While studying neutron deflections produced by a magnetic prism, we have stumbled upon a simple ‘geometric’ formula. For a prism of refractive index n close to unity, the deflection simply equals the product of the refractive power n − 1 and the base-to-height ratio of the prism, regardless of the apex angle. The base and height of the prism are measured respectively along and perpendicular to the direction of beam propagation within the prism. The geometric formula greatly simplifies the optimisation of prism parameters to suit any specific experiment.