Extreme fiber stress in bending for a rectangular timber beam is calculated by the following
f=6M/bh²
=M/S
where
f= maximum fiber stress, lb/in² (MPa)
M= bending moment, lb in (Nm)
h= depth of beam, in (mm)
b= width of beam, in (mm)
S= section modulus (bh2/6 for rectangular section), in³ (mm³)

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As per the guidelines, the allowable unit stresses for any given compression perpendicular to the grain is applied to any length bearings at the ends of beams. But when the length is more than 152.4mm or 6 inches, it is applied to all at other locations.

When we are calculating the bearing at the end of beams we should keep in mind that we don’t take into consideration the bending of beams and also that the pressure on inner edge is more than that at the outer edge.

When the bearings are less than 152.4mm or 6 inches in length and are not close than 76.2 mm or 3 inches to the end of the concerned member, the allowable stress for compression perpendicular to the grain is to be modified by using following formula
(l+3/8)/l

where l is the length of the bearing in mm or inches measured along the grain of the wood.