A medium raw onion (about 110 grams) contains roughly 44 calories. That makes onions one of the lowest-calorie vegetables you can cook with, since they’re about 90% water by weight.
Calories by Size and Serving
Onion sizes vary quite a bit at the grocery store, so the calorie count shifts depending on what you grab. A useful baseline: raw onion runs about 38 calories per 100 grams. From there, the math is straightforward.
- Small onion (about 70g): roughly 27 calories
- Medium onion (about 110g): roughly 44 calories
- Large onion (about 150g): roughly 57 calories
- Half cup chopped (80g): roughly 30 calories
Even a large onion adds fewer calories than a single tablespoon of olive oil. If you’re tracking intake, onions are one of the ingredients you barely need to worry about.
What’s Actually in Those Calories
Almost all of an onion’s calories come from carbohydrates. Per 100 grams of raw yellow onion, you’re looking at about 8.6 grams of carbs, 5.8 grams of sugar, 1.9 grams of fiber, less than 1 gram of protein, and virtually zero fat. The sugars are a mix of glucose, fructose, and sucrose, which is why onions turn sweet and golden when you caramelize them. That natural sugar content is low enough that onions still qualify as a low-glycemic food.
The fiber is worth noting. A whole medium onion delivers close to 2 grams, mostly as a type called fructans. Fructans act as prebiotics, feeding beneficial gut bacteria. They can also cause bloating in people sensitive to FODMAPs, so if onions bother your stomach, that’s likely the reason.
Do Different Varieties Have Different Calories?
Yellow, white, and red onions are nutritionally similar. The calorie differences between them are minimal, generally within a few calories per 100 grams. Red onions contain slightly more of certain plant pigments (the same compounds that give them their color), but the calorie and carb profiles are close enough to be interchangeable for tracking purposes. Sweet varieties like Vidalias have a bit more sugar and taste milder, though the calorie difference per onion is negligible.
Green onions (scallions) are a different story. Because they’re mostly hollow greens with a tiny bulb, they’re far less calorie-dense. A half cup of chopped green onions has only about 10 calories, roughly a third of the same volume of chopped bulb onion.
How Cooking Changes the Numbers
Cooking doesn’t add calories to onions on its own, but it does two things that matter. First, onions lose water as they cook, so a cup of sautéed onions is more calorie-dense than a cup of raw onions simply because you’ve packed more onion into the same volume. Second, most cooking methods involve oil or butter. A tablespoon of oil adds about 120 calories, which can easily triple the calorie contribution of the onions in a dish.
Caramelizing onions concentrates their natural sugars as water evaporates. A large onion that starts at 57 calories still contains 57 calories after caramelizing (minus whatever trace amount evaporates), but it cooks down to a fraction of its original volume. If you’re measuring by the spoonful rather than by the whole onion, the cooked version will appear more calorie-dense.
Onions Compared to Other Vegetables
At 38 calories per 100 grams, onions sit in the middle-to-low range for vegetables. For context, the same weight of raw carrot has about 41 calories, bell pepper about 31, celery about 14, and potato about 77. Onions are comparable to tomatoes (18 calories per 100g) only if you account for how much less onion you typically eat in a sitting. Most people use a quarter to half an onion in a recipe serving, which puts the actual calorie contribution somewhere between 10 and 25 calories per plate.
For anyone counting calories, onions are essentially a “free” ingredient. Their real contribution to a meal is flavor, not energy.