Abstract: The Heat Stress Calculator is a manually operated, mechanical computer and program for determining human thermal discomfort or heat stress, reported as an equivalent heat stress temperature (HST), for any given summer air temperature, relative humidity, wind velocity, sky condition, time-of-day, terrain, and individual physical activity level. The calculator consists of six circular and partial discs concentrically mounted for adjustment of their scales by rotation to various positions relative to each other and to stationary front and back windowed panels. The Heat Stress Temperature is the equivalent air temperature under standard moderate thermal and activity conditions that would result in the same heat stress produced by the actual conditions experienced.