How to Calculate Board Feet: The Formula, Worked Examples and the Traps

Lumber math · Board feet · Updated 2026-09-13

A board foot is the unit hardwood is sold in, and it is a measure of volume, not of length or of area. One board foot is 144 cubic inches of wood: the amount in a piece 1 in thick, 12 in wide and 12 in long. Any board holding that same volume is one board foot, whatever shape it arrives in. A piece 2 in thick, 6 in wide and 12 in long is one board foot. So is a piece 1 in thick, 6 in wide and 24 in long.

The arithmetic is one multiplication and one division, and almost nobody gets that part wrong. What people get wrong is which numbers go into it: the surfaced thickness instead of the rough one, a nominal width instead of the width the board actually is, inches where the formula wanted feet. This guide covers the formula in both of its forms, a worked example you can follow with your own board, tables of common shop sizes, how to tally a whole project, and the four mistakes that turn up later on an invoice.

What a board foot actually is

Hardwood is sold by volume because volume is what you are buying: wood, not board length. Two boards can both be 8 ft long and one of them can hold twice the wood of the other. The board foot puts them on the same footing, and a lumber tally is nothing more than a count of those units. It is worth keeping three units straight before you start, because a yard will use all of them in the same conversation. A board foot is volume. A lineal foot, sometimes called a running foot, is length only, and it is how trim and moulding are normally sold. A square foot is area, and it is how sheet goods are usually priced. Board foot vs lineal foot vs square foot works through the differences and what gets sold which way.

The formula, and why it has two forms

• Length in feet: board feet = thickness in inches x width in inches x length in feet, divided by 12.
• Length in inches: board feet = thickness in inches x width in inches x length in inches, divided by 144.
• Thickness and width are in inches in both forms. Only the length changes units, and the divisor changes with it.

The two forms are the same statement. Multiply a length in feet by 12 and you have it in inches, so the inches form carries an extra factor of 12 in the numerator, and its divisor is 12 times larger to match: 12 x 12 = 144. That 144 is the definition itself, the cubic inches in one board foot. Prove it on a board you already know. Take a piece 1 in thick, 8 in wide and 8 ft long: 1 x 8 x 8 = 64, and 64 / 12 = 5.3333 board feet. The same board measured in inches is 96 in long: 1 x 8 x 96 = 768, and 768 / 144 = 5.3333 board feet. Use whichever form matches the tape in your hand. Rack lumber is easier in feet, cut parts are easier in inches, and neither one is more correct.

Calculate one board, step by step

Here is a single rough 8/4 walnut board, 7-1/4 in wide at its narrowest point and 9 ft long. Follow the steps with your own numbers in place of these ones.

1. Write down the thickness in inches: For hardwood, use the rough quarter thickness, not the thickness the surfaced board measures. 8/4 is 2.0000 in.
2. Measure the width in inches: Measure the board rather than assuming it. If it tapers, use the narrow end. This one is 7-1/4 in, which is 7.25 in as a decimal. Either way of writing the number works in the formula.
3. Pick a length unit, and take the divisor that goes with it: 9 ft goes with a divisor of 12. If you would rather work in inches, 9 ft is 108 in and the divisor is 144. Do not mix a length in feet with a divisor of 144, or the other way round.
4. Multiply the three dimensions: 2 x 7.25 x 9 = 130.5. In the inches form it is 2 x 7.25 x 108 = 1566.
5. Divide: 130.5 / 12 = 10.875 board feet. The inches form gives 1566 / 144 = 10.875 board feet. Same board, same answer, as it has to be.
6. Multiply by the number of pieces: Three boards like this one: 10.875 x 3 = 32.625 board feet.
7. Round once, at the end: 32.625 rounds to 32.63 board feet. Round here and nowhere earlier in the sequence.

Worked examples at common shop sizes

These all use the feet form. Read the middle column as the working: multiply the three dimensions, then divide by 12. Thicknesses written as quarters are rough thicknesses, which is how a hardwood yard will bill them.

Cut parts are easier in the inches form, because a cut list is already written in inches and converting 27 in into 2.25 ft is one more place to slip. Multiply the three dimensions and divide by 144 instead of 12. Everything else is identical.

Quantity, and a whole project

Quantity is a multiplication and nothing more. Work out the board feet in one piece, then multiply by how many of that piece you need. A project is the same idea one level up: every part gets a line, every line is pieces times board feet each, and the project total is the sum of the lines. Keep the lines separate instead of carrying a running total in your head, because a line you can see is a line you can check a week later when the tally does not match the invoice. Here is a small side table, four parts and fifteen pieces, worked both ways: the top and the rails in feet, the legs in inches, because that is how the cut list reads.

That 37.7083 rounds to 37.71 board feet, and it is the net figure: the wood that ends up inside the finished piece. It is not what you buy. Every part has to be cut out of something larger, with the checks, the wane, the snipe and the saw kerf taken off first, and the allowance for that is a waste factor applied on top of the net total. At a 20 percent allowance this project becomes 45.25 gross board feet. At 30 percent it becomes 49.02. How much extra lumber to buy covers how to choose that percentage and how net and gross relate to each other.

