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If you figure you need a board 1 1/2 by 5 1/2 by 96 inches long, or 2 by 6, 8 feet long, everyone will know that you are used to buying soft wood - construction lumber - sometimes called dimensional lumber. Soft wood is normally sold in standard sizes. Hardwood is normally sold in random widths and lengths. Some lumber yards will plane and/or sand it to your specifications (for an extra fee), and a few lumber yards have boards that are already surfaced on two sides (S2S), or planed on two sides with one edge ripped in a straight line (S3S). If you want it finished to a fixed size (S4S), it will often cost quite a bit more (they charge you for the larger board, charge you for planing it, charge you for trimming it, and discard the scrap). Therefore take your plans and go to your lumber yard of choice (take a pair of gloves to avoid slivers) so you can look for the pieces you need to build your project.
Many lumber yards "skim plane" 4/4 wood to 15/16 inches thick, so you can see color and grain over most of the front and back. Skim planed wood is pretty flat, but not as flat as you might want off a good jointer. You can buy 4/4 S2S wood that is 13/16 thick, ready for sanding, or could have the lumber yard prepare it to the final 3/4 thickness (but there won't be extra wood to remove if you get a ding).
Continuing with our example, a board 1 1/2 x 5 1/2 x 96 would be called a 2 by 6 by 8 feet if it were soft wood, because the 2 by 6 dimension is roughly the size of the board before it is planed smooth and straight. Rough cut hardwood is sold in "quarters" thickness, so that board would be 8 quarters thick. If you buy it rough cut, it would have been sawn green to just over two inches thick, and so that when dry it is a full 2 inches thick. If you asked the lumber yard to surface plane it, it would probably be 1 1/2 inches thick. If you were carefully doing it yourself, you might get 1 3/4 finished thickness. But you might also have to buy "10 quarters" thick wood if you really must have 1 3/4 inch final thickness.
Softwood, or dimensional lumber, is often sold to the end user by the piece or by the linear foot. Hardwood, typically in random widths and lengths, is usually sold by the board foot - for wood 4 quarters (one inch) or less thick, a board foot is a square foot. For thicker wood, such as 5 quarters wood (1 1/4 inch thick in rough form) multiply the square feet by 5/4 to get the number of board foot. There are very complex rules involved with how the dimensions of width and length are rounded off - generally to the whole board foot when dealing with hardwood but measuring multiple smaller boards together.
When buying small quantities of hardwood, you pay for each piece, which is measured when you check out. Really wide and thick wood is slightly more per board foot, but a lot more per square foot. For example, Walnut is at least $5 per board foot, but 8/4 (2 inch thick) walnut is about $6.50 per board foot, or $13 per square foot. Walnut over 8-10 inches wide is also more expensive per board foot. Expect cherry to be about $6 for 4/4 and $7.50 for 8/4, so if the board mentioned above were needed in Cherry, and the lumber yard happened to have a piece exactly the right size, that cherry 2x6 would cost about $60. Since the board may be a little wider or longer, I often have paid $85 or more for such a board. (If my 2 x 6 were actually 2 x 7, I would use the "scrap" as a good piece of 1 x 2 cherry lumber.)
If you are buying in larger quantities, you may get a lower price per board foot for wood "off the top" without the privilege of picking just the best pieces. It will also likely be measured with a "scaling stick" that does some of the legally correct rounding of the sizes (don't expect to use a ruler, measuring to a fraction of an inch, and pocket calculator if you are buying more than just a few boards).
Be sure to know what you are looking for before you go. Asking someone at a lumber yard for advice is silly - they may have 50 choices, each of which is best for somebody. (You may find my page on choosing wood helpful, on my furniture site.) If you want Maple, do you want hard maple or soft maple or (probably not) quilted or birds eye maple? For oak, do you want Red Oak, White Oak, or Quarter Sawn White Oak? For your secondary (hidden) wood, consider poplar, soft maple, ash, or even the less-pretty pieces of your primary wood.
When lumber is first cut, green wood, it is larger than it will be after it is dried, and far larger than it will be after it is planed and straight-line ripped. When buying green wood from a lumber mill, the amount of wood you buy is called gross tally or green tally. Take that same batch of lumber from the mill, say 1000 board feet, and (depending on species) after it is dry, you measure and find you have 900 board feet, 7-10% less. The 1000 board feet gross tally is now 900 board feet net tally or dry tally. Sometimes I call this rough measure since it is what the wood measures when it is still rough sawn but dry. And if you plane it, and make one side straight, the 1000 gross board feet from the tree is now only 800 board feet in your shop, before you figure that you may only get 70% utilization (560 bf) after avoiding knots and other flaws, and fitting to your requirements.
Some lumber yards estimate the original size of the boards you buy, when it was first cut, using the government standards for each species. When you measure what you bought, you may find you have 17-25% less lumber than you expected, because at some point the wood was that much larger. What the lumber yard is doing is legal. You order 100 board feet, pay for 100 board feet (gross tally), and when you measure what you got, you find 82 board feet. That unusually attractive price per board foot is because they are charging you for what the wood once was - the amount of wood that they bought back at the sawmill to fill your order, rather than what it is today. It is the normal measure when you buy wholesale at the sawmill, but occasionally reaches the retail level - computing the size when the wood was green. The lumber yard accountants like this because the amount of wood in inventory doesn't change as the wood dries.
Some lumber yards buy their wood "dry measure" but then have most or all of their lumber planed and "straight line ripped" - one edge will be straight. When they are done, they only measure about 90% as much as they started with. If you order 100 board feet, and pay for 100 board feet (dry tally), and measure what you got, you may only find 90 board feet... corresponding to what you would have gotten if you bought rough lumber and milled it yourself. Those lumber yards don't charge much for the milling, but you only get 90% as much wood - the same 90% they got back from the mill, or an estimate of the wood in rough measure. Their accountants like the fact that the inventory that they sell matches the inventory they bought in dry form.
Still other lumber yards charge a higher price per board foot for skip planed and straight line ripped lumber, but charge you only for the wood actually received. I haven't heard an official name for this measure, but I will call it current tally or true measure, since it is what the wood currently is.
Which is legal? All are valid, honorable business approaches. Which is better? Whatever the lumber yard you like chooses to do. I have seen all three approaches in lumber yards in the Austin area. But none of the lumber yards say which way they measure... and all are defensive when you want to talk about it. I wish there were a simple universal answer, but I haven't found it. It would be nice if they were required to advertise or post a sign that says "we sell green measure, or we sell by rough measure."
A few wood stores (largely mail order) are now selling "dimensional" hardwood by the linear inch, arguing that it is cheaper to buy just what you need. My only warning is to compute the price you will have to pay for the wood purchased this way, and compute how much lumber you would have to buy from a traditional lumber yard to do the same project. My examples have come out at least twice as expensive buying by the inch, compared to buying traditional lumber and trimming it myself.
If you are lucky enough to live in Austin, Texas, as I do, then you may ask where I get my wood - as many have. On my other web site, I keep a list of places I have recently used for wood and hardware, together with comments, phone numbers, and directions. Let me know if you have experience to add to my comments.
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