An easy-to-make virtual impactor was developed to be utilized as an accessory to optical particle counters. This simple, yet effective, virtual impactor can be used to optimize the sensitivity of optical particle counters to larger particles. A virtual impactor sized for use with a very low flow optical particle counter was built with a 37-mm cassette and a set of appropriately sized hypodermic needles. When evaluated, it multiplied the number concentration of 2- and 3-μm polystyrene latex particles by factors of 15 and 42, respectively. It magnified the number concentration of particles in polydispersed corn oil and salt aerosols and ambient laboratory air by similar amounts. Instructions on how to construct a virtual impactor with a 37-mm cassette, hypodermic needles, and common laboratory items while adhering to critical design parameters are presented.