Keep the working, not just the answer. Board Foot Calculator₊ runs both forms of the formula, takes fractions or decimals anywhere you type, carries a rough and surfaced toggle for every quarter thickness, and labels every number it hands back. Tap a total and it opens line by line with your own inputs echoed in, so you can check the math at the rack instead of trusting it. Learn more about Board Foot Calculator₊.

Four traps that cost money

The first trap is the one in the callout above: thickness and width stay in inches no matter what the length is doing. The other three are not about the arithmetic at all. They are about which number you write down before the arithmetic starts.

A board is not its nominal size. Softwood construction lumber is named for a size it does not have. A 2x4 measures 1-1/2 by 3-1/2 in, and yet board-foot math for construction lumber is conventionally done on the nominal 2 by 4, because nominal is the size the trade prices. Hardwood works the other way around. It is sold in random widths and you measure what is actually in front of you. A 4/4 board that happens to be 7-1/4 in wide is a 7-1/4 in board, not an 8 in board, and a board that runs wider than you assumed costs you on the same arithmetic in the opposite direction. Yards differ in how they treat part-inch widths on a tally, so ask yours how it measures before you argue about a total, and measure a tapered board at its narrow point.

Hardwood is billed on the rough thickness. This is the expensive one, and it is the single most common reason a shop tally comes up short of the invoice. An 8/4 board is charged at 2.0000 in of thickness even after it has been surfaced two sides and measures 1.7500 in under your caliper. The yard is not shorting you. The surfacing allowance is part of how the trade prices lumber, and the tally was taken before the board went through the planer. Put the rough number into the formula. What 4/4 lumber means goes through quarter notation, rough, S2S and S4S in full.

Rounding too early. Board feet come out as repeating decimals constantly, because 12 divides cleanly into very few of the numbers a shop actually uses. If you round each piece to two places and then multiply by the count, the rounding error multiplies along with it, and if you then add rounded lines together the errors stack. Carry the full value, multiply by the quantity, add the lines, and round once at the very end. The gaps look trivial on one line and stop looking trivial on a tally of fifty pieces.

Where to go next

If you want the numbers without doing the arithmetic at all, the board foot chart holds the reference tables: common widths against common lengths in 4/4 and 8/4, a quarter thickness reference, a board feet per foot of length table you can use at any quarter, plus the 2x nominal stock. If you are about to buy, work out your net total here and then read the lumber waste factor guide, because the figure you hand the yard is the gross one, not the net one. And once you have board feet in hand, how lumber is priced per board foot covers putting two quotes on the same footing. The arithmetic under all of it is what Board Foot Calculator does, if you would rather work it on the phone at the rack.

Frequently asked questions

What is the formula for board feet?

Board feet equal thickness in inches times width in inches times length in feet, divided by 12. If your length is written in inches instead, divide by 144 rather than by 12. Thickness and width are in inches in both versions, and only the length unit ever changes. The 144 is simply the number of cubic inches in one board foot.

How many board feet are in a 1x6x8?

A nominal 1x6 that is 8 ft long is 4.00 board feet: 1 x 6 x 8 = 48, and 48 / 12 = 4. Construction lumber is figured on the nominal size even though the board really measures 3/4 by 5-1/2 in. A hardwood board of roughly that size would instead be measured and billed on its actual width and its rough thickness.

How many board feet are in a 2x4x8?

An 8 ft 2x4 is 5.33 board feet: 2 x 4 x 8 = 64, and 64 / 12 = 5.3333. The stick measures 1-1/2 by 3-1/2 in, but softwood board-foot math is conventionally done on the nominal 2 by 4. A 2x4 that is 10 ft long is 6.67 board feet on the same basis.

Do I use rough or surfaced thickness to calculate board feet?

Use the rough thickness. Hardwood is tallied and billed before it is surfaced, so an 8/4 board is charged at 2.0000 in even though it arrives measuring 1.7500 in. The surfacing allowance behind that is 3/16 in for stock 1-1/2 in and under, and 1/4 in for stock 1-3/4 in and over. Figuring on the surfaced thickness will make your total come out short of the invoice every single time.

Is a board foot the same as a square foot?

No. A board foot is a unit of volume, a square foot is a unit of area, and a lineal or running foot is length only. Hardwood is normally sold by the board foot, sheet goods by the square foot or by the sheet, and trim and moulding by the lineal foot. Mixing them up is how a quote ends up looking far cheaper or far dearer than the one beside it.

How do I calculate board feet for a whole project?

Work out each part on its own line, multiply by the number of pieces, then add the lines together. Keeping parts separate lets you check any single one later, and lets you handle species separately if the project mixes them. Carry full decimals through the multiplication and the addition and round only the final total. Then add a waste allowance on top, because the sum of the finished parts is the net figure, not the amount you need to buy.

Why does my board foot total not match the lumber yard's?

The usual causes are thickness and width, in that order. The yard bills hardwood on the rough quarter thickness rather than the surfaced thickness you can measure, and it tallies the width the board actually is rather than the nominal width you assumed. Rounding each piece before multiplying by the quantity is a smaller but real third cause. Check those three before you check the arithmetic, because the arithmetic is almost never where it went wrong.

Keep reading

• Board Foot Chart: Common Hardwood and Softwood Sizes
• What Does 4/4 Lumber Mean? Quarter Thickness, Rough and Surfaced
• Lumber Waste Factor: How Much Extra Lumber to Buy, and Net vs Gross Board Feet