The serving size sits at the top of the Nutrition Facts label, and it controls every other number on it. Misreading it is the easiest way to underestimate what you're actually consuming.
It's a reference, not a recommendation
A common misconception is that the serving size is the amount you should eat. It isn't. By law it is a Reference Amount Customarily Consumed (RACC) — an amount based on how much people typically eat in one sitting, established so that labels are consistent between products. A serving of soda is set at 12 fluid ounces because that's a typical amount, not because 12 ounces is the recommended dose.
Everything is per serving
If a pint of ice cream lists 250 calories per serving and 4 servings per container, eating the whole pint is 1,000 calories. This is why reading the serving size first matters so much. The gap between the serving on the label and the portion you actually eat is where most surprises hide. See serving size vs. portion.
What changed in 2016
The 2016 label update made serving sizes more honest. Since the original rules were written in the 1990s, portions had grown. The FDA updated many reference amounts to reflect what people really eat today — for example, a serving of ice cream went from ½ cup to ⅔ cup, and a serving of soda from 8 to 12 ounces. Serving size and calories were also made larger and bolder.
Dual-column labels
For packages that could be eaten in one sitting or several — a 20-ounce soda, a pint of ice cream — the FDA introduced dual-column labels showing nutrition both 'per serving' and 'per package'. This closes the loophole where a single-sitting item was technically labeled as multiple servings.