We obtained directly measured maximal oxygen uptake (VO₂ max) by open-circuit spirometry in 1,453 patients with chronic heart failure (HF) who completed a treadmill test (n = 1,453) or cycle ergometry (n = 1,838), as participants in The Fitness Registry and the Importance of Exercise National Data Base (FRIEND) dataset. We developed a new equation to predict measured VO₂ max in those using a treadmill by randomly sampling 70% of the participants from each of the following age categories: <40, 40 to 50, 50 to 70, and >70 and used the remaining 30% for validation. Multivariable linear regression analysis was applied to identify the most relevant variables and construct the best prediction model for VO₂ max. Treadmill speed and treadmill speed * grade were considered in the final model as predictors of measured VO₂ max and the following equation was generated: VO₂ max in ml O₂ kg/min = speed (m/min) * (0.17 + fractional grade * 0.32) +3.5. To assess the efficacy of the equation, we applied it to 1,612 patients in the HF-ACTION cohort. To assess the efficacy of the FRIEND cycle ergometry equation developed for healthy individuals we applied it to 1,838 HF patients in the FRIEND cohort and 306 patients in a Greek population of HF patients with directly measured VO₂ max_. The FRIEND equations were superior to ACSM equations in predicting VO₂ max regardless of the cohort or exercise mode used (treadmill or cycle ergometry) to access VO₂ max.