Board Footage Calculator

What a board foot measures

A board foot is a volume, not a length. One board foot is 144 cubic inches: a piece 12 inches wide, 12 inches long and 1 inch thick. Every price quoted per board foot is a price for that volume, which is why a wide board costs more than a narrow one of the same length, and why thickness counts as much as width.

Hardwood dealers, sawmills and millwork shops sell by the board foot. Home centers sell construction lumber by the piece, so the board footage on the tag is there for the contractor's estimate rather than the checkout scanner. The National Institute of Standards and Technology publishes the voluntary product standard that governs softwood sizes, PS 20-20, the American Softwood Lumber Standard, and the USDA Forest Service covers the same ground for commercial lumber in chapter 5 of the Wood Handbook. Both use the formula this calculator applies.

The formula, worked three ways

Thickness in inches times width in inches times length in feet, divided by 12. The divisor is 12 because a board foot is 12 inches of width at 1 inch thick for every foot of length. Multiplying the three dimensions gives cubic inches per foot of run, and dividing by 12 turns that into board feet.

• 2x4 at 8 feet, dressed 1.5 by 3.5 inches: 1.5 x 3.5 x 8 / 12 = 3.5 board feet
• 2x10 at 16 feet, dressed 1.5 by 9.25 inches: 1.5 x 9.25 x 16 / 12 = 18.5 board feet
• 4/4 hardwood, 6 inches wide, 8 feet long: 1 x 6 x 8 / 12 = 4 board feet

Watch the units, because this is where most hand calculations go wrong. If you measure length in inches, divide by 144 rather than 12. If you measure thickness in quarters, 4/4 is 1 inch, 5/4 is 1.25 inches and 8/4 is 2 inches. A 5/4 board is not a 1-inch board, and the 25% difference shows up on the invoice.

Length also has to be the real length. A "2x4x8" from a home center is usually 8 feet, but studs are often cut to 92-5/8 inches so they fit under an 8-foot ceiling with plates. That shorter stud is 3.37 board feet, not 3.5, which matters when you are ordering hundreds.

Nominal size vs actual size: the 2x4 problem

The number on the tag is a name, not a measurement. A 2x4 starts as a rough board close to 2 by 4 inches, then gets planed smooth and dries, and it arrives at 1-1/2 by 3-1/2 inches. PS 20 sets that dressed size, along with every other standard softwood size in the table below.

The gap is not small. On a 2x4 at 8 feet the label claims 5.33 board feet against 3.5 board feet of wood, 52% more. On a 2x12 the gap is 42%, and on a 1x4 it is also around 52%. The thinner the board, the bigger the share lost to planing, because the same 1/4 inch comes off a 1-inch board and a 1/2 inch comes off a 2-inch board.

None of this is a pricing trick. Dressed sizes are the size the board is when you cut it, so they are the ones to use for board footage, for shelf loads and for fitting. Use the label only when you are asking for the board at the counter.

Board feet by common size and length

This table saves you the arithmetic on the sizes that show up in most orders. Values are board feet for a single piece at the dressed size, rounded to two decimals.

One pattern is worth memorizing for 2-inch material: at 8 feet, the board footage of a 2xN equals the actual width in inches. A 2x6 at 8 feet is 5.5 board feet, a 2x10 is 9.25, and the arithmetic falls out of 1.5 x W x 8 / 12 = W. That makes a quick site estimate easy, and it is close enough for a lumberyard run.

Longer boards are not cheaper per board foot, and they are harder to find straight. A 16-foot 2x12 is 22.5 board feet, so a single board can carry a real share of a project budget. If the design allows a joint, two 8-foot boards usually cost less and waste less.

Adding up a stack, with waste

Board footage adds linearly, so a stack is one multiplication. Ten 2x4s at 8 feet: 3.5 board feet each, 35 board feet total. Add 10% for cuts and defects and the order becomes 38.5 board feet. At $4.50 a board foot that is $173.25.

Waste is not padding. Every miter, every defect you cut around and every board that arrives bowed eats into the total, and the smaller the parts, the larger the share. Cabinet parts cut from wide stock can lose 20% before the first assembly.

Round the order up to what the yard sells. Most suppliers price in whole boards, not fractions of a board foot, so 38.5 board feet of 2x4x8 becomes 11 boards and 38.5 board feet on the invoice. The calculator gives you the volume; the counter gives you the count.

What board feet does not tell you

Board footage measures volume and nothing else. Two boards with the same count can differ in every way that matters to a project.

• Grade. Hardwood grades such as FAS, #1 Common and #2 Common describe how much clear, defect-free face a board carries. A #2 Common board can be half knots and still count as board feet.
• Moisture. Kiln-dried stock at 6% to 8% is stable in a house. Green lumber at 20% or more will shrink and cup as it dries, and the width can drop by 1/16 inch or more per 4 inches.
• Surfacing. Rough, S2S (surfaced two sides) and S4S (surfaced four sides) are different products at different prices, and only the surfaced ones match the dressed table above.
• Figure and color. Curly maple and plain maple have the same board footage and a price gap that can run three to one.

That is why a quote of "$6 a board foot" means little on its own. Ask for the grade, the moisture content and the surfacing, then compare. The Wood Handbook chapter linked above explains how each of these is measured and why a grading rule works on the clear face rather than the whole board.

Cost per board foot, and comparing quotes

Once you have board feet, price comparison is one division. Take the price of a single board and divide by its board footage.

• A $9.98 2x4x8 stud: 9.98 / 3.5 = $2.85 a board foot
• A $120 walnut board, 8/4 by 8 inches by 8 feet: 10.67 board feet, so $11.25 a board foot
• A $42 white oak board, 4/4 by 6 inches by 8 feet: 4 board feet, so $10.50 a board foot

The stud looks cheap per board foot because it is a softwood commodity sold by the piece, and the walnut looks expensive because it is a hardwood sold by volume with a grade attached. Neither number is wrong; they are different markets. The comparison only becomes useful when you hold the species, grade and surfacing constant and compare two suppliers of the same material.

For a project budget, work in this order: finished part sizes, then board footage with waste, then price per board foot, then the board count the yard will sell you. Doing it in that order keeps the estimate honest, because the waste allowance and the rounding both land before you commit to a number.

When to buy by the piece instead

Board feet is the right unit for hardwood and millwork, and the wrong unit for a lot of what a home project needs.

Construction lumber is sold by the piece at a unit price, and the board footage is informational. Two studs of the same length can carry different prices because of grade and straightness, and the yard's system prices the piece, not the volume.

Sheet goods are sold by the sheet. A 4x8 panel is 32 square feet, and thickness is in the product name (1/2 inch, 3/4 inch) rather than in a board foot count. Plywood is never quoted per board foot.

Molding and trim are sold by the lineal foot, because the profile is fixed and only the length varies. Baseboard at $3.20 a lineal foot tells you nothing about volume, and that is fine, since you buy it to cover a wall length.

Firewood is sold by the cord, which is 128 cubic feet of stacked wood. That is a volume measure like the board foot, but the conversion depends on how the wood is stacked, so the two units never meet.

Use the calculator for anything a mill would quote by volume, and use the unit the seller quotes for everything else. Mixing them is how a $200 estimate turns into a $340 invoice.