The reason you get different numbers is that the calorie calculators often use different formulas. Some use Harris-Benedict, some use Mifflin-St Jeor, etc. Also, all calorie calculators are mere estimates because we're all different. For instance, if I put in all my stats and use several different calorie calculators online, I'll get results that state that I can eat anywhere from 1800 - 2400 calories to maintain my weight. Through trial and error, I realize that I can eat 2200 calories per day to maintain my weight.
Here's a decent calorie calculator:
My advice is to start with their number, and adjust your calories according to your progress (without putting too much stock into daily scale fluctuations if you happen to weigh daily).