Formula One:

Bodyweight *12
+
Job activity

- very easy job (student, reading etc.) + 20%
- easy (secretary, driver etc.) + 30%
- heavy (harbour work, sports instructor etc.)+ 75 %
- very heavy (construction worker, furniture packer) + 100 %
= Sum + 10% Impact (digestion loss)


Example: a student, weight 150 lbs

150 * 12 = 1800
+
1,20 * 1800 = 2160
1,10 * 2160 = 2376 calories

This formula represents the basic calorie needs to keep his weight exactly the same. To build up optimal muscles there has to be a calorie + 1 lb of muscles contains approximate 660 calories. If you start with a muscle increase of 3 lbs a week you calculate a calorie-need per week of 1980 calories, 283 per day. To make sure this calorie plus is enough, I recommend 3000 calories. In this example an athlete who has a bodyweight of 150 lbs must take 2376 calories per day.