If you exercise on an exercise machine wearing a heart rate monitor and then read the number of calories burned from your watch and then compare it to the exercise machine, you will probably see a big difference, maybe hundreds of calories different. The machine will nearly always be higher than your heart monitor.
Your heart monitor allows you to enter much more data on yourself than the machine, such as weight, height, sex, age and so on. The heart monitor is also likely to be more accurate as it is being read from a chest strap and not as in most cases a handgrip pulse sensor. However all calorie readings should be viewed as an estimate.
Unless you go to a lab, it is almost impossible to get a fully accurate reading.
A calorie is a measure of energy: When you read a food label, the number of calories reflects the amount of energy that is packed in there. Once that food is in your body, the potential energy gets unleashed through a few processes, but the dominant mechanism involves the use of oxygen to break down carbohydrates, fats and (sometimes) even protein into other chemicals. Though different fuel sources require different amounts of oxygen and produce different amounts of energy, the boiled-down version is that each litre of oxygen reflects the body’s use of about five calories.
So to really count calories it’s necessary to measure the amount of oxygen used – hence the mask they hook you up to during exercise and stress tests, that’s hardly practical at the gym.
However you do have one organ that is a very good guide to the amount of oxygen you are using, and that is your heart. When you exercise your body needs oxygen and your heart has to beat faster to supply it. The amount of oxygen used is different for different people. Factors such as weight, sex, age, body composition and a number of other factors all play a role. Manufacturers of heart rate monitors have developed formulas based on these factors to give a close estimate of calorie usage.
The computers on cardiovascular equipment do not allow you to enter so much data as a heart rate monitor and therefore the reading will be less accurate. The other problem is that some units use handgrip pulse monitors which results in you having to grab hold of the sensors, which is very unreliable. Those that allow you to wear a telemetric chest strap and pick up the heart rate from that are more accurate, but without the original data they are never going to be exact.
One company that takes a slightly different approach is Life Fitness. They have done studies in house on lab equipment and worked out calorie expenditure formulas, which they build into their machines, for different exercise intensities that are independent of heart rate and then combine that with the heart rate for a more accurate reading. It is accurate to around 5-10% but the best way is to keep using the same machine and compare the readings and the variations from the same computer will be pretty accurate.
According to the American College Of Sports Medicine who tested these machines most machines err on the high side, on average 26%. But they did find machines that went from 39-79% higher. At those kinds of percentages the readings are meaningless.
So going back to the original question ‘are calorie counters accurate on exercise machines’ the answer is not really, they are only an approximate guide and on average they will give a higher reading than they should and they are not as accurate as a personal heart rate monitor. But if you workout on a machine and it reads 350 calories burned and then use the same machine for your next workout and it reads 400 calories, then you have done better.