Flexure Formula

σ_{max} = \frac{M c}{I}

σ = - \frac{M y}{I}

σ = - \frac{σ_{max}}{c} y

where:

$σ_{max}$ is the maximum normal stress (at the point farthest away from the neutral axis)
$M$ is the resultant internal moment, about the neutral axis of the cross section
$c$ is the perpendicular distance form the neutral axis to a point farthest away from the axis
$I$ is the moment of inertia of the cross sectional area about the neutral axis
$y$ is the intermediate distance of an element on the cross section