Sure, you know how to find out the number of calories in your morning bowl of cereal. But below the calorie count, you may be confused by the mg’s, %’s, and odd definitions. Here are the most important elements on a nutrition label and what they mean to you.

Serving Size

This number is at the top for a reason: The nutritional information on the rest of the label applies to one serving. The FDA sets serving sizes for all foods — they are measurements, not recommendations. Total calories are calculated per serving, as are total calories from fat — so be sure to look at the servings per container. A bag of potato chips might say it has 150 calories per serving, but the entire bag might be three servings, or 450 calories.

Percent of Daily Value

This is calculated for a moderately active woman, or a fairly sedentary man, who eats 2,000 calories a day. (Highly active women, moderately active men, and growing teen boys may need closer to 2,500 calories a day.) A serving of Cheerios with 1/2 cup of skim milk gives the average adult just 3 percent of the daily value of fat intake and 11 percent of the daily value of fiber intake recommended by the U.S. Department of Agriculture (USDA).

Fat

More important than total fat are the numbers for saturated, polyunsaturated, and monounsaturated fats. (Trans fats will be added to labels by 2006.) You want to see that the food contains relatively little saturated fat and trans fat, and relatively more polyunsaturated and monounsaturated. Keep in mind that “fat-free” doesn’t equal “calorie-free.” Many fat-free and low-fat foods have added sugar.

Cholesterol

This is a fatlike chemical that’s an essential component of cell membranes, a covering for nerve-cell fibers, and a building block of hormones. Only animal products contain cholesterol. Adults are advised to limit their daily intake to 300 milligrams. Too much can elevate your blood cholesterol, raising your heart-disease risk.

Sodium

The recommended daily limit for an average adult is 2,300 milligrams (it was lowered from 2,400 for the 2005 guidelines), because too much sodium can cause high blood pressure. By the USDA’s reckoning, a food is low in sodium if it contains no more than 140 milligrams. (A serving of Cheerios has 210 milligrams and is therefore not low in sodium.) A single serving of soup or a frozen dinner may contain 1,000 milligrams or more of sodium, which is nearly half the daily limit.

Potassium

Getting enough of this mineral — 4,700 milligrams a day for adults — may help prevent high blood pressure. Low potassium can lead to an irregular heartbeat.