HR zones are based either on your last FTP Check ride or, if that does not exist, they are based on your age. You should see an indication of which method we are using just below the Training Zones chart on the Stats tab. Our calculations are as follows:

For FTP Check based HR ranges, we multiply your average HR during your FTP Check workout segment by the following percentages to get each zone. This method is described by Hunter Allen in "Training and Racing with a Power Meter."

Zone1: 0 - 68%
Zone2: 69 - 83%
Zone3: 84 - 94%
Zone4: 95 - 105%
Zone5: > 106%

If you haven’t done an FTP Check recently, we calculate your HR zones based on age, multiplying your max HR (220 - age) times the following percentage ranges to get each zone. This method is based on the British Cycling Heart Rate Zones and appears to be very similar to the method employed by Strava.

Zone1: 0 - 76%
Zone2: 76 - 83%
Zone3: 83 - 90%
Zone4: 90 - 95%
Zone5: > 95%