When performing precision spectroscopy experiments, it is often necessary to incorporate a stabilized, high finesse Fabry-Perot etalon to provide frequency markers. We show that when the experiments use diode lasers, this technique can be replaced by precision measurements of the laser injection current. We examine the relationship between injection current and optical frequency for three TJS single-mode AlGaAs diode lasers and show that, to the limit of our measurement accuracy, the relationship is linear. By using the hyperfine structure of the D 2 line of cesium as our frequency reference, we are able to determine optical frequencies inside a 9.4 GHz range to within a few MHz; furthermore, we show that we can extrapolate this calibration several GHz beyond the calibration range. This optical frequency-calibration technique is very simple to implement and should be widely applicable.