To calculate beam spans, first measure the length of the beam using a measuring tape. Measure the length and width of the room, and multiply them together to calculate the room's area in square feet. Multiply the area of the room by 50 to derive the maximum load of the floor of the room in pounds.

Multiply the floor's maximum load with the beam length, and divide the result by 8 to arrive at the maximum bending movement or fiber strength of bending for the beam. Divide this number with the fiber stress of the beam. The fiber stress of a wooden beam differs between the species of wood used. On an average, a wooden beam's fiber stress is 1,100 pounds per square inch.

Finally, multiply the length of the beam with the result of the beam's maximum bending movement divided by its fiber stress. This figure reflects the beam span.

Wooden beam spans depend on various factors such as the weight of the load the beam is to bear, the type of wood used to make the beam and the beam's size. These factors must be determined before calculating beam spans.