How Many Calories Are In An Onion?

The humble onion is a cornerstone of global cuisine, forming the base of countless soups, stews, sauces, and sautés. Yet for those tracking their caloric intake—whether for weight management, athletic performance, or medical reasons—one question arises frequently: just how many calories does an onion actually contribute to a dish? The answer is both reassuring and practical: onions are remarkably low in calories while delivering significant flavor, aroma, and nutritional value. Understanding the precise caloric load of onions, how it varies by type and preparation method, and how to leverage this ingredient without tipping the energy balance is essential knowledge for home cooks and health-conscious eaters alike.

Onions are not only foundational in cooking but also metabolically neutral in most recipes—they enhance taste without substantially increasing calorie density. This makes them ideal for building flavor in reduced-calorie dishes. However, portion size, variety, and cooking technique can influence the final caloric impact. To fully grasp the role of onions in nutrition and culinary design, it’s important to examine their composition, compare types, and understand how they interact with other ingredients in real-world meals.

Definition & Overview

An onion (Allium cepa) is a biennial vegetable belonging to the Alliaceae family, which includes garlic, leeks, chives, and shallots. It grows underground as a bulb composed of layered fleshy scales that store nutrients. Onions have been cultivated for over 5,000 years, originating in Central Asia before spreading globally due to their hardiness, storability, and culinary versatility.

Culinarily, onions serve as an aromatic base—often combined with celery and carrots in what’s known as *mirepoix* in French cuisine or *sofrito* in Latin American and Mediterranean traditions. They provide depth, sweetness, pungency, and umami when cooked, and sharp brightness when served raw. Their ability to transform under heat—from harsh and acrid to deeply sweet and complex—makes them indispensable across cultures.

Nutritionally, onions are primarily composed of water (about 89%), carbohydrates (including natural sugars and dietary fiber), and trace amounts of protein and fat. They contain no cholesterol and are naturally free of sodium, making them suitable for heart-healthy and low-sodium diets. Beyond macronutrients, onions are rich in antioxidants such as quercetin, sulfur compounds linked to anti-inflammatory effects, and vitamin C.

Key Characteristics

Characteristic	Description
Flavor Profile	Raw: Pungent, sharp, slightly spicy. Cooked: Sweet, mellow, savory, with caramelized notes when browned.
Aroma	Pronounced sulfurous scent when cut; mellows significantly during cooking.
Color & Form	Bulbous root with papery outer skin; flesh ranges from white to yellow to red/purple.
Heat Level	Mild to moderate pungency; varies by variety and preparation.
Culinary Function	Aromatic base, flavor enhancer, textural component (raw or cooked), thickening agent (when reduced).
Shelf Life	Whole bulbs: 1–3 months in cool, dry storage. Sliced: 7–10 days refrigerated. Frozen: up to 6 months (blanched recommended).

Caloric Content by Type and Serving Size

The number of calories in an onion depends on its size, variety, and whether it's consumed raw or cooked. Water loss during cooking concentrates flavors and slightly increases calorie density per gram, though total calories remain nearly unchanged unless fat is added.

Here’s a detailed breakdown based on USDA FoodData Central averages:

Yellow Onions (Most Common)

• Per 100 grams (raw): 40 kcal
• Medium whole onion (~110g edible portion): ~44 kcal
• 1 cup chopped (160g): ~64 kcal
• Sautéed in 1 tsp olive oil (added fat): ~115 kcal (40 from onion, 75 from oil)

White Onions

• Per 100 grams (raw): 42 kcal
• Slightly crisper and more pungent than yellow; often used raw in salsas and salads.

Red (Purple) Onions

• Per 100 grams (raw): 40 kcal
• Higher in anthocyanins (antioxidants); milder when raw, excellent for pickling.

Green Onions (Scallions)

• Per 100 grams: 32 kcal
• One medium scallion (~15g): ~5 kcal
• Much lower in sugar and volume than bulb onions; used as garnish or mild flavor accent.

Shallots

• Per 100 grams: 72 kcal
• Denser and sweeter than common onions; often used in vinaigrettes and fine sauces.

