Why Do To-Go Coffee Cup Lids Have That Tiny Hole?

If you’re a frequent takeout coffee drinker (and even if you’re not, for that matter), you’ll likely have noticed that the plastic lids of takeout cups typically have not one, but two holes.

One of them, of course, is the larger hole through which you can actually drink your coffee. But often on the opposite side of that—or else somewhere in the center or side of the lid—there will be a far smaller, almost pinprick-sized hole that sometimes looks like it has been punched into the plastic. 

At first glance, this second hole might appear to serve little purpose—so what exactly is it? Decoration? Some quirk or hangover from the manufacturing process? Well, as unassuming as it might appear, without this second hole, you would likely not be able to actually drink your coffee at all (or at least, not particularly easily). 

Small But Mighty

On the one hand, this tiny perforation works as little more than an air vent. The contents of a takeout coffee cup are often very hot, of course, and as such, they give off a great deal of steam. This tiny vent hole, therefore, allows some of this steam to escape, preventing it from building up inside the cup, without cooling your drink down too quickly. 

But this tiny vent hole has a second, rather more important purpose. You might not realize it, but placing your lips around the drinking hole of a coffee cup’s closed lid forms such a perfect seal that then drinking from the cup creates a mild vacuum inside of it, as its contents are drained away.

Nature abhors a vacuum, of course, and so naturally something has to take the liquid’s place inside the cup (or else it risks crumpling in on itself, and spilling or leaking all over you in the process). This tiny vent hole—dubbed the “pressure equalization port”—ultimately not only allows steam from the drink out, but a steady flow of air in too, replacing the contents of the cup as you drink it, and thereby maintaining equilibrium inside. 

All About the Air Flow

The steadier the flow of air in through this vent hole, the steadier and smoother the flow of coffee out of the mouth hole will be. And so without it—or if the hole were to be too small, or even choked up with foam from your cappuccino—you’d find yourself drinking in a series of messy glugs as the airflow is disrupted, a vacuum begins to form, and the coffee doesn’t drain out quite so smoothly. 

In essence, this is precisely the same process as lies behind that old high school science experiment to find the fastest way to empty a bottle of water. Simply tipping a bottle upside down causes the water to spill out in a series of messy, sputtering glugs as the water pours out while the air tries to force its way in. Quickly swirling the bottle around, however, creates a vortex inside that allows both the water and the air to move past one another smoothly, making the emptying process faster. 

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