Britney Spears aside, math is HARD. Also, calorie counting is hard. Combine the two and you have giant headaches. Luckily, them there fancy scientists have solved this problem for us. They have all these cool things called calculations and tests to find out if their assumptions (insert joke about ASS here) are right or not.
I've been asked: how do you know you need 1400 calories per day? So here's what the fancy science-people (Hi, Mom!) told me when I researched this:
To start I'm a lady so my basal metabolic need is 655 (the science people told me so). For a dude this would be 66. I weigh 145 pounds (and damn proud of it). I'm 5 feet 5.5 inches tall (I like that last .5 inch) and I am 26 years old (ignoring the 11 months for now). The thing about the fancy science people is they like the metric system so...
Step 1. Divide weight in pounds, for me this is 145, by 2.2 to find my weight in kilograms. This equals 65.9(ish).
Step 2. Multiply height in inches, for me this is 65.5 (5 feet times 12 is 56 plus 5.5), by 2.54 to find height in centimeters. This equals 166.37.
Step3. Figure out your age in years. For me this is 26. This will be the easiest part, I hope.
(I know what you're thinking, you've clearly burned enough brain calories at this point that the whole thing is moot. I promise to make it easier at the end with some handy links.)
SO! Lets make it more complicated. Now you have to multiply each of these steps by a somewhat arbitrary (read: science-y) number.
Step 4. Multiply step 1 by 9.6 for a woman and 13.7 for a man.
Step 5. Multiply step 2 by 1.8 for a woman and 5 for a man.
Step 6. Multiply step 3 by 4.7. for a woman and 6.8 for a man.
THEN! After you've put the heating pad on your brain to rest your fattiest body part because, man this is harder than it seems like it should be (I recently learned your brain is like, solid fat. I bet you Britney didn't know.)
So THEN! Basal rate + step 4 + step 5 - step 6.
Again, for me. 655 (basal rate) + 9.6 times 65.9 (kilos) + 1.8 times 166.37 (centimeters) - 122.2 (age times 4.7) = 1466.91.
I know, the brain, it hurts. Do this when you have time and patience.
So anyway. Why do I eat 1400 calories everyday when it says 1466.91? Well, I don't really. Yea, I know your way confused. But I have logic for this part! 1466.91 is what my body needs everyday to function healthfully and boy do I love to help it do that. But some days I go over 1400. Some days I'm not that hungry and weirdly end up going to bed before I've even hit 1000. Rare, yes, but it happens.
I also have a habit of taking nibbles of food around me and either not writing it down or thinking its too tiny to bother. Those things add up so I build them in by rounding down a bit.
This also means that on the occasional Saturday night where I have more wine than my liver cares for and maybe an extra cookie or five, its no big deal because most days during the week I hit my basal rate and the weekend indulgence isn't going to send me into a crazy downward spiral where I end up throwing the scale from our 6th floor apartment.
That said, this plan works for me. It might not work for you. But the kind science-folks at SparkPeople laid it all out for me here: http://www.sparkpeople.com/resource/calorie_calculation101.asp so I tried it and I'm happy. I know some people who journal all their calories and change their caloric in-take every single day. Personally, I'd go crazy working hard to get to 2000 calories today and working hard to get under 1200 the following day. For you, it might be a fun game.
The moral of this story is: math is hard and you gotta find what works for you!