Pro Tip: When calculating calories in a recipe, remember that one medium onion contributes roughly 40–50 calories—less than half the energy of a banana. But if you’re sautéing it in butter or oil, the fat source will dominate the calorie count, not the onion itself.

Variants & Types: Choosing the Right Onion for the Job

While all edible onions share core characteristics, each variety brings distinct qualities to cooking. Selecting the right type enhances both flavor and efficiency in meal planning.

1. Yellow Onions

The workhorse of the kitchen. When cooked, yellow onions caramelize beautifully due to their high sugar content. They form the backbone of gravies, soups, stir-fries, and roasted vegetable medleys. Best stored at room temperature; keep away from potatoes to prevent sprouting.

2. White Onions

Common in Mexican and Southwestern cuisines. They have a cleaner, sharper bite when raw and hold their texture well when cooked. Ideal for fresh pico de gallo, ceviche, and tacos where a bright onion note is desired.

3. Red Onions

Valued for color and moderate heat. Their vibrant hue fades when cooked but remains striking in raw applications. Excellent sliced thin on burgers, in Greek salads, or quick-pickled with vinegar and sugar for use on sandwiches and grain bowls.

4. Sweet Onions (Vidalia, Walla Walla, Maui)

Grown in low-sulfur soils, these onions are exceptionally mild and juicy. Lower in pungency, higher in water content. Can be eaten entirely raw—grilled like fruit or used in onion rings with minimal frying time. More perishable; best used within two weeks.

5. Pearl and Boiler Onions

Small, round varieties often sold frozen or jarred. Used in stews, casseroles, and cocktails like the classic Gibson martini. Boiled or braised until tender, they absorb surrounding flavors beautifully.

6. Dried Onion Products

• Onion Powder: 302 kcal per 100g—concentrated due to water removal. Use sparingly: 1 tsp ≈ 5 kcal.
• Dehydrated Chopped Onion: 310 kcal per 100g. Reconstitutes with liquid; useful in spice blends and rubs.
• Fried Onion Strips (commercial): Up to 500+ kcal per 100g due to oil absorption—common on green bean casserole or baked potatoes.

Comparison with Similar Ingredients

Onions are sometimes substituted or confused with related alliums. Understanding differences ensures accurate flavor outcomes and nutritional calculations.

Ingredient	Calories (per 100g)	Flavor Difference	Best Uses
Garlic	149 kcal	More intense, spicy-sweet when roasted	Flavor accent, not base; mince finely
Leek	61 kcal	Milder, grassier, less sweet	Stocks, soups, gratins; requires thorough cleaning
Shallot	72 kcal	Sweeter, more delicate, wine-like complexity	Vinaigrettes, pan sauces, fines herbes
Chives	30 kcal	Very mild, herbal, no crunch	Garnish, egg dishes, potato topping
Green Onion	32 kcal	Balanced between mild bulb and green stem	Stir-fries, garnishes, kimchi

“Onions are the scaffolding of flavor. You don’t taste them directly in a finished bolognese, but remove them and the dish collapses.” — Chef Elena Ruiz, Culinary Institute of America

Practical Usage: How to Use Onions Without Adding Excess Calories

Because onions contribute so little in terms of calories yet so much in flavor, they are a strategic tool in healthy cooking. Here’s how to maximize their impact:

1. Build Flavor Bases Without Fat

Sauté onions in water, broth, or non-stick spray instead of oil. While traditional methods call for fat to prevent sticking and promote browning, modern techniques allow for flavorful results with minimal or no added lipids. Use a heavy-bottomed pan and cook over medium-low heat, deglazing with liquid as needed.

2. Caramelize Smartly

True caramelization takes time—30 to 45 minutes—and develops deep sweetness through slow Maillard reactions. To reduce calories:

• Avoid adding butter or oil beyond 1 tsp per large onion.
• Add a pinch of baking soda (1/8 tsp) to speed up browning (increases pH).
• Use a splash of balsamic vinegar or apple juice to enhance sweetness naturally.

3. Use Raw Onions Strategically

Soak sliced red onions in ice water for 10 minutes to reduce sharpness and improve texture for salads. This removes volatile sulfur compounds without diminishing nutrient content. For even milder results, quick-pickle with vinegar, a touch of honey, and spices.

