I have always wondered how treadmills calculate the amount of calories we burn. Are they accurate? Do different people burn different amount of calories when exercising? According to strength coach and fitness trainer Arnav Sarkar Treadmill calculators use your bodyweight and the work done to calculate how many calories are burnt during the session. The problem with this is that it does not take your unique factors like body fat and fitness level into consideration. For e.g. if two men weigh 75 kg but one has a higher fat percentage then he will burn less calories for the same effort.