Well, lets look at the numbers...
To lose 10 lbs, you need to roughly have a defecit of 35000 calories over whatever time period you want to lose that 10 lbs in. So, for a month, that turns out to be a defecit of ~1200 calories per day.
A defecit means to eat less calories than your body needs to burn in a day - NOT burn more calories (through exercise) than you eat. As Matt posted, your BRM is the amount of calories your body burns just to survive, but you need to adjust for activity and exercise. A good estimate for how many calories your body burns in an average day is to multiply your bodyweight times 15, and create your defecit by eating less than that.
Now, to figure out if losing 10lbs per month is a realistic goal for you, take your current weight and multiply by 1% - a healthy amount of weight to lose is 1% of your weight every week. So for me, I'm currently flucuating ~310-320, so I can healthily lose 3.1-3.2lbs/week. Times 4 weeks in a month, and thats a potential FOR ME to lose 12-13 lbs this month.
That means, assuming I was just starting out, and had maintained my current weight for a long time, I would start out by trying to create a defecit of ~10500 calories per week (3*3500), or ~1500 per day, through a combination of diet and excercise.
Another check is to take you current weight and multiply by ~12-15 to figure out how many calories your body will burn on an average day - I use a smaller number because I sit at a desk all day, but I exercise 3 times a week, so some days I'll burn more. For me, that amounts to ~3800-4800 calories. To calculate how many calories you should eat to lose weight, you can multiply your weight by ~10. For me, thats ~3200 calories - but I know that if I eat 4000 calories a day, I will at worst not gain any weight (well, I probably would because I've gone through a long period of undereating, which my body adjusted to by slowing my metabolism *some*, but if my body were *normal* right now, I wouldn't)
Now, all that is nice to know, but I typically do things a little backwards, for a couple reasons. I typically moniter my weight loss, and use that to determine how big my defecit is. The reason I do this is because I'm coming off of a long period of undereating, so my body is burning less calories than it should be. I know this because I typically eat between 2000-3000 calories per day, and have been losing weight at a rate of ~3 lbs per week. That tells me the amount of calories my body needs to maintain is somewhere between 3500 and 4500, which is roughly 300 calories less than it should be. Could be more, could be less, and i'll never know for sure because I don't keep an accurate calorie count. Only a rough "in my head" count, and I let the scale tell me whether or not I'm being honest with myself.
Now that I've said all that, heres the short answer you were probably looking for:
If you weigh.....................You should lose no more than..............
125.................................5 lbs/mo.
150.................................6 lbs/mo.
200.................................8 lbs/mo.
250.................................10 lbs/mo.
300.................................12 lbs/mo.
350.................................14 lbs/mo.
400.................................16 lbs/mo.
450+...............................18+ lbs/mo.
So there you go... Don't take these numbers as written in stone, but at the start of every month, I consider how much I currently weigh, and use this chart to determine how much weight I should lose in a month. 90% of the time, I'll come up a pound or 2 short for 2 reasons - as you lose weight, the amount of weight you can lose in a month decreases, and the amount of calories need to decrease as well. For example, I'll lose weight slightly faster at 320, than I will at 310, than I will at 300, etc. Also, every 10 lbs you lose, you need to cut another 100-150 calories out of your diet, so until you make that cut, you'll lose weight slower.
Finally, make sure you're eating enough. Multiply your body weight times 10 and eat that many calories, and you will lose weight, regardless of whether you exercise a lot or not at all. (unless you're like me and have been severely undereating for a while). If you're running 5 miles a day, you might even want to go (your body weight) x 12 or more. And if you have been undereating - you probably have if you've been eating 1200 calories a day or less, combined with lots of running - then you should work on upping your calories to 10-12 x your body weight or more. Actually, it'd probably be better to up your calories to maintenance for a month or so. Your weight loss might stall or you might gain some weight in the process, but your body will thank you in a month or 2 when you're able to be eating more, feeling better, and still losing weight. "Maintenance" is just the term for eating as much as your body needs, or basically, eating to not lose or gain weight.
So... thats that. I write a lot, but its not all to complicated.
