The Active Zones defined by Fitbit Charge 5 are not in line with what is defined universally, including Apple Watch. The differences are in the percentages used in each zone. Due to this, Fitbit gives a higher number of active zone minutes thus giving false sense of satisfaction even though the real active zone minutes is much lower. This is explained below.
Everywhere else (including Apple Watch), the zones are defined as follows:
Zone 2 (Fat burning Zone): Bottom of Zone: (0.6 * Heart Rate Reserve + Resting Rate Reserve), Top of Zone: (0.7 * Heart Rate Reserve + Resting Rate Reserve)
Zone 3 (Cardio Zone): Bottom of Zone: (0.7 * Heart Rate Reserve + Resting Rate Reserve), Top of Zone: (0.8 * Heart Rate Reserve + Resting Rate Reserve)
Zone 4 (Peak Zone): Bottom of Zone: (0.8 * Heart Rate Reserve + Resting Rate Reserve), Top of Zone: (1 * Heart Rate Reserve + Resting Rate Reserve)
But in Fitbit Charge 5, the Zones are defined as:
Zone 2 (Fat burning Zone): Bottom of Zone: (0.4 * Heart Rate Reserve + Resting Rate Reserve), Top of Zone: (0.59 * Heart Rate Reserve + Resting Rate Reserve)
Zone 3 (Cardio Zone): Bottom of Zone: (0.6 * Heart Rate Reserve + Resting Rate Reserve), Top of Zone: (0.84 * Heart Rate Reserve + Resting Rate Reserve)
Zone 4 (Peak Zone): Bottom of Zone: (0.85 * Heart Rate Reserve + Resting Rate Reserve), Top of Zone: (1 * Heart Rate Reserve + Resting Rate Reserve)
