We cut, count, save and splurge on calories. But in a recent Johns Hopkins University study, more than half the participants didn’t know how many calories they should consume each day. What’s the point in obsessing over the numbers if a key part of the equation is missing?

Soon, calorie counts will be splashed across fast food menu boards, restaurant menus and snack-food vending machines from coast to coast. A new rule from the Food and Drug Administration mandates that food sellers reveal calories in an attempt to help rein in the nationwide obesity epidemic and its alarming health consequences.

“Americans eat and drink about one-third of their calories away from home, and people today expect clear information about the products they consume,” said FDA Commissioner Margaret A. Hamburg, MD, in a statement. “Making calorie information available … is an important step for public health that will help consumers make informed choices for themselves and their families.”

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Making that informed choice — between, say, the 700-calorie grilled cheese-and-bacon sandwich or the 400-calorie spinach salad with grilled chicken — requires knowing those numbers but also knowing how many calories you can afford to take in each day without packing on pounds. That sum is different for everyone. Menus will note, “2,000 calories a day is used for general nutrition advice, but calorie needs vary.” Your requirements are likely lower or higher.

Here are three things you need to know.

1. How many calories it takes to gain or lose a pound. To stay at your current weight, the number of calories you take in must equal the number of calories you burn. Take in 3,500 calories too many and you’ll gain a pound. Cut or burn 3,500 calories by eating less or exercising and you’ll lose a pound.

2. Your basal metabolic rate. The human body burns calories in two main ways. You burn some through activity, whether it’s working out at the gym or standing at the sink doing the dishes. You burn even more through basic body functions like thinking, breathing, digesting, producing hormones and pumping blood throughout your system. How fast your body burns calories in this non-exercise way is your basal metabolic rate (BMR).

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Your genes, age, sex and body size work together to set your body’s BMR. You can’t change your genes — you may have inherited a fast or slow metabolism. Taller, heavier people burn more calories than shorter, lighter people. Men burn about 5 to 10 percent more calories a day than women, even if they’re the same size, because men have more muscle mass. Muscle mass diminishes with age, which is why we need fewer calories as we get older.

3. How to calculate your calorie needs. The easiest way to get your magic number is to plug your info into an online calorie calculator. Rather do it yourself? The first step is figuring out your BMR:

For women: 655 + (4.35 × weight in pounds) + (4.7 × height in inches) – (4.7 × age in years) = BMR

For men: 66 + (6.23 × weight in pounds) + (12.7 × height in inches) – (6.76 × age in years) = BMR

Then multiply your BMR by your physical activity level to get your daily calorie needs:

  • If you are sedentary (little or no exercise): BMR x 1.2
  • If you are lightly active (light exercise/sports 1 to 3 days a week): BMR x 1.375
  • If you are moderately active (moderate exercise/sports 3 to 5 days a week): BMR x 1.55
  • If you are very active (hard exercise/sports 6 to 7 days a week): BMR x 1.725
  • If you are extra active (very hard exercise/sports and a physical job): BMR x 1.9 

Related: How to Choose the Best Diet for You

Once you have the answer, you’ll know whether you can afford that grilled-cheese-and-bacon sandwich. Trying to lose weight? Forget the sandwich. Order the salad instead.

calories infographic (Photo: Sydney Herwig/SafeBee) 

Sari Harrar is an award-winning health, medicine and science journalist whose work appears in Dr. Oz The Good Life magazine, Good Housekeeping, O--Oprah Magazine, Organic Gardening and other publications.