How to Calculate Lumber for Projects?

Article - July 14, 2009

This question comes from Zach who asks:

Hey Marc, I’m a new woodworker and I’m having trouble trying to figure out how much lumber I need for each project. I know how to calculate the amount of board feet needed, but that doesn’t factor in how much material is lost from saw blades and the countless errors I’m bound to make. So, I guess what I’m trying to ask is do you have any sort of formula for calculating the amount of material needed for a project?

And this was my response:

calculatorBasically there are two ways I approach this. The “lazy” way and the “accurate” way. The lazy way is to take that board foot number and add about 15-20% to it. Then go to the lumber supplier and buy that many board feet. Now the drawback to this is the fact that you might wind up with the wrong set of boards. For instance, if your project calls for a bunch of long pieces, you might have the right total number of board feet but the boards might not be long enough. So you may wind up making another trip to the lumber supplier if you use this method.

Now the accurate method is to plan everything out on paper. Lets assume you have access to rough boards that are 8′ long and 6-7″ wide. I would draw a bunch of “blanks” on a piece of paper and then try to plan out the cuts for all of your project parts. Keep in mind things like milling and kerf widths since the goal is to be as realistic as possible. You may have to erase a few boards and reorganize them for the sake of saving wood, but eventually you should have a nice cut diagram. The interesting part of this process is that you don’t really need to be concerned with board feet. Instead you want to make sure you are getting the right number of boards so you can make cuts according to your diagram. This process takes a while but it can be worth it since the “lazy” way can be frustrating and disappointing. And furthermore, it makes it much easier to select that perfect wood grain while you are at the lumber yard. Don’t be a afraid to bring a piece of chalk with you so you can mark the boards up as you place them in your cart or vehicle.

Most times, I do a bit of both techniques. If I have a bunch of smaller parts, I lump them into a board foot number and make sure I have that number plus 15%. For larger pieces, I make sure I have the right number of boards. And for certain parts of projects, you want to take it one step further by picking very specific boards. For instance, a table top. You don’t want to let fate decide which boards make a bold statement in your project. So ultimately, a combination of those two techniques is what works for me.

Note that when calculating board feet in terms of Thickness x Width x Length  you should be using the rough thickness of the boards. So if your project calls for 3/4″ thick stock and you’re buying rough 4/4, you’ll want to base your calculations off of 1″ thickness since that’s how the lumber yard will charge you.  Even if the wood is under 1″ (and it will be), if it’s marked at 4/4 you’ll be charged for 1″. You should have no trouble using your finished length and width dimensions for the remainder of the calculation.