Here's Exactly How Many Onions You Need To Chop To Equal A Cup - Tasting Table

"For example, if a recipe calls for 1 cup of chopped onions, do you know exactly how many onions you'll need to buy or ultimately, chop? Often, you won't know until you get deep into your recipe's ingredient list, whether you'll need a specific volume or number of onions (sometimes with a size specification, but oftentimes - annoyingly - without). This leads to further confusion as small onions weigh around 6 ounces, medium about 8 ounces, and large roughly 16 ounces."

"Onions are one of the most basic and versatile staples used in cooking. Whether raw, sauteed, grilled, roasted, caramelized, or fried, they impart their unique flavor in subtle and not-so-subtle ways in everything from French onion soup to Tennessee onions. But following a recipe that calls for these aromatic, allium bulbs (as many do), can certainly bring one to tears for more than one reason, like the fact that many recipes have less-than-stellar instructions."

Onion quantity and measurement vary with bulb size and how the onion is cut. Small onions are roughly 6 ounces, medium about 8 ounces, and large roughly 16 ounces. One medium-sized onion (tennis ball-sized) yields about 1 cup of chopped onions, with chopped piece size ranging from finely chopped 1/4-inch to roughly chopped 1-inch pieces. Measuring by volume depends on density and how tightly pieces pack into a cup. Recipe ingredient lists often omit size or cut details, so knowing approximate conversions helps avoid confusion when following cooking instructions.

Read at Tasting Table