The radius of curvature is a fundamental parameter of optical surfaces. Improving the measurement tolerance is critical for an increasing number of applications. Interferometry is potentially a very accurate technique, but careful implementation is critical to achieving full potential. To this end, the error budget for radius of curvature measurement by interferometry is examined. The goal is to achieve 0.001% (10 ppm) measurement tolerance. The major errors, Abbé errors, are typically 10 to 100 μm, and can be virtually eliminated using a distance-measuring interferometer. The remaining major errors are cavity null errors and axial alignment errors. These are quantified and corrections are described. Other errors including environmental and tooling errors are also cataloged.