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)