Onion Calories: Exact Count Per Type & Size (2024 Data)

A medium-sized onion (110g) contains approximately 44 calories, with virtually no fat and just 10g of carbohydrates. This comprehensive guide breaks down calorie counts by onion type and size, provides verified nutritional data from USDA sources, and explains how onions fit into balanced diets.

Curious about onion nutrition? Whether you're tracking macros, managing diabetes, or simply cooking smarter, knowing the exact calorie content of onions helps make informed dietary choices. This guide delivers science-backed facts you can trust, with practical applications for everyday cooking and meal planning.

Onion Calorie Breakdown by Type and Size

Not all onions are created equal when it comes to calories. The exact count depends on variety, size, and preparation method. Here's the verified nutritional data from the USDA FoodData Central database:

Onion Type	Size/Weight	Calories	Carbohydrates	Key Nutrients
Yellow Onion (raw)	Medium (110g)	44	10g	Vitamin C (11% DV), Fiber (15% DV)
Red Onion (raw)	Medium (110g)	42	9.8g	Anthocyanins, Quercetin (antioxidants)
White Onion (raw)	Medium (110g)	43	10.2g	Higher sulfur compounds
Green Onions (scallions)	1 cup (100g)	32	7.3g	Vitamin K (207% DV), Vitamin A
Caramelized Onions	1/2 cup (80g)	85	20g	Concentrated natural sugars

This nutritional comparison reveals why yellow onions remain the most commonly used variety in cooking—they offer the best balance of flavor and nutritional value. The USDA's National Nutrient Database for Standard Reference provides these standardized measurements that nutrition professionals rely on for accurate dietary planning.

What Affects Onion Calorie Content?

Several factors influence the actual calories you consume:

• Preparation method: Raw onions maintain their natural calorie count, while caramelizing concentrates sugars and increases calories per volume
• Size variations: Onions range from 50g (small) to 150g (large), directly impacting total calories
• Water content: Onions are 89% water, which affects density and portion measurements
• Storage conditions: Older onions develop higher sugar content as starches convert

According to research published in the Journal of Food Composition and Analysis, the carbohydrate composition of onions changes during storage, with fructose levels increasing by up to 15% over three months. This means that an onion stored for several weeks contains slightly more calories than when freshly harvested.

Nutritional Benefits Beyond Calories

Onions deliver impressive nutritional value far beyond their modest calorie count:

• Fiber content: One medium onion provides 15% of your daily fiber needs, supporting digestive health
• Antioxidant power: Red onions contain anthocyanins that provide 2x the antioxidant activity of yellow varieties
• Vitamin C: Essential for immune function and collagen production
• Quercetin: A flavonoid with anti-inflammatory properties studied for cardiovascular benefits

The American Journal of Clinical Nutrition highlights onions as part of the "Allium vegetable" family, which research associates with reduced risk of certain cancers. Their low glycemic index (10) makes them suitable for diabetes management when consumed in appropriate portions.

Practical Applications for Healthy Eating

Understanding onion calories helps optimize their use in various dietary approaches:

For Weight Management

With just 44 calories per medium onion, they add volume and flavor to meals without significant caloric impact. Try these strategies:

• Replace half the ground meat in burgers with finely diced onions
• Add raw onions to salads for crunch instead of croutons
• Use caramelized onions as a flavor base instead of oil or butter

For Diabetes Management

The American Diabetes Association recommends onions as a non-starchy vegetable. One medium onion contains about 10g carbohydrates, which fits within most meal carb budgets. Pair onions with protein sources to minimize blood sugar impact.

Keto Diet Considerations

While onions contain carbohydrates, small portions (1-2 tablespoons of minced onion) can fit within keto macros. Green onions offer a lower-carb alternative with just 0.7g net carbs per tablespoon.

Common Misconceptions About Onion Nutrition

Several myths persist about onion calories and nutrition:

• Myth: Cooking significantly increases onion calories
  Fact: While caramelizing concentrates sugars, the actual calorie increase comes from added fats, not the onion itself
• Myth: All onion varieties have identical nutritional profiles
  Fact: Red onions contain significantly more antioxidants than yellow varieties
• Myth: Onions cause weight gain
  Fact: At 44 calories per medium onion, they're actually weight-loss friendly when used to enhance flavor without added fats

Registered dietitians emphasize that the sulfur compounds in onions may actually support metabolism. A study in Nutrition Research found that Allium vegetables like onions positively influence gut microbiota composition, which affects how our bodies process calories.

Maximizing Nutritional Value in Cooking

How you prepare onions affects their nutritional benefits:

• Raw consumption: Preserves maximum vitamin C and quercetin content
• Light cooking: Brief sautéing (3-5 minutes) maintains most nutrients while improving digestibility
• Avoid overcooking: Prolonged high heat destroys heat-sensitive compounds
• Pair with healthy fats: Combining onions with olive oil enhances absorption of fat-soluble compounds

Food science research shows that chopping onions and waiting 10 minutes before cooking allows beneficial compounds to fully develop. This simple technique can increase the quercetin content by up to 25% according to studies published in the Journal of Agricultural and Food Chemistry.

How Onions Compare to Other Common Vegetables

Understanding where onions fall in the vegetable spectrum helps with meal planning:

• Lower calorie than: Potatoes (163 calories per medium), corn (134 calories per cup)
• Similar calorie range to: Bell peppers (31 calories each), zucchini (20 calories per 100g)
• Higher fiber than: Cucumbers, lettuce, celery

This positioning makes onions an excellent choice for adding flavor and nutritional value without excessive calories—a key consideration for those following calorie-controlled diets.

How many calories in a small onion?

A small onion (50g) contains approximately 20 calories. This size is commonly used in single-serving recipes and provides about 4.5g of carbohydrates and 1g of fiber.

Are cooked onions higher in calories than raw onions?

Cooking doesn't significantly increase onion calories unless you add fats. A medium raw onion has 44 calories, while the same onion caramelized with no added fat contains about 45 calories. The perceived calorie increase comes from water loss concentrating the natural sugars.

Which onion variety has the fewest calories?

Green onions (scallions) have the lowest calorie count at 32 calories per cup (100g). Red onions follow closely with 42 calories for a medium onion, slightly fewer than yellow or white varieties.

Can onions help with weight loss?

Yes, onions can support weight loss efforts. With only 44 calories per medium onion, they add flavor and volume to meals without significant calories. Their high water and fiber content promotes satiety, helping you feel full longer while consuming fewer calories overall.

How many carbs are in an onion for keto diet?

A medium onion contains about 10g total carbohydrates with 8g net carbs. For strict keto diets (20-50g net carbs daily), use onions sparingly. Green onions offer a lower-carb alternative with just 0.7g net carbs per tablespoon.