How to Calculate Loft Beam Sizes

Written by alex smith
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How to Calculate Loft Beam Sizes
Calculating the size of your beam is easy to do. (man measuring lumber image by palms from

If you are installing beams into a loft, it is important to be able to calculate various aspects of their size. This information is vital to purchasing the correct hardware, as well as to make sure that you are following proper safety protocols. These calculations are not complicated, requiring only a tape measure, a calculator and a few charts. If you fell asleep in grade school math class, now is the time to learn what you need to know.

Skill level:

Things you need

  • Measuring tape
  • Calculator
  • Average weight chart

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  1. 1

    Measure the height and width of one end of the beam. This will provide the measurement you need to purchase the proper wall joists to hang them from.

    Round those numbers up to the nearest whole inch, as the actual sizes of lumber are slightly smaller than their names. For example, a "2-by-4" actually measures 1 1/2-by-3 1/2 inches.

  2. 2

    Measure the length of the beam and multiply it by the actual measurements from Step 1 to obtain the total cubic inches. For example, a beam that is 5 inches wide, 8 inches tall and 120 inches wide has a total of 10,800 cubic inches (5 x 8 x 120). Divide this number by 12 to obtain the cubic feet (in this case 900).

  3. 3

    Calculate the weight of the beam by multiplying the cubic inches or feet against the average weight of that kind of wood. The average weights can be found on charts available to purchase at lumberyards. This will only give you an approximate weight of the beam, but will help determine if you are close to the maximum weight load of the joists.

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