The field normal to the plane of a wire of n turns and radius r which carries a current i is measured on the axis of the coil at a small distance h from the centre of the coil. This is smaller than the field at the centre by the fraction

Field at the centre, B1 =μ04π ×   2πinr  =  μ02 .  nirField at a distance h from the centre,B2 =μ04π .   2πnir2r2 + h23/2  =  μ02 .  nir2r3 1+h2r23/2=B1  1+h2r2−3/2 =  B1  1−32.h2r2                                                    (By binomial theorem)Hence B2 is less than B1 by a fraction = 32  h2r2