Half a cup of chopped onion is about 75 grams (2.6 ounces), which works out to roughly half of a medium onion. If you’re mid-recipe and don’t have a measuring cup handy, that’s the simplest rule: one medium onion gives you about 1 cup chopped, so cut it in half and you’re there.
What Counts as a Medium Onion
The USDA classifies onion sizes by diameter. A medium onion measures 2 to 3¼ inches across, and that’s the size most recipes assume when they call for “one onion.” A small onion tops out around 2¼ inches and yields closer to ½ cup on its own. A large or jumbo onion starts at 3 inches across and can give you 1½ to 2 cups chopped, so you’d only need about a quarter of one.
Grocery stores aren’t always consistent with labeling, so diameter is the most reliable way to judge. If you can wrap your hand around the onion and your fingers nearly touch, it’s probably medium.
How Chopping Style Changes the Amount
The way you cut an onion affects how much fits into a half cup. Minced onion packs more tightly than a coarse dice, so the same weight fills less volume when the pieces are bigger. Industry yield data shows that a pound of onion produces roughly 4.9 cups when minced but only about 2.7 cups when diced. That’s a significant difference.
In practical terms, if your recipe calls for ½ cup and you mince very finely, you’ll end up with a stronger onion presence than if you chop into larger pieces. Most recipes that say “chopped” mean a roughly ¼-inch dice, which is what the 75-gram standard assumes. If you’re mincing, you could ease back to about 60 grams and get the same onion intensity in the finished dish.
Raw vs. Cooked Volume
If a recipe calls for ½ cup of cooked onion, you need to start with more. Onions lose a substantial amount of moisture during cooking. Frying causes somewhere between 37% and 43% weight loss depending on the variety, and sautéing produces similar shrinkage. Half a cup of raw chopped onion will cook down to roughly ¼ to ⅓ cup.
So when a recipe says “½ cup sautéed onion,” start with about ¾ to 1 cup raw, which is one full medium onion. When it says “½ cup onion, chopped” and the cooking step comes later in the instructions, that means ½ cup raw.
Quick Reference by Onion Size
- Small onion (under 2¼ inches): yields about ½ cup chopped, so use the whole thing
- Medium onion (2 to 3¼ inches): yields about 1 cup chopped, so use half
- Large onion (3+ inches): yields 1½ to 2 cups chopped, so use roughly a quarter
Nutrition in Half a Cup
Half a cup of raw chopped onion has about 30 calories, 5 grams of natural sugar, 1 gram of fiber, and 5 milligrams of vitamin C. It’s a low-calorie way to add a lot of flavor. The sugar content is worth noting if you’re watching carbohydrates closely, since onions are sweeter than most people expect, especially when caramelized and that sugar concentrates.
Measuring Without a Measuring Cup
If you’re cooking and don’t want to dirty a measuring cup, a cupped handful of chopped onion is close to ½ cup for most adult hands. You can also use a standard coffee mug, which holds about 1 cup, and fill it halfway. For weight-based precision, 75 grams on a kitchen scale is exact and faster than measuring by volume, especially if you’re prepping multiple ingredients.
Onion measurements in home cooking are forgiving. A tablespoon more or less won’t ruin a soup or stir-fry. The exception is baking or pickling, where ratios matter more. For those, weigh your onion.