimensions (like a 2×4) don't match actual dimensions, and whether you use nominal or actual measurements changes your total. Get this wrong on a large framing order and your material costs are off before the first board is cut.
This guide explains the board foot formula, clears up the nominal vs. actual dimension confusion, and shows the per-linear-foot board footage for common lumber sizes. Use the BidFlow Board Foot Calculator to enter any combination of thickness, width, length, and piece count - it returns board feet per piece, total board footage, and optional cost at your current price-per-board-foot.
Nominal vs. actual dimensions - the source of most ordering errors
When a lumber yard sells you a "2×4," neither dimension is accurate. The nominal size refers to the rough-sawn green dimensions before the board is dried and planed. After milling, a 2×4 is actually 1.5 inches thick by 3.5 inches wide. A 2×6 is 1.5 × 5.5. A 1×6 is 0.75 × 5.5.
This creates a genuine estimating question: do you calculate board feet using nominal dimensions or actual dimensions? The answer depends on context. Lumber is priced and sold using nominal dimensions - the industry standard since the 1960s. When you order 100 board feet of 2×4, the supplier uses nominal figures to count what you owe. But when you're calculating how much wood you actually have for a finish carpentry application - say, matching linear footage of paneling to a wall - actual dimensions matter.
The BidFlow Board Foot Calculator uses the formula with whatever dimensions you enter. For purchasing, enter nominal dimensions. For coverage calculations against actual area, use actual dimensions. The table below shows both for common sizes.
Common lumber sizes: nominal vs. actual and board feet per linear foot
| Nominal Size | Actual Size (inches) | BF/Linear Ft (nominal) | BF/Linear Ft (actual) | Common Use |
|---|---|---|---|---|
| 1×4 | 0.75 × 3.5 | 0.33 | 0.22 | Furring, paneling, shelving |
| 1×6 | 0.75 × 5.5 | 0.50 | 0.34 | Trim, fence boards, shelving |
| 1×8 | 0.75 × 7.25 | 0.67 | 0.45 | Fascia, siding, trim |
| 1×12 | 0.75 × 11.25 | 1.00 | 0.70 | Shelving, cabinet sides |
| 2×4 | 1.5 × 3.5 | 0.67 | 0.44 | Wall framing, studs |
| 2×6 | 1.5 × 5.5 | 1.00 | 0.69 | Exterior walls, floor joists |
| 2×8 | 1.5 × 7.25 | 1.33 | 0.91 | Floor joists, rafters |
| 2×10 | 1.5 × 9.25 | 1.67 | 1.16 | Floor joists, headers |
| 2×12 | 1.5 × 11.25 | 2.00 | 1.41 | Stair stringers, beams |
| 4×4 | 3.5 × 3.5 | 1.33 | 1.02 | Posts, deck framing |
| 6×6 | 5.5 × 5.5 | 3.00 | 2.52 | Heavy posts, timber framing |
BF/linear ft calculated as: (thickness × width) ÷ 12. Nominal column uses published nominal dimensions; actual column uses milled dimensions.
Calculating board feet, step by step
1Board feet per piece
FormulaBF per Piece = (Thickness (in) × Width (in) ×
Length (ft)) ÷ 12
Length must be in feet, not inches - the division by 12 normalizes thickness and width (in inches) to produce a cubic measurement in board feet. A single nominal 2×6 at 8 feet long: (2 × 6 × 8) ÷ 12 = 96 ÷ 12 = 8 board feet.
2Total board feet for the order
FormulaTotal BF = BF per Piece × Number of Pieces
The BidFlow Board Foot Calculator takes piece count as a separate input, so you can spec one size and count without doing repeated multiplication. 40 pieces of that 2×6×8: 8 BF × 40 = 320 board feet total.
3Estimate material cost
FormulaTotal Cost = Total BF × Price per Board Foot
Price per board foot varies by species, grade, and market conditions. Framing lumber (SPF #2) typically runs $0.60–$1.20/BF. Clear pine and fir for finish work runs $2.00–$5.00/BF. Hardwoods like oak and walnut start at $5–$8/BF and go up sharply for figured grain. Enter your current supplier quote in the BidFlow Board Foot Calculator to get a real cost estimate alongside the board footage.
Practical notes for lumber orders
Always add 10–15% for waste and cut-off. Framing allows you to plan cuts fairly efficiently, but finish carpentry, stair stringers, and cabinet work generate significant offcuts. The waste factor is higher for shorter pieces (more end cuts relative to material used) and lower for long straight runs.
Standard lengths limit your options. Lumber yards stock specific lengths - typically 8, 10, 12, 14, 16, and 20 foot. If your board foot calculation implies a 13-foot piece, you're buying a 14-footer and paying for the extra foot. Optimize your cut list against available lengths before finalizing the order.
Mixed-size orders require separate calculations per size. Each lumber dimension has its own price per board foot, and prices don't scale linearly by size. A 2×8 is not simply twice the price per BF of a 1×8. Run the BidFlow Board Foot Calculator separately for each size, then sum the costs. This takes 2 minutes and prevents the common mistake of applying one price per BF to an entire mixed-species order.
Hardwood lumber uses different standard lengths. Unlike dimensional framing lumber, hardwood is sold in random widths and lengths at the hardwood dealer. Board footage is still the unit, but you're calculating from whatever specific boards you're selecting off the rack - measure each board individually and add them up.
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