Heat energy (HE) of small ruminants in free-moving settings such as grazing is often measured indirectly from heart rate (HR) and the ratio of HE to HR determined when situated once or periodically in a stationary calorimetry system. Therefore, feasibility of use of a portable indirect calorimetry system with goats while grazing, for a direct estimate of HE based on gas exchange without reliance on HR, was investigated in this experiment. Ten yearling Boer goat wethers (44.4 ± 0.95 kg) were used to determine HE and the grazing activity energy cost (GAEC) while standing or grazing Sudangrass pasture with a portable indirect calorimetry system. The method entailed use of a partial face mask that allowed unrestricted grazing to measure oxygen consumption and carbon dioxide emission for 30 min while restrained in a stanchion near the grazing area, followed by 60 min of grazing with other members of the group. The face mask was attached to a 15-m tether along with a corrugated plastic hose through which exhaled air was passed to a FlowKit Mass Flow Generator and FoxBox Respirometry System (Sable System, Las Vegas, NV) that were carried by a researcher who allowed unrestricted goat movement. Prior to measurements, animals were trained to become accustomed to presence of personnel and use of the equipment, with observed grazing behavior similar among all animals of the group. Measurement periods were during morning and afternoon grazing bouts. Heat energy while restrained was 18.7 kJ/kg BW^0.75/h or 446 ± 10.7 kJ/kg BW^0.75/d. Grazing HE increased to 35.1 kJ/kg BW^0.75/h or 843 ± 39.3 kJ/kg BW^0.75/d, implying that the GAEC was 16.4 kJ/kg BW^0.75/h. Goats spent 8.5 h/d grazing; therefore, the daily GAEC was 138 ± 17.3 kJ/kg BW^0.75. A very similar GAEC of (165 ± 10.4 kJ/kg BW^0.75/d) was determined from the difference in HE estimated indirectly from HR between times when grazing a 0.8-ha pasture and confined in nearby 1.2 × 1.2 m pens and fed fresh forage. In conclusion, this method offers promise for relatively simple and direct estimates of the sizable fraction of total HE comprised by GAEC.