This paper describes the preparation, measurement and use of microspheres for calibrating individual test sieves. Using a 63 mu m sieve as an example, 2.5 million apertures, or 80% of the surface is examined in under 2 minutes. Because of the narrowness of the size distribution, a 5% difference in percent passing only results in a mean aperture difference of 1 mu m, indeed, the measurement uncertainty for all 24 tests performed was only 0.7 mu m The calibration is independent of the method of shaking and can be used for most sieves.