We examine the “input capacitance,” CSETT, of a single-electron tunneling (SET) transistor. We note that this quantity is crucial in quantifying the sensitivity of a SET transistor used as a charge electrometer. Further, we point out that CSETT is not the same as the “gate capacitance,” CG, usually taken to be e/ΔVG, where ΔVG is the period of the oscillation in current versus gate voltage. While CG is indeed the average value of CSETT over one period, CSETT can in fact differ substantially from that value, depending on the applied voltages. This has important consequences for maximizing the sensitivity of SET charge electrometers when a large stray capacitance is present.