Common lumber sizes (nominal):
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What is a Board Foot?
A board foot (BF) is a unit of volume used for lumber. One board foot equals 144 cubic inches, or a piece of wood 12" × 12" × 1".
Board Feet Formula
Where thickness and width are in inches, and length is in feet.
Why Board Feet Matter
Hardwood and specialty lumber is priced per board foot. Understanding this measurement helps you compare prices and estimate project costs accurately.
On this page
Nominal vs Actual Lumber Sizes
Lumber is sold by nominal size, but actual dimensions are smaller due to drying and planing.
Common Conversions
- 2×4: Actual 1.5" × 3.5"
- 2×6: Actual 1.5" × 5.5"
- 2×8: Actual 1.5" × 7.25"
- 2×10: Actual 1.5" × 9.25"
- 2×12: Actual 1.5" × 11.25"
- 4×4: Actual 3.5" × 3.5"
- 1×6: Actual 0.75" × 5.5"
The rule behind it: dimension lumber loses 1/2 inch off nominal up to 6 inches wide, and 3/4 inch above that. So a 2×4 is 1.5 × 3.5, but a 2×8 is 1.5 × 7.25, not 7.5. This is why a "2×4" wall framed 16 inches on center gives you a 3.5 inch cavity for insulation.
| Nominal | Actual (in) | Board Ft per Linear Ft | 8 ft Piece | Typical Use |
|---|---|---|---|---|
| 1×4 | 0.75 × 3.5 | 0.33 | 2.67 BF | Trim, shelving |
| 1×6 | 0.75 × 5.5 | 0.50 | 4.00 BF | Fence boards, trim |
| 2×4 | 1.5 × 3.5 | 0.67 | 5.33 BF | Wall framing |
| 2×6 | 1.5 × 5.5 | 1.00 | 8.00 BF | Framing, decking |
| 2×8 | 1.5 × 7.25 | 1.33 | 10.67 BF | Joists, headers |
| 2×10 | 1.5 × 9.25 | 1.67 | 13.33 BF | Joists, stair stringers |
| 2×12 | 1.5 × 11.25 | 2.00 | 16.00 BF | Beams, long-span joists |
| 4×4 | 3.5 × 3.5 | 1.33 | 10.67 BF | Posts |
| 6×6 | 5.5 × 5.5 | 3.00 | 24.00 BF | Structural posts |
Board feet always use the nominal size, even though the board is actually smaller. A 2×4 is billed as 2 × 4, not 1.5 × 3.5. That's the convention across the industry, and it's why the "Board Ft per Linear Ft" column above is a clean 0.67 for a 2×4.
Rough-Sawn and Quarter Sizing
Hardwood is a different world. It's sold rough-sawn in quarters of an inch: 4/4 ("four quarter") is 1 inch thick, 5/4 is 1.25, 8/4 is 2 inches. Rough boards measure close to their stated thickness, but surfacing takes off about 1/4 inch — so 4/4 planed on both faces lands near 3/4 inch. Order rough if you're milling it yourself, S2S or S4S if you aren't.
Lumber Grades and What They Mean
Grade is about structural strength and appearance, and it changes the price substantially. Buying a higher grade than the job needs is one of the easiest ways to overspend.
| Grade | Description | Best For |
|---|---|---|
| Select Structural | Highest strength, few defects | Long spans, engineered loads |
| No. 1 | Strong, limited knots | Joists, rafters, beams |
| No. 2 | The framing standard | Most walls, joists, rafters |
| No. 3 / Stud | More knots, lower strength | Non-load-bearing walls |
| Construction / Common | Appearance-graded boards | Shelving, fences, utility |
No. 2 is what most framing lumber on the rack actually is, and it's what span tables assume. Don't substitute Stud grade into a load-bearing application to save money.
Treated, Kiln-Dried, and Green
- Kiln-dried (KD): Dried to about 19% moisture. Stable, straight, and what you want indoors.
- Green: Undried. Cheaper and heavier, and it will shrink and twist as it dries. Fine for some outdoor framing, a poor idea for finish work.
- Pressure-treated (PT): Chemically treated for rot and insect resistance. Required for any wood in contact with the ground or concrete. Usually sold wet, so it will shrink — let deck boards dry before final fastening if you can.
- Ground contact vs. above ground: Two different PT ratings. Anything touching soil, or less than 6 inches above it, needs ground-contact rated lumber.
