To measure the influence of proximity effects in electron-beam lithography on the optical properties of planar photonic crystal (PPC) waveguides we propose a PPC structure called the “PECmeter.” The PECmeter consists of nearly identical PPC waveguides which only differ in the number of rows of holes along the waveguide. The difference in the number of rows does not influence the modal properties directly but changes the diameter of the holes neighboring the waveguide through the proximity effect. The operation principle of the PECmeter is demonstrated using energy-intensity simulations of a W3 waveguide (three missing rows of holes) mini stop band. The principle is confirmed experimentally with structures fabricated in the InP-based material system and measured by the end-fire transmission technique. The results clearly show that the application of proximity-effect correction (PEC) is crucial for the fabrication of PPC waveguides. We demonstrate that when using the midpoint-equalization PEC method a near-to-perfect correction with sub-nm hole-radius uniformity can be achieved. We show the PECmeter to be sensitive enough to detect hole-radius changes as small as ∆R=0.4 nm.