It sounds counter-intuitive, but just because you're working out harder, increasing your heart rate and burning calories doesn't mean you're burning fat. Your body's ideal fat-burning zone is only 60 to 70 percent of your maximum heart rate.
Work any harder than that, and the percentage of fat calories burned during your workout actually drops.
You can burn fat faster if you stay within 60 to 70 percent of your maximum heart rate.
Maximum Heart Rate
Your maximum heart rate depends on your age. The younger you are, you'll have a maximum heart rate higher than someone a few decades older.
Find your heart rate by placing your index and middle fingers along the side of your neck, just under your jawbone near your glands —other places to take your pulse would be on the top of your foot, the fold of your groin or behind your knee, says ExRx. You can also feel your pulse on the thumb-side of your wrist. Count the number of beats for 15 seconds, then multiply this number times four to get your total beats per minute.
Read more: Fat Burning Vs. Cardio Heart Rate
Healthy Heart Rate Zones
The first heart rate level reached during a workout is the healthy heart zone. In this zone, your heart rate should be at 50 percent of its max. Using our example from above, a 32-year-old would be 94 bpm.
The healthy heart zone is ideal for individuals just starting a workout routine, or for experienced athletes during workout warm-up, according to the American Heart Association. In addition to promoting overall wellness — such as lowering blood pressure and cholesterol levels — 85 percent of calories burned while in this zone are fat calories.
Fat Burning Zone
The second tier you'll reach during a workout is the fitness zone, also called the fat-burning zone. Just as with the healthy heart zone, 85 percent of calories burned in this zone are also from fat. In this zone, your heart rate should be at 60 to 70 percent of its max. Using the same subject, 113-132 bpm.
The key difference between the calorie burn between this level and the healthy heart zone is the amount of calories burned. Because you are working at a higher intensity level, you will burn more calories during your workout, provided you are doing the same exercise for the same amount of time.
Aerobic and Anaerobic Zones
The third and fourth levels — called the aerobic zone and anaerobic zones, respectively — are geared towards more serious athletes. These tiers are also referred to as the endurance and performance training zones. The aerobic/endurance zone provides all the benefits of the fitness zone, but also works your cardiovascular and respiratory systems to a greater degree. At 70 to 85 percent of your max heart rate (132-160 bpm), only half the calories burned will be from fat.
However, since a higher intensity workout also means you'll be burning more calories overall, it is possible you could burn as many fat calories during an aerobic workout as during a fitness zone workout — the difference being a lower proportion of fat calories burned in the aerobic workout. The anaerobic/performance zone also conditions your cardiovascular and respiratory systems, but to a higher degree.
The target heart rate for this zone is 80 to 90 percent of your max — 150-169 bpm. In this zone, just 15 percent of calories burned are from fat, although the overall number of calories burned in this workout — provided it is the same form of exercise and same duration — is greater than during any of the others. However, since it forces the body to work at such a high intensity —such as a HIIT workout — it is harder to work out in this zone for prolonged periods of time, says ACE Fitness.