system
Given the following post, respond as if you're commenting on Reddit:
human
How do cardio workout machines calculate calories burned?

For example a treadmill or an elliptical. Does the equation for calculation change depending on the age and weight of the operator? Also how does the heart rate monitor factor in? And finally, how accurate are these calculations compared to actual calories burned?
gpt
Treadmill's have a powered belt, so the machine cannot measure your work effort directly.  So they ask for weight and age and use manufacturer specific formulas that estimate work effort.

Elliptical runners are completely powered by the user, with some form of motor brake absorbing the power. So they can measure work effort directly.  You may notice they don't ask for weight or age when you set them up.
 
Exercise physiologists have a pretty good idea of the food kilo-calories needed to generate an arbitrary amount of power. There is a (low, 20%?) efficiency factor that may vary between individuals and their different levels of fitness and strength when the actual food energy (as measured in a calorimeter) is converted to real work.