How to calculate board feet
Nominal thickness in inches, times nominal width in inches, times length in feet, over twelve. That is it. A board foot is 144 cubic inches, so if your length is in inches you divide by 144 instead and get the same answer.
The arithmetic is not the hard part. The hard part is that the two numbers you put in are sizes the wood is not, and has not been since it left the mill.
A 2x4 has not been 2 by 4 for a century
It is 1-1/2 by 3-1/2. Not approximately: exactly, as a published minimum. Voluntary Product Standard PS 20-25, the American Softwood Lumber Standard issued by the US Department of Commerce, defines nominal size as the label designation that does not reflect the dressed size, and gives the 2 by 4 as its own example of a piece surfaced to 1-1/2 by 3-1/2 inches.
Britain does not do this. A British sawn 4x2 is sawn at 4 by 2, and planed timber is sold a few millimetres under with the finished size stated. North America sells everything by a name it stopped measuring a very long time ago, which is fine once you know, and expensive on the day you find out.
The half inch rule is wrong on wide boards
Most people learn the deduction on framing: take half an inch off. It holds up to a nominal 6 inch width and then it stops. From PS 20-25 Table 3, dressed dry widths:
- Nominal 4 is 3-1/2. Half an inch off.
- Nominal 6 is 5-1/2. Half an inch off.
- Nominal 8 is 7-1/4. Three quarters off.
- Nominal 10 is 9-1/4, nominal 12 is 11-1/4.
So the deduction changes at 8 inches and stays changed. A quarter of an inch does not sound like much until it is every board across a deck, and then it is a whole extra board every twelve or a gap at the wall that has to be scribed out.
Timbers, which PS 20 defines as 5 inches or more in the least dimension, are a third rule again: half an inch off up to 6 inches, three quarters from 7 to 15, and a full inch at 16 and over. A 6x6 post is 5-1/2 square. An 8x8 is 7-1/4.
You are billed for wood that was never there
Put the two facts together and the consequence is uncomfortable and entirely legitimate. An 8 foot 2x4 is 5.33 board feet on the invoice and 3.5 board feet of lumber in your hands. The board foot is defined on the nominal size, so the invoice is right and there is nothing to dispute.
The gap is biggest on small sections, because the fixed loss is a bigger share of a small piece: about a third on a 2x4, around a quarter on a 2x10, under a tenth on a 6x6. This tool prints both figures because that is the honest way to answer how much wood you are getting, and because nobody else prints the second one.
Linear feet, board feet and square feet answer different questions
A linear foot is how far the stick goes. A board foot is how much wood is in it. A square foot is how much surface it covers. They are not convertible without knowing the size, which is why "convert linear feet to board feet" has no answer on its own: ten feet of 1x4 is 3.33 board feet and ten feet of 2x12 is 20.
Coverage is the one that bites on decking and siding, because it depends on the ACTUAL width. Ten 8 foot nominal 6 inch boards cover 36-2/3 square feet, not the 40 the nominal size implies. The tool works coverage off the dressed width for that reason.
Even lengths, and why an odd run costs more than it looks
Softwood is stocked in 8, 10, 12, 14 and 16 foot lengths. Twenty exists and is not always on the rack. There are no odd lengths, so a 13 foot run is bought as a 14 and you pay for a foot you throw away, which is 7 percent of that stick before a single cut.
This is worth knowing while a deck or a wall is still on paper. An even-length layout wastes almost nothing. It is the cheapest saving available on a lumber order and it is gone by the time you are at the yard.
Britain has no unit like the board foot
The British tool works in millimetres, in 300mm stock steps, with a 3mm saw kerf, and it never mentions board feet because nothing in Britain is sold that way. This one works in nominal inches and even feet, quotes a US federal standard, and leads with a unit that does not exist in British practice. Same arithmetic underneath, genuinely different questions on top, so they get their own pages rather than one page with a units toggle.
What this does not do, and why it is not a lumber estimator
It does not price lumber. The rate field is yours, and the cost shown is your number times the nominal board feet, which is how a yard bills. It does not grade or span anything: board feet are a quantity, not a structural property, and how far a joist reaches is a different question on a different page. And the dressed sizes are the published minimums at 19 percent moisture content, so a real piece can be a shade over.
It is not an estimating package either. Those price a whole job from a drawing, keep supplier rates and produce a bid, which is a subscription product and a full time job to keep current. This turns a list of pieces into the numbers that list is worth, for nothing, in a browser.
Common questions
How do you calculate board feet?
