Calories in Onions: Nutrition Facts & Serving Sizes
Per 1 slice, medium (1/8" thick) (14g)
Per 100g
Source: USDA FoodData Central, SR Legacy, March 2026.
Onions are a fundamental cooking ingredient and a low-calorie vegetable with solid nutritional value. One medium onion (110g) has just 44 calories, 1.2g protein, 10.2g carbs, and 0.1g fat. Per 100g, onions deliver 40 calories, 1.1g protein, 9.3g carbs, and 1.7g fiber. Onions are particularly rich in quercetin, a flavonoid antioxidant linked to anti-inflammatory effects. They also provide 7.4mg of vitamin C per 100g and 19mcg of folate. Sodium is extremely low at just 4mg per 100g, making onions one of the most sodium-friendly flavor bases in cooking. Onions can be eaten raw in salads, caramelized for depth of flavor, or cooked into soups, stir-fries, and sauces. For anyone trying to eat clean and track macros, onions add flavor and nutrients to meals for almost no calorie cost.
Nutrition Highlights
Calories by Serving Size
| Serving | Cal | Protein | Carbs | Fat | Fiber |
|---|---|---|---|---|---|
| 1 cup, chopped (160g) | 64 | 1.8g | 14.9g | 0.2g | 2.7g |
| 1 slice, medium (1/8" thick) (14g) | 5.6 | 0.2g | 1.3g | 0g | 0.2g |
| 1 medium (2-1/2" dia) (110g) | 44 | 1.2g | 10.2g | 0.1g | 1.9g |
| 1 large (150g) | 60 | 1.7g | 14g | 0.2g | 2.6g |
| 10 rings (60g) | 24 | 0.7g | 5.6g | 0.1g | 1g |
| 1 tbsp chopped (10g) | 4 | 0.1g | 0.9g | 0g | 0.2g |
| 1 cup, sliced (115g) | 46 | 1.3g | 10.7g | 0.1g | 2g |
| 1 slice, thin (9g) | 3.6 | 0.1g | 0.8g | 0g | 0.2g |
| 1 small (70g) | 28 | 0.8g | 6.5g | 0.1g | 1.2g |
| 1 slice, large (1/4" thick) (38g) | 15.2 | 0.4g | 3.5g | 0g | 0.6g |
| 100g | 40 | 1.1g | 9.3g | 0.1g | 1.7g |
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Full Nutrition Facts (per 100g)
| Nutrient | Amount | % Daily Value |
|---|---|---|
| Calories | 40 | 2% |
| Protein | 1.1g | 2% |
| Total Fat | 0.1g | |
| Total Carbohydrates | 9.3g | 3% |
| Dietary Fiber | 1.7g | 6% |
| Sugars | 4.2g | 8% |
| Sodium | 4mg |
Vitamins & Minerals (per 100g)
| Nutrient | Amount | % Daily Value |
|---|---|---|
| Calcium | 23mg | 2% |
| Iron | 0.2mg | 1% |
| Potassium | 146mg | 3% |
| Vitamin C | 7.4mg | 8% |
| Vitamin B6 | 0.1mg | 6% |
| Magnesium | 10mg | 2% |
| Phosphorus | 29mg | 2% |
| Zinc | 0.2mg | 2% |
| Selenium | 0.5mcg | 1% |
| Folate | 19mcg | 5% |
| Niacin (B3) | 0.1mg | 1% |
% Daily Values based on a 2,000 calorie diet (FDA reference).
Health Benefits of Onions
Onions are one of the best sources of quercetin, a plant antioxidant that has been studied for its anti-inflammatory and immune-supporting properties. Chronic low-grade inflammation is linked to poor recovery, fat gain, and reduced performance. Including quercetin-rich foods like onions regularly in your diet is a practical way to support the body's natural inflammatory response.
The vitamin C content (7.4mg per 100g) supports collagen synthesis, which keeps joints and connective tissue healthy. This matters for anyone training at high intensity. Vitamin C also helps with iron absorption from plant-based foods eaten in the same meal.
Onions contain prebiotic fibers, including fructooligosaccharides, that feed beneficial gut bacteria. A healthy gut microbiome improves digestion, immune function, and nutrient absorption. The fiber content (1.7g per 100g) also supports bowel regularity.
Allicin and other sulfur compounds in onions have been studied for cardiovascular benefits, including modest effects on blood pressure and platelet aggregation. These same compounds give onions their pungent smell and flavor. Cooking reduces allicin levels, so raw onions in salads or dressings provide more of these compounds than heavily cooked preparations.
Dietary Considerations
Onions are naturally vegan, gluten-free, dairy-free, and fit every major dietary pattern. At only 40 calories per 100g with minimal fat and no cholesterol, they are ideal for weight loss and calorie-controlled eating. The carb count (9.3g per 100g) is moderate but the fiber reduces the glycemic impact. Onions can fit keto diets in small amounts, though a full cup has about 15g carbs. Sodium is negligible at 4mg per 100g, making onions one of the best low-sodium ways to add bold flavor to any dish without seasoning packets or sauces.
Daily Intake (100g)
100g of onion provides 2% of a 2,000-calorie diet.
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Frequently Asked Questions
How many calories in an onion?
Onions have 40 calories per 100g. A medium onion (about 110g) contains around 44 calories. A large onion (150g) has roughly 60 calories. Raw onion added to meals contributes very few calories.
Are onions good for weight loss?
Yes, onions are excellent for weight loss. They are very low in calories, add big flavor to meals, and contain fiber that supports satiety. Using onions generously in cooking helps make low-calorie meals more satisfying and flavorful.
How much protein is in an onion?
Onions contain 1.1g of protein per 100g, or about 1.2g in a medium onion. They are not a protein source, but they are a great low-calorie base for high-protein meals like stir-fries, omelets, and ground beef dishes.
Are onions keto-friendly?
Onions can fit a keto diet in moderate amounts. A tablespoon of chopped onion has less than 1g of carbs, making it fine for flavoring. A full cup (160g) adds about 15g of carbs, which is significant on a strict keto limit. Use smaller amounts to stay within your carb goals.
What makes onions healthy?
Onions are rich in quercetin, a powerful antioxidant with anti-inflammatory properties. They also contain prebiotic fibers that feed healthy gut bacteria, vitamin C for immune function and collagen production, and sulfur compounds linked to cardiovascular benefits.
Should I eat raw or cooked onions?
Both have benefits. Raw onions retain more quercetin, allicin, and vitamin C. Cooked onions are easier to digest and develop a sweeter flavor from caramelization. For maximum nutritional benefit, include both raw and cooked onions in your diet regularly.
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