The pressure exerted by an electromagnetic wave of intensity I(Wm^-2) on a non-reflecting surface is (c is the velocity of light):

I/c

When a surface intercepts electromagnetic radiation, a force and a pressure are exerted on the surface. As the surface is non-reflecting, so it is completely absorbed and in such case the force is

$F = \frac{IA}{c}$

The radiation pressure is the force per unit area, $P = \frac{F}{A} = \frac{I}{c}$