To work out your Base Metabolic Rate (how many calories you burn from just sitting there, not eating, not keeping yourself warm etc): http://en.wikipedia.org/wiki/Basal_metabolic_rate
use the second formula, looks more daunting than it is. Eg i'm male 90kgs, 180 cm 25 yrs old so to use the formula:
(9.99*90)+(6.25*180)-(4.92*25)+5= 1906 so my BMR is 1906 Kcals per day
To add calories you'll burn in exercise: http://www.bmi-calculator.net/bmr-ca...dict-equation/
so as i participate in moderate to hard exercise 5-6 day a week i multiplied my BMR by 1.65 (halfway between moderate and hard) so i need approx 3144.9 Kcals per day.