How much is a small/medium/large onion?

Question

By size or weight - I never seem to know ...

Answer

Here's some general guidelines I have used in the past:

Small onion = 4 ounces by weight or about ½ cup chopped

Medium onion = 8 ounces, or about 1 cup chopped

Large onion = 12 ounces, or about 1½ cups chopped

Jumbo onion = 16 ounces, or about 2 cups chopped

source

The terms are not always completely accurate so use your best judgement. I've often seen large onions that were as big or bigger than "jumbo" according to this data.

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