4. Substitute in High-Calorie Applications

Replace half the ground meat in burgers or meatloaf with finely diced sautéed mushrooms and onions. This reduces saturated fat and overall calories while maintaining moisture and mouthfeel.

5. Measure Before Cooking

Volume changes dramatically when onions cook down. One cup of raw chopped onion yields about 1/3 cup after prolonged sautéing. For precise nutrition tracking, weigh onions before cooking or use pre-cooked values in databases.

Actionable Ratio Guide: In most savory dishes, use a 2:1 ratio of onion to garlic. For every 1 clove of garlic, include 2 tablespoons (about 30g) of chopped onion. This creates balanced depth without overpowering.

Storage, Shelf Life & Substitutions

Optimal Storage Methods

• Whole Bulbs: Store in a cool, dark, well-ventilated area (not the refrigerator). Mesh bags or baskets work best. Avoid plastic, which traps moisture and promotes mold.
• Cut Onions: Wrap tightly in plastic or place in an airtight container; refrigerate for up to 10 days. Expect some odor transfer.
• Freezing: Chop and freeze raw onions for cooked dishes only—they lose crispness. Blanching is optional but helps preserve color. No need to thaw before use.

Common Substitutions

• No Onion Available? Use 1 tbsp onion powder per 1/2 cup chopped fresh onion—but reduce salt since powders often contain anti-caking agents with sodium.
• Allergy or Sensitivity? Leeks (green parts only) or fennel bulb offer similar aromatic profiles with different chemistry. Note: Fennel has a licorice note.
• Reducing Gas & Digestive Discomfort? Cooking onions thoroughly reduces fructans, the fermentable carbs responsible for bloating in sensitive individuals.

Practical Tips & FAQs

Q: Do onions gain calories when caramelized?

A: No. Caramelization concentrates flavor by removing water, but the total caloric content remains the same unless fat or sugar is added. A 100g raw onion (~40 kcal) becomes about 30g of caramelized mass with the same ~40 kcal.

Q: Are onions keto-friendly?

A: Yes, in moderation. One small onion contains about 5–6g net carbs. For strict keto (under 20g/day), limit to 1/4 to 1/2 cup cooked. Shallots and green onions offer lower-carb alternatives.

Q: Why do onions make me cry?

A: Cutting ruptures cells, releasing enzymes that convert sulfoxides into sulfenic acid, which quickly becomes syn-propanethial-S-oxide—a volatile gas that irritates the eyes. Chill onions for 30 minutes before chopping to slow this reaction.

Q: Can I eat onions on a low-FODMAP diet?

A: Only in limited quantities. Fructans in onions are high-FODMAP. However, using the green tops of scallions or infusing oil with onion (then removing solids) allows flavor without triggering symptoms.

Q: Which onion has the most antioxidants?

A: Red onions, particularly varieties like Ruby Ring and Red Zeppelin, contain up to six times more flavonoids than yellow onions. Quercetin and anthocyanins are concentrated in the outer layers—so peel minimally.

Summary & Key Takeaways

Onions are among the most efficient flavor-building ingredients in the culinary world—delivering robust taste and functional benefits with negligible caloric cost. A typical medium onion contains just 40–50 calories, mostly from natural sugars and fiber, making it compatible with weight-loss, diabetic, and heart-healthy diets when used appropriately.

Key points to remember:

• Yellow, white, and red onions average 40–42 kcal per 100g; shallots and dried forms are more calorie-dense.
• Raw onions add crunch and bite; cooked onions build savory depth and sweetness.
• The real caloric risk comes not from the onion itself, but from added fats during cooking.
• Proper storage extends shelf life and maintains quality; freezing is viable for cooked or raw use in future dishes.
• Substitutions exist for allergies, sensitivities, and dietary restrictions—including low-FODMAP and keto adaptations.

Whether you're constructing a three-star reduction sauce or tossing together a weeknight stir-fry, understanding the caloric reality of onions empowers smarter, more flavorful cooking. Embrace them not as a hidden source of calories, but as a powerful ally in creating satisfying, nutritious meals with minimal energy input.

Next time you dice an onion, remember: you're adding flavor, not fat—and certainly not excess calories.