Here's How Many Onions You Need For One Cup: Sliced, Diced, Or Chopped

In a huge range of recipes, the first ingredient in the list is onions. Whether you're caramelizing, getting the base going for a stir fry, or building your sofrito or mirepoix, onions are one of the building blocks of our favorite recipes. Despite that, recipes seem to be eternally vague on how much onion you need for a recipe. Onions come in a wealth of shapes and sizes, but recipes insist on calling for simply "1 onion" or for an onion that is small, medium, or large, or for it measured by cups with no mention of how many onions you should put on your shopping list.

Using slightly more or less onion probably isn't going to be a game changer for the final dish. But it can be frustrating when you need more information and they don't give it. 

The first question we need to settle is the matter of how big an onion is. When recipes call for small, medium, and large onions, what does that really mean? For the most part, this comes from people measuring things with their heart, and there's a lot of guesswork involved. But if you want to be on the safe side, there are some approximate measurements you can use to go by.

The most reliable way to measure an onion is by weight, though the United States Department of Agriculture provides federal onion size guidelines based on the diameter in inches. As a rule of thumb, a small onion is around 4 ounces (1- to 2¼-inch diameter) A medium onion is around 5 to 8 ounces (2 to 3¼ inches). A large onion is any onion that's 9 ounces or more (over 3 inches), though some might also use the term extra-large onion to refer to anything 16 ounces or more. It's still not an exact science, but it helps to get into the right ballpark. 

You can achieve perfectly sliced onions, also known as julienned onions when cut thin, in one of two ways. You can cut rings in half or cut lengthwise through an onion that's been topped and tailed (had the root and tip cut off), peeled, and sliced in half lengthwise (through the root end).

To get 1 cup of sliced onion, you need 4 to 5 ounces, which is about one small to medium onion or half a large onion.

A rough chop or large dice is probably the go-to cut for most recipes. The primary difference is accuracy: With a large dice, you're shooting for roughly cubed pieces of onion in about the same size, whereas a rough chop is just a way to get from point A (whole onion) to point B (cut onion) quickly. Ideally, you'll be looking at about half-inch pieces of onion for this style.

In this case, 1 cup of chopped onion is equal to about 5 ounces, or one medium onion, preferably on the smaller side to avoid leftovers. The onion should be around the same size as a billiard ball or large lemon, and the measuring cup should be completely filled without space to push the onion down too much.

When a recipe calls for a fine dice, it pays to know how to dice onions like a pro. At about an eighth of an inch square, finely diced onions are an efficient cut. If the onion is truly finely diced, then it will have almost no air pockets when measured by cup. 

For a fine dice, 1 cup equals 6½ to 7 ounces, which is one medium onion or three-quarters of a large onion. If you do more of a medium dice, around a quarter of an inch, you can probably get there with only 5 ounces of onion, which is also medium.