What all web based calorie calculators do is estimate your BMR based upon various formulas. These formulas came from various research studies on large groups of people. Several problems with this, these formulas come up with BMR (Basal Metabolic Rate) formulas that work fairly well for average people. The problem is, very few people are average so for most people the equations will over or under estimate the caloric requirements. How far are the formulas off? Hard to say, maybe +/-10% would be my guess. So, how do we improve accuracy and approach the precision that can be obtained in a medical laboratory thru O2/CO2 respiration measurement? By calibration! What is calibration? Here is the procedure:
1) Use the equations to determine how much you should eat and the predicted weight in 30 days
2) For 30 days, eat the specified amount
3) At the end of the 30 days compare your expected weight with your actual weight. From this weight difference, calculate the calibration factor
The calibration procedure takes a month and I wont lie, the calorie counting required is a lot of work. For most people, just using the equations that get you within 10% is close enough and its not worth the bother to calibrate. For serious bodybuilders though, having this accurate information is critical.