How many calories in 1/3 cup diced red onion?

November 23, 2025 •

3 min read

According to nutrition data, a 1/3 cup of chopped red onion contains approximately 22 calories, making it a very low-calorie and flavorful addition to any meal. This nutrient-dense vegetable adds flavor without significantly increasing the caloric content of your dishes.

Quick Summary

A 1/3 cup of diced raw red onion contains about 22 calories, with macros mainly from carbs and negligible fat. This serving also provides beneficial vitamins, minerals, and potent antioxidants like anthocyanins and quercetin.

Key Points

Low Calorie: A 1/3 cup of diced red onion contains approximately 22 calories.
Nutrient-Dense: Despite its low-calorie count, red onion is rich in essential vitamins like C and B6, and minerals such as potassium.
High in Antioxidants: Red onions contain higher levels of powerful antioxidants like anthocyanins and quercetin compared to white or yellow onions.
Supports Heart Health: The antioxidants in red onion have been linked to potential benefits for heart health and blood pressure regulation.
Versatile Ingredient: Enjoy red onions raw in salads and salsa, or cooked in dishes like stir-fries and roasts, with raw consumption offering maximum nutritional benefits.
Source of Prebiotic Fiber: Red onions contain prebiotic fiber, which helps feed beneficial bacteria in the gut, supporting digestive health.

The Nutritional Profile of Red Onion

Beyond its minimal calorie count, the red onion is a nutritional powerhouse packed with vitamins, minerals, and health-promoting antioxidants. Its composition makes it a valuable addition to a balanced diet, offering flavor and health benefits in every serving.

Macronutrient Breakdown for 1/3 Cup Diced Red Onion

The 22 calories found in a 1/3 cup serving of diced red onion come almost entirely from its carbohydrate content. The macronutrient distribution is highly favorable for those managing their weight or simply eating healthy.

Carbohydrates: Approximately 5 grams, with 1 gram of this being dietary fiber. The remaining net carbs are derived from natural sugars, primarily glucose and fructose.
Protein: Around 0.3 to 1 gram. While not a significant protein source, it contributes to the overall nutritional value.
Fat: Less than 0.1 gram. The virtually fat-free nature of red onion makes it an excellent ingredient for low-fat cooking.

Rich in Vitamins and Minerals

Red onions are a decent source of several key vitamins and minerals. The vibrant color isn't just for aesthetics; it's a sign of the vegetable's potent nutritional content. A 1/3 cup serving contributes to your daily intake of:

Vitamin C: An antioxidant that supports immune function, skin health, and collagen production.
Vitamin B6: Essential for metabolism and the creation of red blood cells.
Folate (B9): Important for cell growth and metabolism, especially crucial for pregnant women.
Potassium: A mineral that helps maintain fluid balance, nerve transmission, and healthy blood pressure.

Antioxidants: The Red Onion's Secret Weapon

What truly sets red onions apart from their yellow and white counterparts is their higher concentration of certain antioxidant compounds.

Anthocyanins

These are powerful antioxidant pigments that give red and purple onions their deep color. Anthocyanins have been linked to a lower risk of heart disease and may offer protection against some forms of cancer.

Quercetin

Red onions are particularly rich in quercetin, a flavonoid with potent anti-inflammatory properties. Quercetin has been shown in studies to potentially help lower blood pressure and improve overall heart health. Cooking can reduce some antioxidant levels, so eating raw red onion maximizes these benefits.

Red Onion vs. Other Onion Varieties

While all onions offer health benefits, red onions stand out for their high antioxidant content. The table below compares the key nutritional differences (per 1/3 cup, raw).


Feature	Red Onion	Yellow Onion	White Onion
Calories	~22 kcal	~22 kcal	~22 kcal
Carbohydrates	~5 g	~5 g	~5 g
Antioxidant Content	Higher (rich in anthocyanins and quercetin)	Lower than red, but more than white	Lowest of the three common varieties
Flavor Profile	Mild, sweet, and crisp	Pungent, slightly sweet when cooked	Sharp, slightly milder than yellow

Culinary Uses and Preparation Tips

The mild, sweet flavor of red onion makes it versatile for a wide range of dishes. To get the maximum health benefits, it's best to eat them raw when possible, as cooking can diminish certain nutrients.

Raw Applications:

Salads: Thinly slice and add to any fresh salad for color, flavor, and crunch.
Salsas and Guacamole: Finely dice and mix into fresh salsa or guacamole.
Sandwiches and Burgers: Add a few raw rings to your favorite sandwich or burger for extra zest.
Pickled Onions: Quickly pickle slices in vinegar, sugar, and salt for a tangy condiment on tacos or sandwiches.

Cooked Applications:

Roasting: Chop into chunks and roast with other vegetables for a mellow, sweet flavor.
Sautéing: Use as a flavorful base for sauces, stews, and stir-fries.
Caramelizing: Slowly cook over low heat until deeply browned and sweet, perfect for topping focaccia or pairing with meats.

Considerations for Digestive Sensitivity

Red onions contain fructans, a type of carbohydrate that can cause digestive issues for some individuals, particularly those with Irritable Bowel Syndrome (IBS). If you have a known sensitivity to FODMAPs, you may need to limit your intake of raw red onion. Cooking can sometimes make them more digestible, but individual tolerance varies.

Conclusion

For a small caloric investment of around 22 calories per 1/3 cup, diced red onion delivers a significant amount of flavor and a concentrated dose of valuable nutrients. Its high antioxidant content, notably anthocyanins and quercetin, offers powerful anti-inflammatory and heart-health benefits. Whether enjoyed raw for its crisp texture and maximum nutrient retention or cooked to enhance its sweetness, the red onion is a healthy and versatile kitchen staple that can help you achieve your dietary goals. For more in-depth nutritional information, authoritative sources like the USDA's SNAP-Ed Connection are a great resource.

Frequently Asked Questions

Cooking red onion does not significantly alter its own calorie count, but adding oil or other fats during cooking will increase the total calories of the dish.

While nutritionally similar in basic macros and calories, red onions are richer in certain antioxidants like anthocyanins and quercetin, making them a more antioxidant-dense choice.

Yes, a 1/3 cup serving provides about 1 gram of dietary fiber, including prebiotics that are good for gut health.

While generally safe, consuming large quantities of raw red onion may cause digestive discomfort, gas, or bloating in some sensitive individuals due to its fructan content.

To minimize eye irritation, try chilling the onion in the refrigerator for about 30 minutes before chopping or cut it under running water.

Eating red onions raw preserves the maximum amount of beneficial sulfur compounds and water-soluble vitamins, which can be diminished by the cooking process.

While 'chopped' can refer to a variety of sizes, 'diced' implies a more uniform, smaller cube size. For calorie counting purposes, the volume (1/3 cup) is the determining factor, so the calorie count remains the same.