The complete calibration of indirect calorimetry systems involves simultaneous checks of gas analyzers, volume device, and software, and this requires a machine that can mimic accurately and precisely the ventilation and expired gases of an athlete. While previous calibrators have been built successfully, none have matched the ventilatory flows produced by athletes during high intensity exercise. A calibrator able to simulate high aerobic power (VO2max calibrator) was fabricated and tested against conventional indirect calorimetry systems that use chain-compensated gasometers to measure expired volume (VE systems) and calibrated electronic gas analyzers. The calibrator was also checked against a system that measures inspired volume (VI system) with a turbine ventilometer. The pooled data from both VE and VI systems for predicted VO2 ranging from 2.9 to 7.9 L.min-1 and ventilation ranging from 89 to 246 L.min-1 how that the absolute accuracy (bias) of values measured by conventional indirect calorimetry systems compared with those predicted by the calibrator was excellent. The bias was < 35 mL.min-1 for VO2 and carbon dioxide production, < 0.50 L.min-1 for ventilator (VE BTPS), -0.02% absolute for the percentage of expired O2 and +0.02% absolute for the percentage of expired CO2. Overall, the precision of the measured VO2, VCO2, and VE BTPS was approximately 1%. This VO2max calibrator is a versatile device that can be used for routine calibration of most indirect calorimetry systems that assess the ventilation and aerobic power of athletes.