Heart rate zones in this app are calculated from Karvonen Formula.
The Karvonen method factors in Resting Heart Rate (HRrest) to calculate Target Heart Rate (THR), using a range of 50%–85%:
THR = ((HRmax − HRrest) × %Intensity) + HRrest
Example for someone with a HRmax of 180 and a HRrest of 70:
50% intensity: ((180 − 70) × 0.50) + 70 = 125 bpm
85% intensity: ((180 − 70) × 0.85) + 70 = 163 bpm