I did some basic calculations to figure out how calories are used in the body while lifting weights. Here is a picture showing how energy is used in my body. The sizes of the arrows shows how many calories are used:

Usable energy can only enter the body through food and drinks. About 60% of the total amount of the energy during lifting is used to sustain life. About 30% of the energy reserved for lifting the weight is lost to transform the food energy into muscle work. Only the remaining 10% is used to move the actual weight. Talking about an inefficient machine! Some take home points:

• The calories used to sustain life varies with body size – A large body use more calories than a small body.
• You still burn about 60% of the calories even if you skip the workout. Controlling the food intake is thus the easiest way to lose weight.
• A regular calorie burn calculator use average energy use for the specific activity to calculate the calories burnt, but do not account for differences in body types or how the exercise is performed. IronChart on the other hand measure the lift, the green 10% of the total amount of calories shown in the picture, and does not say anything about how much total amount of calories are burnt. My advice is to multiply the IronChart calorie measurements with 10 for an ok estimate.

Here are links to the calculations used:

Calculation of basal metabolic rate or how many calories it takes to live:

Wikipedia Basal Metabolic Rate

Base metabolic rate calculator

Approximation of how many calories exercise burn:

T-nation article

Wikipedia article containing info about muscle efficiency:

Wikipedia muscle efficiency