If you are devoting a chunk of your day to a cardio workout, you want to burn as many calories in that time as possible. Different heart rates while performing aerobic activity result in different ranges of calorie burn. One way to measure how many calories you are burning is to estimate the volume of your oxygen consumption, also known as your VO2 rate. Maxing out your VO2 will result in the greatest calories burned over the course of your workout.

Oxygen Consumption

When pushed to your aerobic limit, such as in cycling, running and skiing, your heart beats as close to its maximum as you can survive, which delivers the maximum amount of oxygen to your muscles. Oxygen is key to calorie burning because it is an essential ingredient in the breakdown of carbohydrates and fat into energy for your muscles. Increasing the rate of oxygen consumption, by doing activities that raise your heart rate, helps you convert more energy, helping your body use up all those unwanted calories. Your VO2 rate tells you just how much oxygen you are getting.

How to Measure Your VO2 Rate

The rate of VO2 can vary greatly depending upon gender, age, fitness level and genetic predisposition. Measuring your VO2 with complete accuracy requires expensive machinery and testing. An average, healthy 35-year-old male should have a VO2 max of 2.5 liters per minute. An athlete's VO2 max would be higher. If you are just starting out with exercise, your VO2 rate may be lower, but will increase with improved athleticism. In general, you can assume an average VO2 of 3 liters of oxygen a minute.

Using Your Heart Rate to Determine VO2 Levels

If you know your VO2 rate, you can use your heart rate to approximate how much oxygen your activity is causing you to consume. Knowing this, you can estimate how many calories you are burning. First, you must figure out your maximum heart rate. The easiest way to calculate that is to subtract your age from 220. For instance, if you are 30 years old, your MHR would be 190 beats per minute.

Calculating the Calories

Use the pulse reader on cardio equipment or purchase a watch with a heart rate monitor to keep track of your heart rate during exercise to do the math. The closer you get to your maximum heart rate, the more oxygen you consume and the more calories you burn. If your MHR is 190 beats per minute, then 50 percent of your maximum is 95 beats per minute. At 50 percent of your MHR, the average VO2 level of 3 liters will likely be at 20 percent of its maximum. This burns three calories a minute. At the heart rate of 90 percent, the highest you can safely exercise, your maximum VO2 level is at 83 percent, which causes you to burn 12 calories per minute.

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