NOVELTY - The method comprises selecting exercise type and inputting user physiological background parameters, collecting heart beat data indicating relative intensity of exercise and performance data during an exercise session, segmenting the collected heart beat data to data segments with coherent intensity, calculating reliability of data and calculating weighting coefficients for the data segments based on their reliability, and forming an estimate of user cardiorespiratory fitness level based on dependency between the heart beat data segments and performance data.    USE - The method is useful for evaluating cardiorespiratory fitness of a user during a freely performed exercise such as running and cycling outdoors, bicycle ergometer, rowing ergometer and treadmill.    ADVANTAGE - The method is capable of effectively, reliably and accurately evaluating the cardiorespiratory fitness of the user during any user performed exercise session, and allows the user to freely exercise.    DETAILED DESCRIPTION - The method comprises selecting exercise type and inputting user physiological background parameters, collecting heart beat data indicating relative intensity of exercise and performance data during an exercise session, segmenting the collected heart beat data to data segments with coherent intensity, calculating reliability of data and calculating weighting coefficients for the data segments based on their reliability, and forming an estimate of user cardiorespiratory fitness level based on dependency between the heart beat data segments and performance data using the calculated weighting coefficients. The heart beat data describing relative intensity of exercise is inputted by the user, and is a heart beat derivable parameter, rating of perceived exertion, blood lactate level, respiration rate or ventilation. The set data segments have duration of 30 seconds to 4 minutes. An INDEPENDENT CLAIM is included for a system for evaluating cardiorespiratory fitness of a user during a freely performed exercise.    DESCRIPTION OF DRAWING(S) - The diagram shows a linear equation of the chosen segments to estimate cardiorespiratory fitness of user.