Use hot-dipped galvanized or stainless fasteners with pressure-treated lumber. Modern treatments are corrosive to plain steel, and standard screws will fail in a few years.
Estimating Lumber for Common Projects
Wall Framing
Studs at 16 inches on center: divide the wall length in feet by 1.333 and add one, then add extra for corners and openings.
A 20 ft wall needs about 16 studs, plus plates: one bottom plate and usually two top plates, so 60 linear feet of plate stock for that wall.
Deck Framing
- Joists at 16 in on center: (deck width in ft ÷ 1.333) + 1
- Decking boards: deck area in sq ft ÷ board coverage. A 5/4×6 board covers about 0.46 sq ft per linear foot once you allow a 1/4 inch gap
- Posts: typically every 6–8 ft along the beam
Our fence calculator handles post-and-rail layouts, and the square footage calculator is the fastest way to get a deck's area.
How Much Waste to Add
Add 10% for straightforward framing and 15% for decking, trim, or anything with angled cuts. Hardwood should carry 20–30%, because you're also cutting around defects to get clear material.
Buy long and cut down. Two 8 ft pieces cost more than one 16 ft piece and give you an extra joint you didn't want.
Frequently Asked Questions
Should I calculate with nominal or actual size?
For purchasing lumber, use nominal sizes. For calculating if lumber will fit your project, use actual dimensions.
How much extra lumber should I buy?
Plan for 10-15% waste for most projects. Lumber can have defects, knots, or warping that make some pieces unusable.
What's the difference between board feet and linear feet?
Linear feet measures length only. Board feet measures volume. A 2×4×8' is 8 linear feet but 5.33 board feet.
How do you calculate board feet?
Multiply thickness in inches by width in inches by length in feet, then divide by 12. A 2×6 that's 10 ft long is (2 × 6 × 10) ÷ 12 = 10 board feet. Always use the nominal size, not the actual planed dimensions.
Why is a 2×4 not actually 2 by 4 inches?
Because the nominal size is measured before drying and planing. A 2×4 comes off the saw near 2 × 4 inches, then loses material as it dries and gets surfaced smooth, finishing at 1.5 × 3.5 inches. Lumber loses 1/2 inch off nominal up to 6 inches wide and 3/4 inch above that.
How many board feet are in a 2×4?
A 2×4 contains 0.67 board feet per linear foot. So an 8 ft stud is 5.33 board feet, a 10 ft is 6.67, and a 12 ft is 8.
What does 4/4 mean in hardwood?
It's the rough thickness in quarter-inches. 4/4 is 1 inch, 5/4 is 1.25 inches, 6/4 is 1.5, and 8/4 is 2. Surfacing removes about 1/4 inch, so 4/4 planed on both faces ends up near 3/4 inch thick.
What lumber grade do I need for framing?
No. 2 is the standard for framing and is what span tables assume. Use No. 1 or Select Structural for long spans and heavy loads. Stud grade is only appropriate for non-load-bearing walls.
Do I need pressure-treated lumber?
Yes for anything in contact with soil, concrete, or masonry, and for most exterior structures. Note there are two ratings — ground contact and above ground. Anything touching soil or within 6 inches of it needs the ground-contact grade.
What fasteners should I use with treated lumber?
Hot-dipped galvanized or stainless steel. Modern wood treatments corrode plain and electroplated steel, and standard screws can fail within a few years. This applies to joist hangers and connectors too.
How many studs do I need for a wall?
At 16 inches on center, divide the wall length in feet by 1.333 and add one. A 20 ft wall needs about 16 studs, plus 3 extra per corner and 2 per door or window opening, plus plate stock for the top and bottom.
Is it cheaper to buy longer boards?
Usually yes per board foot, and you get fewer joints. Two 8 ft boards almost always cost more than one 16 ft board of the same dimension. Buy long and cut down where the project allows it.
Sources & Standards
- American Lumber Standard Committee — Softwood lumber nominal vs. dressed dimensions
- American Wood Council — Wood framing span and design guidance
Estimates for planning only. This calculator sizes material quantities. It does not size structural members, verify load paths, or confirm compliance with the code in your jurisdiction. Local requirements — frost depth, footing sizes, permits, and inspections — override anything here. For structural or safety-critical work, have the design checked by a licensed engineer or contractor. See our full method and assumptions.