If you've ever finished a workout on a treadmill or an elliptical trainer, you may wonder - Did I really burn *all* those calories?

First, it's important to remember that the calorie counts on machines (or those you get from using any formula) are just estimates and will never be 100% accurate. There are so many factors involved

### What Affects Your Calorie Burn

To understand how machines work, it helps to understand the factors that affect how many calories you burn.

If you're on a treadmill or other cardio machine, it will probably ask for your gender, age, and your weight, all things that affect how many calories you burn. However, there are things the machine doesn't account for such as:

- Fitness Level - The newer you are to an activity, the more calories you burn. As you learn the movement, your body becomes more efficient and it burns fewer calories doing the same activity.
- Body Composition - The more muscle mass you have, the more calories you burn, something else a cardio machine doesn't factor in.
- Body Size - Body size also makes a different. You might have two people who weigh the same, but if one has a larger frame, that may affect how many calories he or she is burning.

### The Mystery of Cardio Machine Formulas

Here's the other thing: Every company uses a formula to figure out how many calories you're burning with exercise. What we don't know is exactly what the formula is for whatever machine we're using.

What most experts suggest is that most machines use a formula based on The Compendium of Physical Activities. This document assigns a value to each activity you do and you use that number to compute how many calories you burn per kilogram of body mass.

They base the value on what's called a MET, which stands for Metabolic Equivalent, which is defined like this: One MET is 1 kcal per kilogram per hour and is about the equivalent of sitting and doing nothing.

Using that as a base, values are assigned to everything from walking and running to fishing and dancing.

If you look at the Compendium for the category of walking, you can see just how many options there are. For example, if you're walking and carrying a 15-lb load, you burn 7.5 METS, or 7.5 kcal/kg/hour.

Of course, there's always a drawback when you use one formula for everyone and that is that it's not going to be accurate for everyone.

Some experts estimate that cardio machines can overestimate calories burned by up to 10-15% so it's best not to take these numbers as gospel but, instead, simply use them as a benchmark from workout to workout.

For example, let's say you run for 30 minutes at 5 mph and the treadmill shows you burned 300 calories. Maybe it's not exactly 300 calories, but you now at least have a number to work with and, from week to week, you can tweak that to burn more calories - Running faster, for example, or for a longer period of time.

Instead of relying on calories burned, go by your perceived exertion and/or keep track of your target heart rate. If two machines show the same number of calories burned but one feels easier and keeps your heart rate lower, that's a sign that you're probably burning less calories than you think.