How to figure "Board Foot" measures by Rex Rothing

Many new woodworkers and some experienced construction carpenters often find figuring board foot measures confusing. Read this article and you will learn why there is confusion, and how to figure a board foot correctly.

The formula is simple: Thickness x Width x Length (in inches) divided by 144 is equal to the board foot measure of those dimensions. A board foot is equal to 144 cubic inches; in other words, a 1" thick piece 12" x 12", or a 1" x 6" x 24", or a 1" x 3" x 48", or a 1" x 1" x 12 feet long would each be one board foot. When you start to figure different thicknesses, like 5/4 or 6/4 stock, you need to use a calculator. If you use thickness and width in inches and the length in feet, then you only divide by 12 on the calculator.

So here it is again:  Thickness x Width x Length (in inches) ÷ 144 = Board Feet

The confusion comes in when people are familiar with buying construction grade building materials at a lumber yard and then at a hardwood dealer, find the pricing different. Hardwoods are sold and graded differently from softwoods and construction grade materials.

Softwoods and constuction grade materials are usually sold by the piece or by the lineal foot, and they are sold with "nominal dimensions". A 2 x4 is not really a 2 x 4. A two by four is 1.5" x 3.5" in thickness and width, and so it is with 2 by 8, 1 by 6, etc. So when a one inch board is called 4/4 it is really 3/4. And what they call 5/4 is one inch thick. A person could assume they have ripped and planed the board already so part of your purchase has been turned into sawdust already. You are paying for the size they cut it out of, not the size you are getting.

Hardwoods of a furniture grade are sold by the board foot. These woods are sold as they come off the tree; not all the same size, but rather by random widths and lengths. Instead of buying by the piece, you buy by the cubic inch. Each piece is measured and calculated seperately. A board called 4/4 is an inch thick rough cut and usually finished off the planer at 7/8 to 3/4". A 5/4 board is 1.25" thick and usually finishes at 1.125 to 1". The same with 6/4 and 8/4 boards which are inch and a half or two inches thick and plane out to 1/8 to 1/4 inch less, depending on how flat they have dried out and how well you need the second side to look.

Here's How to Order exotic, domestic and native hardwoods:

Figure how thick you need the finished piece to be, then add 1/8 to 1/4" to that for surface planing. For 3/4 finished you order 1 inch rough. Woodworkers call 1 inch 4/4 (four quarters). Woodworkers are allowed to use improper fractions ( 5/4, 6/4, and 8/4) because they understand them and because that is the way woodworkers do it. Construction workers are allowed to use nominal measures because that is what they understand and that is what they do.

Next, figure how wide the board needs to be finished at, then add 1/4" to that for waste that is lost on the jointer and the saw cut for each piece. If you need two pieces the same, you either double the width or double the length depending on what size is available. Continue to figure additional cuts till you arrive at a total size board or boards you need. There is more than one way to cut your order depending on what sizes are available and what is the best use of the material. Bringing a complete "rough cut list" with you to a hardwood dealer can be helpful in determining the least amount of waste allowance and your optimal purchase of the wood available.

Figure how long the boards you need by adding an inch to the length of each finish-sized cut. Then figure how many cuts you will get out of the length boards available. Adding that extra inch allows you to rough cut the pieces, then joint, plane, and route them before making the final square trim and exact length cuts. Sometimes the planer or router will chip off the ends of a board so it is better to make the final length cut last. 

Now you know how much waste to count in your order if you can buy the right size board. If you do the math calculation you will know how many board feet to order. For a big job order when you can't plan all your cuts beforehand, add an additional 10 to 20 percent for allowance of waste. There might be some knots, cracks, waned edges, and uneven cuts to allow for.

What is shrinkage? Shrinkage is when you order a board foot and you get only nine tenths, or when you pay for 1.08 board feet and you only get one board foot. Hardwood wholesalers like to charge for the wood size it was before it dried out. They could sell a board foot as a board foot and just figure the shrinkage in the price but they are hoping you don't really notice the shortage in your delivery. They are really trying to disguise the price as being lower than it is in reality. When you order your hardwoods it is a good idea to ask if there is a shrinkage charge. Some woods, such as teak, are sold as "net tally", meaning you are getting all you are paying for.

At WoodShop 102, you can order any wood you like and we will cut it for you to thickness, width, and length, in any number of pieces, and we will charge you only for that amount of wood you actually use. The shrinkage charge, the incoming freight, and cutting to size is all included in our price. Labor is added to orders with extra ripping, planing, and crosscutting involved, as in multiple piece orders.

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