If the surface is level and no external forces other than gravity act on the object, the normal force on the object is equal to
- $ F_N = mg $
where m is the mass of the object and g is the strength of the gravitational field (measured in N/kg or m/s2; on earth it is 9.81 m/s2).
If the surface is slanted θ degrees past horizontal, and normal force will be equal to the force of gravity perpendicular to the surface, which is
- $ F_N = mg \cos \theta $
Unless the object penetrates the surface, the normal force will always be equal to the force pushing the object into the surface.