On its face, weight loss seems simple: As long as you burn more calories than you eat, you should shed pounds. But almost anyone who has tried to reclaim her waist can point to weeks or months when it doesn't seem to work that way. Here are four vital statistics for helping you meet your weight-loss goals.

Daily calorie count

Once you know your Resting Metabolic Rate [will link to: Managing Your Weight: Calories In vs. Calories Out], you will need to account for physical activity to determine the total number of calories you expend each day. Here, an equation is the most practical method to gauge your calorie burn. Multiply your RMR by the appropriate activity factor:

If you are sedentary (little or no activity) - RMR x 1.2

If you are slightly active - RMR x 1.375

If you are moderately active (moderate exercise/sports 3-5 times a week) - RMR X 1.55

If you are very active - RMR x 1.725

The number you get represents the minimum number of calories you need to eat daily to maintain your current weight. Researchers believe that you have to burn roughly 3,500 calories to lose a pound of fat, so to lose 1 pound a week, a safe rate of weight loss, you'd need to diet or exercise your way to a 500-calorie deficit every day.

Maximum heart rate

Maximum heart rate is a measure of your body's ability to use oxygen, and it equals the number of times your heart would beat in a minute if you were running as fast as you possibly could. While the most precise tests are done in a lab, a more feasible approach in determining this number involves an equation created by researchers at the University of Colorado at Boulder.

To get an idea of your maximum heart rate, the researchers recommend the following formula: 208 - 0.7 x age = heart rate max. For example, a 35- year-old woman would have a maximum heart rate of 183.5. See Target Heart Rate (below) for ways to use this figure to determine your ideal exercise intensity for weight loss.

Target heart rate

One persistent myth about exercising to lose weight is that low-intensity exercise -- working at less than 55 percent of your maximum heart rate -- is the best way to burn fat. While your body is burning a greater percentage of calories from fat when your heart rate is lower, the overall number of calories you expend during a workout is what counts. In fact, some scientists believe exercising harder burns more calories both on the treadmill and off. A study in the journal Metabolism-Clinical and Experimental suggests post-workout burn lasts three times longer (up to 101?2 hours!) for those who work out at 75 percent of their maximum heart rate than for those who coast at 50 percent.

If you're a beginner, aim for between 50-70 percent of your maximum heart rate (just multiply your max heart rate by 0.5 and 0.7). A heart-rate monitor with a chest strap, costing between $80-$120, is the best way to tell if you're in your target. But the heart-rate grips on many fitness machines are a good substitute. They work best if your hands are slightly damp with sweat (water helps to conduct the electrical signals from your heart), your arms are relatively still and your grip is light.

More advanced exercisers should shoot for at least 70 percent of their max heart rate, but don't go above 92 percent. At this point, most of us cross our aerobic threshold, according to a recent study by researchers at the University of Birmingham, England. Almost all your calorie burn comes from stored carbohydrates. After about an hour at that pace (depending on how many carbs you're storing), your muscles will run out of fuel, causing you to feel weak and fuzzy-headed – an experience athletes call "hitting the wall."

Body fat percentage

Without exercise, once you hit your 25th birthday you'll begin to lose lean muscle mass and replace it with fat at the rate of up to 3 percent per year. By age 60, an inactive woman might weigh the same as she did at age 20, but have twice as much body fat. Excessive body fat, especially in areas such as the abdomen, is increasingly recognized as an important risk factor for killers like heart disease and diabetes.

That's why experts now suggest that women ditch body weight as a fitness benchmark and look to body composition as a better gauge of how healthy they are. The most practical and accurate way to measure body fat is a skin-fold caliper test. This can be up to 96 percent accurate if the average of three tests is used and it's done by an experienced tester. The test is offered at most gyms. However, results on people of color may be skewed by an additional 1-3 percent because the formulas most commonly used in health clubs are derived from research performed primarily on white subjects.

For optimum fitness, a study in The Physician and Sportsmedicine points to an ideal body-fat-percentage range between 16 and 25. Less than 12 percent can be dangerous to your health, while more than 32 percent puts you at higher risk for disease and a shorter life span.