A good thread to read:
Originally Posted by Owly
Calculating Calorie & Macronutrient Needs - Bodybuilding.com Forums
If you look, you'll probably find most online calorie calculators use the Harris-Benedict formula. In addition, when it comes time to select your "multiplier" for activity, people typically choose the one above what they actually are. Your calorie estimates from your calculator are likely 20-25% too high, and your 2,600 figure is probably more like 2,100-2,200. Use the Katch-McArdle equation and use the activity multiplier below what you think your activity really is.
There are then a number of other formula which calculate BMR. This means it calculates what you need should you be in a coma.
1/ Harris-Benedict formula: Very inaccurate. It was derived from studies on LEAN, YOUNG, ACTIVE males MANY YEARS AGO (1919). Notorious for OVERESTIMATING requirements, especially in the overweight. IF YOU CAN AVOID IT, DON'T USE IT!
2/Mifflin-St Jeor: Developed in the 1990s and more realistic in todays settings. It still doesn't take into consideration the differences as a consequence of high BF%. Thus, once again, it OVERESTIMATES NEEDS, ESPECIALLY IN THE OVERWEIGHT.
3/Katch-McArdle:Considered the most accurate formula for those who are relatively lean. Use ONLY if you have a good estimate of your bodyfat %.
Calorie calculators peg me around 2,400 maintenance. I'm really more like 2,100-2,200.
Don't put your trust in anyone on this forum, including me. You are the key to your own success.