How many calories you can eat and still lose weight depends on a number of factors -- not only on your current weight. The number of calories you should eat per day also depends on how quickly you would like to lose weight, how active you are, how old you are and your gender. The key to losing weight is to create a calorie deficit by burning more calories with physical activity than the number of calories you eat.

No matter how quickly you'd like to lose weight, it's advisable to eat a certain number of calories -- and not fewer than this number -- because doing so might affect your metabolism and make it difficult to get enough of your essential nutrients. For women, this number is 1,200 calories per day; for men, it's 1,800. Once you cut calories to this level, you'll need to increase the amount of exercise you get to create a greater calorie deficit instead of eating fewer calories.

If you don't want to go down to the minimum number of calories -- which can be a drastic cut for some people -- start by figuring out how many calories you need to maintain your current weight of 200 pounds. A quick way to do this is to take your current weight in pounds and multiply it by 15 -- which works out to about 3,000 calories each day to maintain 200 pounds of weight, if you don't do a lot of exercise.

A more accurate way to estimate calorie needs and to also account for exercise is to calculate your basal metabolic rate or BMR. To do this, plug it into a formula that takes exercise into account.

BMR for Women = 655.1 + (9.6 x weight [kg]) + (1.8 x height [cm]) − (4.7 x age [years])

For women, start with 655.1 and add this to your weight in kilograms times 9.6. Add the result to your height in centimeters times 1.8, and then calculate your age in years times 4.7 and then subtract from the total.

BMR for Men = 66.47 + (13.7 x weight [kg]) + (5 x height [cm]) − (6.8 x age [years])

For men, start with 66.47, and multiply your weight by 13.7, your height by 5 and your age by 6.8, using the same formula.

Once you have your BMR, you multiply it by your activity factor. If you don't exercise much, multiply the results by 1.2. If you exercise a couple of days a week, multiply the results by 1.375. If you exercise three to five days a week, multiply the results by 1.55. If you exercise every day, multiply your BMR by 1.725. If you have a very physically active job, by 1.9.

The final result is the number of calories you need to maintain your current weight.

For a quick estimate of how many calories to eat to lose weight, you could also plug in your desired weight to the BMR calculation. This will give you approximately the number of calories you need if you want to maintain the lower weight you're aiming for.

Typically, men need 2,000 to 3,200 calories a day and women need between 1,600 and 2,400 calories to maintain a healthy weight, depending on their age and activity level.

To achieve a specific rate of weight loss, keep in mind that each pound has 3,500 calories. To lose 1 pound a week, you need to eat 500 fewer calories each day; to lose 2 pounds a week, you need to eat 1,000 fewer calories a day. This is now being called into question, however, as it doesn't take into account people's decreasing calorie needs as they lose weight, which is most likely due to having less body mass.

Using a new formula created to take more factors into consideration, hypothetically, if a 40-year old who's 5 feet, 5 inches tall and weighs 200 pounds cuts 500 calories a day, he or she will lose about 20 pounds a year, not the expected 52.

While you may be able to lose weight by cutting calories alone, you'll lose more fat and find it easier to maintain weight loss if you exercise as well. Without exercising, about 25 percent of any weight you lose will consist of muscle rather than fat. A combination of strength training and aerobic exercise will help minimize muscle loss. The Centers for Disease Control and Prevention recommend adults get at least 150 minutes of moderate-intensity exercise and at least two strength training workouts each week.

For weight loss, however, you may need as much as 300 minutes per week of aerobic exercise. This exercise makes it easier to create the calorie deficit needed for weight loss as well. For example, a 200-pound person who bikes for an hour burns about 364 calories, and one who walks for the same amount of time at 2 miles per hour burns 255 extra calories.