What's the Difference Between a Serving Size and Portion Size?

Whether you're trying to adopt healthier snacking habits or simply keep yourself from veering into food coma territory after dinner—admit it, we've all been there—it's important to pay close attention to your serving size. Or is it your portion size? To be totally honest, it can be a little confusing.

The truth is, a serving size of food is not the same as a portion size of food. So what's the difference? And how does your serving size affect your portion size, and vice versa? To get to the bottom of it, we tapped Jessica Green, a registered dietitian and nutritionist, to break it down.

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What Is a Serving Size?

Simply put, a serving size is a specific and standardized amount of food like 1 cup, two slices, or 30 grams.

"Think about a box of graham crackers," Green explains. "The nutrition facts label on the box may say that the serving size is eight crackers. All of the information listed on that box of graham crackers nutrition facts label is in reference to those specific eight crackers."

A nutrition facts label will also share the amount of sugar, sodium, carbs, and calories in each serving, which will help you understand if the snack in question is healthy for you. (Read: Anyone who needs to stick to a low-sodium diet should stay away from snacks with a lot of sodium per serving.)

Another way to think about serving size is as a guide to how much you should eat over the course of the day.

"For example, the U.S. Department of Agriculture's MyPlate program suggests that women 31 years-old or older [should] aim for two to three cups of vegetables per day," she adds. "These are examples of a standardized amount or serving size."

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What Is a Portion Size?

That said, just because that box of graham crackers has a serving size slapped on the Nutritional Facts label doesn't mean we'll eat exactly eight crackers. Enter the portion size, which is best known as how much you eat at once. (Whether you had one slice of pizza or sprung for a second helping, the total amount of slices is your portion size.)

"Because a portion is not standardized, it could cause you to eat more or less than a serving size," Green explains. "For instance, the serving size for a regular soda may be 8 fluid ounces, but if you fill a 20-ounce cup with soda, you are drinking more than the serving size."

Unlike a serving size—which food companies are legally obligated to disclose—your portion size is entirely up to you. So go ahead and spring for that second helping if you're hungry; there's no judging here!

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How to Measure Serving Size vs. Portion Size

So how are you supposed to convert your portion size to a serving size? The answer is right at your fingertips—literally.

"While there are different ways to align portion size to serving size, one simple way is to use your hand as a guide," Green explains. "The fist is about one cup; one scooped hand (from tip of your fingers to the base of the palm) is about ½ cup; ¼ cup is about a palm full, one thumb is about 1 tablespoon; and your palm is about 4 ounces. So if you are trying to eat more veggies, aim for two to three fistfuls of raw or cooked veggies per day."

Sure, serving size and portion size might be two different things, but they can still work together to give you a better sense of your overall diet. "If the recommendation is to aim for 2 to 3 cups of vegetables per day, you can use this serving size as a goal by matching your portions of vegetables each day to the recommended amounts," Green says.