We calculate your exercise calories using your profile statistics, and exercise effort values knows as "METs." Because your specific statistics are part of the equation, your calories burned will vary if you gain or lose weight, and will be different from other users of the site. (If your exercise calories have changed dramatically, please see this article for some troubleshooting steps)
These calculations, while reasonably accurate, are not as accurate as the feedback from a cardio machine in the gym, or an activity tracker, that can calculate your calories burned based on data collected moment to moment. For this reason, our system allows users to input their own values for calories burned, in case they have a more accurate figure from such a source.
If you know exactly how many calories you burned from a machine at the gym, you can manually enter that number into your diary. Just search for the exercise in our database, select it from the search results, and type in the exact number of calories you burned in the "Calories Burned" box to the right. That number will be saved in your diary.
If you are interested in devices that can track your actual use of calories, may we suggest looking into our third party integrations? Several of our integrations offer solutions for monitoring your calorie burn over the course of the day, and can update your MyFitnessPal calorie goals based on this data.