SynecdocheSam wroteOne useful measure that not too many people consider is calorie density. If a food has more calories per gram, you're going to be able to eat more of it to get the same calories as from something with fewer.
This chart is a good visualization of this.
wisegeek.com/what-does-200-calories-look-like.htmAlso, various web pages exist for the purpose of calculating your 'Basal Metabolic Rate'. (This is one of them:
bmi-calculator.net/bmr-calculator/.) The BMR is how many calories you expend every day without even trying. Now, how many calories you expend besides that typically ranges from 20% of the BMR (if your primary form of exercise is eating) and 90% of the BMR (if you run a marathon daily). So, your daily calorie use is somewhere between 1.2 and 1.9 times your BMR, and that's your call on where exactly that falls. If you're especially muscular, go a bit higher on that range than you might think, and if you're especially fat, go a bit lower.
Now, one pound of fat is about 3500 calories. So, the natural conclusion to draw here is that, for every 500 calories you eat daily that exceeds what you use, you will gain one pound per week. However, the body's energy efficiency is about 70%, so that number is probably more like 700 calories. Of course, neither does this factor in all the fanatical adjustments one's metabolism is prone to making when one alters the amount of food entering the system. Also, a digestive system accustomed to eating 2000 calories per day won't be able to process a sudden influx of 5000 calories in a day, and that is a good way to make an enemy of your digestive system.
I guess I had a lot to say on this topic. Apologies to the bored.