How to Calculate Lumber Beams

Written by jenny westberg
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How to Calculate Lumber Beams
Wood beams provide structural support in some of the most important parts of a building. (construction details image by Greg Pickens from

Whether you're remodelling a bedroom or building your dream home from the ground up, you might find you need lumber beams. Beams not only provide structural support for walls, roofs and ceilings, but many people also find that exposed beams add a touch of beauty to a room. Beams are a type of timber measuring 5 inches or more in thickness and width, and often they are made from western red cedar, Douglas fir, larch, pine, spruce or California redwood. When you go to the lumberyard, carry your calculator so you can calculate the board footage. This lets you estimate the price. In addition, you'll be able to double-check the yard's calculations. That way you can save several dollars per board that you would otherwise spend on rounding errors.

Skill level:

Things you need

  • Calculator

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  1. 1

    Multiply the beam's thickness by its width. Both of these dimensions in the lumberyard will be in inches, such as 6 inches by 8 inches. The next number is the beam's length in feet. For this example, we'll use a beam 5.5 inches by 8 inches and 10 feet long. Multiplying 5.5 by 8 gives 44.

  2. 2

    Divide the result by 12 to convert inches to feet. For the example, 44 divided by 12 equals 3.667, rounded off.

  3. 3

    Multiply the result by the beam's length in feet. Using the example, multiply 3.67 by 10 to give 36.67 board feet.

  4. 4

    Round the result to the nearest board foot. For example, 36.67 rounds to 37 board feet.

Tips and warnings

  • Measurements for some beams, especially antique timber, are by their "nominal size." Due to drying or other factors, an 8-inch by 12-inch antique timber may actually be 7.25 by 11.25 inches. It is still called an 8 by 12, and still priced as an 8 by 12.
  • You can use board footage to estimate price, but not to calculate whether the beam will provide support. It is extremely complex to calculate beam capacity. Do not skip this step, or your wall or ceiling could collapse. Instead, let a professional handle it.

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