The invention relates to a device and method that provide an individualized guideline for fluid replacement during normal activity or inactivity. Preferred embodiments calculate the user's fluid loss using the equation: {[(% humidity.sup.2-1600)*time]/J}+[(temp .degree. F.*total caloric expenditure)/K], wherein when time units are in percent of an hour, J is a value selected from 1250-1290 and K is a value selected from 1610-1650. Preferred embodiments of the claimed device measure loss of fluid through the use of a pedometer and include a housing having a keyboard and a display screen, an ambient temperature sensor, an ambient humidity sensor, a step counting apparatus, a timing apparatus and a microprocessor. A fluid replacement value is communicated via a numerical display, graphic display, or both, on the display screen.