Nominal thickness in inches, times nominal width in inches, times length in feet, divided by 12. An 8 foot 2x4 is 2 times 4 times 8 over 12, which is 5.33 board feet. If you have the length in inches instead, divide by 144, because a board foot is 144 cubic inches. The part that catches people is which sizes go into it: the nominal ones, the numbers on the label, not what the piece measures.
Why is a 2x4 not 2 inches by 4 inches?
Because it is planed after it is sawn. It leaves the mill at roughly 2 by 4 and arrives surfaced to 1-1/2 by 3-1/2. The US Department of Commerce standard PS 20-25 states it plainly in its definition of nominal size: the nominal size is greater than the dressed size, and a dry 2 by 4 is surfaced to 1-1/2 by 3-1/2 inches. It is not a tolerance or a cheat, it is the published standard, and the same nominal names are used in Canada under the NLGA rules.
Do board feet use the actual size or the nominal size?
Nominal, and that is the definition rather than an oversight. So you are billed for 5.33 board feet of 8 foot 2x4 and about 3.5 board feet of wood turns up, a difference of a third. This tool prints both numbers next to each other because both are true and every other calculator gives only the first. If you are pricing a job, use the nominal figure, because that is what the invoice will say.
How many board feet are in a 2x10?
A 12 foot 2x10 is 2 times 10 times 12 over 12, so 20 board feet. What is worth checking on a 2x10 is the actual width: it is 9-1/4 inches, not 9-1/2. The deduction is half an inch up to a nominal 6 inch width and three quarters of an inch from nominal 8 upward, so anybody applying the half inch rule they learned on 2x4s is a quarter of an inch out on every wide board. Across a floor of joists at 16 inch centers that adds up.
How do I convert linear feet to board feet?
You cannot, until you say what the stick is, and that is the useful answer rather than a formula. Ten linear feet of 1x4 is 3.33 board feet. Ten linear feet of 2x12 is 20 board feet. Same length, six times the volume. Linear feet measure how far the wood goes; board feet measure how much wood there is. Trim and moulding are sold by the linear foot for that reason, and rough or hardwood stock by the board foot.
What is the difference between boards, dimension lumber and timbers?
PS 20 divides them by nominal size and the dressing rules differ for each. Boards are under 2 inches thick and 2 inches or more wide. Dimension is 2 inches up to but not including 5 inches thick, which is where framing, joists, rafters and studs live. Timbers are 5 inches or more in the least dimension. It matters because a timber loses half an inch up to 6 inches, three quarters from 7 to 15, and a full inch at 16 and over, so a 6x6 is 5-1/2 square while an 8x8 is 7-1/4 square.
What does green lumber mean, and is it a different size?
Green means over 19 percent moisture content, for anything under a nominal 5 inch thickness. It is slightly larger when dressed, because it has not shrunk yet: a green 2x8 is 7-1/2 wide against 7-1/4 dry. That is deliberate, so it lands near the dry size once it has dried out. The practical consequence is movement. A green frame is built expecting shrinkage, and a green board fitted tight is a board that cups.
Why are lumber lengths always even numbers?
Convention, and it costs you on odd runs. North American softwood is stocked in 8, 10, 12, 14 and 16 foot lengths, with 20 available and not always on the rack. A 13 foot run is bought as a 14 and a foot goes in the skip. Worth knowing at the layout stage rather than the till: a deck or a wall designed to an even length wastes almost nothing, and one designed to 13 feet wastes 7 percent of every stick.
How much lumber do I need?
That depends on what you are building, and this page answers the second half of the question rather than the first. Work out the pieces from the structure, which for a wall is the stud-wall page and for a floor is the joist span page, then bring the list here to get the board feet, the linear feet and what it actually measures. Splitting it that way is deliberate: a single box promising to size a deck and total the lumber gets one of the two jobs wrong.
Can it work out lumber cost?
From your own rate, yes. Put in your price per board foot and it multiplies by the nominal board feet, which is how a yard bills. It does not know what lumber costs where you are, and it never will: prices move weekly, vary by species and grade, and a figure baked into a web page is a figure that is wrong within a month. Getting the quantity right is the part a calculator can actually do.
Does this work in Canada?
Yes, and the sizes are the same. Canadian mills grade to the NLGA rules under the same American Softwood Lumber Standard, so a Canadian 2x8 is the same 1-1/2 by 7-1/4 as an American one and a board foot is the same 144 cubic inches. Prices, grades stocked and species mix differ, which is why the price field is yours to fill in rather than something this page pretends to know.