Standard drying conditions in the laboratory have been related to the diurnal fluctuations of temperature and humidity to define a standard drying day for forest fuels. The mean equilibrium moisture content of a fuel on a standard drying day is slightly different from the equilibrium moisture content for standard conditions, but the difference is small enough to be negligible. This relationship may be used to compare the moisture change of a specific fuel during a specified time, or the time required to reach a specified moisture content, under field conditions, to the moisture change or time under standard conditions. Forest Sci. 16:121-128.