How to Calculate Wooden Beam Spans

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The distance over which a wood beam bears a load when used for support is its span. The span of any wood beam depends on many factors, such as type of wood, weight of the load and overall size of the beam. Calculating the span of a beam requires using various factors that you need to first determine for a structure where the beam is being used.

Measure the length of the beam with the tape measure and make a note on a piece of paper. Calculate the square footage of the room by multiplying the width of the room by its length.

Multiply the square footage of the room by 50 to calculate the maximum load of the floor in pounds.

Multiply the length of the beam by the maximum load. Divide this number by 8. This will give you the “fiber strength of bending” or maximum bending movement for the beam

Divide the maximum bending movement by the fiber stress of a wooden beam. The fiber stress varies depending on the species of wood. The average fiber stress for a beam is 1,100 pounds per square inch or PSI.

Multiply the length of the beam by its maximum bending movement. Divide this number by 1,100 to calculate the section modulus or beam span.


  • Heavily used rooms in a home typically are 50 lbs of load per square foot.

    If a beam is used to support the floor for a second story it needs to carry 1/3 of the total load of the floor.