Whats the difference between ZWO, FIT, ERG and MRC Files?
All these file formats contain training and structured workout data for bicycle trainers.
Popular soft- and hardware in this space uses different file formats that are not compatible. Fortunately often these file formats are converted autoamtically in the background when exchanging structured trainings between platforms.
ERG – Structured workout data
ERG files are simple text files that contain definitions of structured workouts for cycling or other workout types. Basically they define a power target (in Watts) and how long to stay at this target in minutes. This format is widely used by aproviders of structured workouts like Trainer Road, Training Today or Strava and can be imported by most cycling computers except Garmin. For Garmin Edge cycling computers ERG files need to be converted to FIT files first.
ERG files contain the Watt data in absolute values in Watts. This means at creation of the ERG file the individual FTP value of the user needs to be known to tailor the structured workout to their profile.

In this examle the first block of the structured training is kept at 162 Watts for 5.0 minutes.
The FTP value this training is based on is published in the “FTP=” line.
MRC

MRC is bascially identical to ERG files but contains the power target in percentage of FTP. This means the MRC file is portable and the actual power target is calculated by the device controlling the smart meter during the training according to the FTP value of the user.
The first block of this structured training is supposed to be at 58% of FTP. If set to the same FTP value as in the above ERG file this would also be 162 Watts, but could be different for another user.
ZWO – Zwift Cycling Workouts
ZWO Files are a proprietary format used by Zwift to define individual workout sessions. They are XML based. ZWO is used for Zwift workouts. There is a list of pre defined workouts that are stored within Zwift but they can also be externally loaded.
FIT – Garmin Flexible and Interoperable Data Transfer
FIT is a file format defined by Garmin and aims to be a universal file format that can contain definitions for workouts like target power for smart trainers. FIT files can also contain data of finished workouts that is recorded by bicycle computers or smart watches. FIT files use a proprietary binary format but at the moment Garmin provides libraries for common programming languages to access the content.