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A serving size is a measured amount of food or drink, such as 2 to 3 ounces of meat or 1 cup of milk. Serving sizes are standardized so that nutrient levels in each serving are comparable. Consumers and dietitians may use serving sizes to determine how much of the recommended daily levels of certain substances (vitamins, minerals, fats, calories, cholesterol) a particular food represents.

Dietitians often use visual approximations to help consumers determine by sight the appropriate serving size for many foods. Rather than using a scale or measuring cup, people can estimate the correct serving size by comparing the food to commonly used items. For instance, one serving of meat, fish or poultry (3 ounces) is about the size of a deck of cards.

Serving sizes may vary depending on how certain foods are prepared (e.g., raw, cooked, canned), as well as the nutrition guide or source consulted. Servings listed on a Nutrition Facts Label (found on the back of nearly all packaged food products) are different from those used as the basis of food guidelines for healthy eating plans, such as food guide pyramids like MyPyramid. They were developed by different government agencies to serve different purposes.

Serving sizes should not be confused with portion sizes or helpings of food, which are the amounts of food typically served or eaten during a meal or snack. Portions can vary from meal to meal or person to person. Although the terms are sometimes used interchangeably, serving sizes are usually much smaller than portion sizes. Most foods are sold and consumed in larger portions than is recommended by government standards. For example, the average bagel consists of two serving sizes, pasta is almost five servings and a steak may be two and a quarter servings.

Many experts believe large meals have contributed to obesity because consuming more than the recommended serving sizes adds excess calories, salt and fat to the diet. All of this potentially contributes to bigger waistlines and an increased risk of developing chronic diseases such as heart disease, stroke, high blood pressure (hypertension), diabetes, osteoporosis or obesity.

Large portion sizes in restaurants and at home have also resulted in consumers who are increasingly confused about the amounts of food they should eat. Becoming familiar with what counts as a serving size can help consumers make food choices that help them maintain a healthy weight and body.