How to Calculate Floor Load Capacity

Write down the beam strength formula: Maximum load in pounds = FBd^2 / 9L.

Fill in the values for B, d and L. B is the breadth of the joist, in inches. If your floor system uses standard 2-by lumber, this will be 1.5 inches. d is the depth of the joist in inches, which you will have to measure. Most floors use 2-by-8 or 2-by-10 joists; the depth of a 2-by-8 is usually 7.25 inches, and the depth of a 2-by-10 is usually 9.25 inches. L is the span, in feet. The span is the unsupported distance over which the joist must bear its load.

Determine the value for F, which stands for the fiber stress in bending of the wood. This may be difficult to find, because it depends on the species of tree that your joists came from. If the joists are relatively new, the F value may be stamped right on the piece of lumber. If you know the species of the wood, you can refer to a lumber manual to find the corresponding F value. If all else fails, you can safely use 1,000 for F, as almost all graded lumber will have an F value of 1,000 or higher.

Calculate the maximum load in pounds that one of your floor joists can support. For example, if you have 2-by-10 joists spaced 16 inches on center with a span of 14 feet, the calculation would be the following: ( (1,000) x (1.5) x (9.25^2) ) / (9 x 14) = 1,019 pounds per joist.

Calculate the total area supported by one joist according to this formula: joist spacing (in feet) x span (in feet). Continuing the previous example, 1.333 x 14 = 18.7 square feet.

Divide the maximum load of one joist by the area supported by one joist: 1,019 pounds / 18.7 square feet = 54.5 pounds per square foot. This number tells you how much load your floor can support for each square foot of floor space.

Multiply the maximum load per square foot by the total square footage of the floor. If the example floor is 20 feet x 30 feet, the total area is 600 square feet; 600 x 54.5 = 32,700 pounds. This number tells you the total load capacity of your floor.