Your Fitness Pal website says..Here are three easy ways to calculate your fat-burning zone:
Figure out your max heart rate (Max Heart Rate = 220 – your age). And then determine your fat-burning range, which is 60% to 70% of your max heart rate.
Use a fitness app, like Wahoo Fitness, MapMyFitness, or RunKeeper, to calculate your 5 heart rate zones.
Do a Field Test or Home Test with the Wahoo Fitness app, which will identify your Burn and Burst Zones.