I'm no expert, and I struggle with this myself too. But, are you eating a big enough breakfast? I've found that if I have a good balance of protein and carbs at breakfast, I'm good until lunch. I usually need two packets of oatmeal - one just barely covers the bottom of the bowl!

Most of those calorie calculators do not take into account active peope, because most folks do not exercise. Like others have said, if you're active, you need a few more calories to keep the muscles fueled.