The approximate strength of a wooden beam is governed by a fairly simple formula that involves only the width, depth, span, load and fibre stress rating of the beam. This formula allows you to safely make load-bearing and beam-sizing calculations for many interior and exterior projects.

Determine the "fibre stress in bending" rating of the species of wood used to make your beam. Call this F. Your beam supplier should have this information, but if not, you can find it in various publications from the lumber industry. If all else fails, you can use F = 1000 as an approximate value.

Determine the span of the beam in feet. Call this L. The span is the unsupported distance over which the beam must bear its load.

Choose a value for the beam depth in inches. Call this d.

Calculate the maximum load in pounds the beam will carry. This will vary greatly depending on the use of your beam, but most load-bearing situations can be quickly approximated. For example, a second-story floor load is estimated at 18.1 Kilogram per square foot, and a roof load is estimated at 4.54 Kilogram per square foot plus the snow load in your region. In areas with little snow, consult a carpenter or building inspector for loading guidelines.

Calculate beam width in inches according to the following formula: W = ((Max load) * (9) * (L))/(F * d^2).