To burn fat when exercising you must get your heart rate up to 70% to 85% of your max for 30 minutes. (You can figure your target heart rate by taking 220 minus your age and then multiplying that by 70 to 85 percent. Example if your 30 years old take 220-30 = 190 times .7 to .85 = 133 to 162. So you must get your heart rate between 133 to 162 beats per minute for 30 minutes to burn fat.)