The importance of wood beam calculations
The fetched Omni wood beam span page checks three things for a uniformly loaded beam: deflection, bending stress, and shear stress. The beam passes only if all three checks stay within their allowable values.
Checking deflection, bending, and shear
delta = 5 x w x L^4 / (384 x E x I)
I = b x d^3 / 12
M = w x L^2 / 8
fb = M / S ; S = b x d^2 / 6
fv = w x L / (2 x b x d)
Source-backed default example
Omni's worked example uses a 2 x 10 Select Structural Douglas Fir-Larch beam spanning 8 ft with 240 lb/ft uniform load. The page uses E = 1,900,000 psi, an adjusted allowable bending value of about 1,066.4 psi, and an adjusted allowable shear value of about 139.68 psi.