There is a need for valid measures of pro-environmental behavior, in particular ones measuring actual behavior. In response, a number of such measures have been introduced recently, however each faces limitations such as high costs, practical use and/or only measure indirect environmental behavior, limiting their scope, accessibility and inclusivity. In response, this study introduces the easily administered and low cost Email Conservation Task (ECT), which measures direct pro-environmental behaviour (in this case, email use). Here, participants chose between personal costs (time spent on trivial tasks) or environmental costs (receiving unnecessary emails leading to CO2 emissions) across multiple trials. In a pre-registered study testing the validity of the ECT, it was found (as hypothesized) that participants scored higher on the ECT (incurred more personal costs rather than receiving unnecessary emails) when personal costs to them were lower, and also scored higher when the environmental costs of receiving unnecessary emails were higher. Finally, total scores on the ECT significantly correlated with scores on three self-reported indicators pro-environmentalism. Overall the results of this study support all three hypotheses, indicating that the ECT is a valid measure of pro-environmental behavior that can be implemented across a diverse range of research areas.