We demonstrate nonreciprocal critical current in 65 nm thick polycrystalline and epitaxial Nb thin films patterned into tracks. The nonreciprocal behavior gives a supercurrent diode effect, where the current passed in one direction is a supercurrent and the other direction is a normal state (resistive) current. We study the variation of the diode effect with temperature, magnetic field, and the width of the Nb tracks from 2-10 µm. For both polycrystalline and epitaxial samples, we find that tracks of width 4 µm provides the largest supercurrent diode efficiency of up to ≈ 30%, with the effect reducing or disappearing in the widest tracks of 10 µm. It is anticipated that the supercurrent diode will become a ubiquitous component of the superconducting computer.