The usual formula for finding the strength of a beam when simply supported at both ends is:

W = c bd2/L.

Where W = breaking weight in cwt. in centre, c = constant (3.6 spruce fir, 40 Northern pine, Dantzic, and Memel, 3.5 Riga, 4.3 Baltic oak, 5.0 English oak), b = breadth of beam in inches, d - depth of beam in inches, L = clear span or length of beam between supports in feet. Where the load is distributed a beam will carry double the amount. The safe load for temporary work may be one-sixth of the breaking weight, but for permanent work it is better not to exceed one-tenth.