In order to convert the pressure into a shear stress, it is necessary to determine the component of the pressure that provides shear on the bed. For a channel that is at an angle  from horizontal, the shear component of the stress acting on the bed , which is the component acting tangentially to the bed, equals the total pressure times the sine of the angle .
In natural rivers, the angle  is typically very small. As a result, the small-angle formula states that:
The tangent of the angle  is, by definition, equal to the slope of the channel, .
From this, we can arrive at the final form of the relation between bed shear stress and depth–slope product: