QUICK ANSWER
How many calories are in red onion?
Raw red onion provides 44 kcal per 100 g. Scale from the edible portion and keep other states separate.
44 kcal / 100 gNutrition values describe a reference food state, not every individual item. Variety, water content, preparation and portion weight can change the estimate.
THE NUMBERS
Red onion nutrition facts per 100 g
The values below use the reference state shown at the top of this page. Keeping one gram basis makes it easier to compare foods before translating the values to the portion you actually ate.
- Calories
- 44 kcal
- Protein
- 0.94 g
- Carbohydrate
- 9.93 g
- Fat
- 0.1 g
- Fibre
- 2.2 g
- Potassium
- 197 mg
- Vitamin C
- 8.1 mg
- Magnesium
- 11.4 mg
Source: USDA FoodData Central Foundation record 790577. Values may be rounded for readability.
SERVING SIZE
Calories by portion and serving weight
Words such as “small,” “medium,” “half” and “one piece” are convenient but imprecise. Grams make the estimate reproducible, particularly when natural foods vary in size.
MACROS
Carbohydrate, protein and fat explained
Protein
0.94 g per 100 g. From Onions, red, raw.
Carbohydrate
9.93 g per 100 g. From Onions, red, raw.
Fat
0.1 g per 100 g. From Onions, red, raw.
MORE THAN CALORIES
Fibre, vitamins and minerals
One food does not need to provide every nutrient. These are the characteristics most useful for understanding where red onion fits in a varied eating pattern.
Potassium
197 mg per 100 g in the selected USDA record.
Vitamin C
8.1 mg per 100 g in the selected USDA record.
Magnesium
11.4 mg per 100 g in the selected USDA record.
Manganese
0.119 mg per 100 g in the selected USDA record.
FOOD STATE MATTERS
How preparation changes the nutrition estimate
A reliable calorie entry should describe the food you actually ate—not merely a food with a similar name.
Red, raw
Selected reference.
White, yellow or sweet
Mapped as separate raw varieties.
Boiled or sautéed
Use the matching cooked record and include cooking fat.
Frozen, canned or dehydrated
Match the processed form rather than raw red onion.
PRACTICAL CONTEXT
How red onion can fit different goals
There is no universal “good” or “bad” food. Portion, frequency, the rest of the meal, preferences and individual health needs all matter.
Portion math
At 44 kcal per 100 g, the 50 g example is 22 kcal.
Fibre check
The selected record lists 2.2 g fibre per 100 g.
Protein check
The selected record lists 0.94 g protein per 100 g.
Best logging move
Choose the onion variety and cooked state first, then add oil, sugar, sauce or recipe ingredients separately.
QUALITY & SAFETY
How to choose, store and prepare it
Store
Keep whole dry onions in a cool, dry, ventilated place.
After opening or cooking
Keep cut onion covered and chilled.
Prepare
Use a clean knife and board, especially when onion will be eaten raw.
Safety boundary
Discard onion with mould, leaking decay or a strong spoilage odour; the nutrient record does not set storage duration.
PHOTO LOGGING
How to log red onion more accurately
A food photo can suggest the ingredient, but it cannot always reveal exact weight, hidden oil or preparation details. These checks turn an image guess into a better estimate.
- 01
Match the exact reference state: Onions, red, raw.
- 02
Use edible grams. Calories are scaled from the selected per-100-g record; named portion weights come from the same local USDA record.
- 03
Log oil, sauce, sweetener, salt and recipe ingredients separately.
- 04
Use a separate record when preparation changes the food state.
COMMON QUESTIONS
Red onion calorie and nutrition questions
How many calories are in 100 g of onion?
44 kcal in the selected Onions, red, raw record.
How much fibre is in onion?
2.2 g per 100 g in the selected record.
Which onion state does this page use?
Onions, red, raw. Preparation and edible state are part of the answer.
Can cooked onion use the raw red-onion entry?
No. Boiled and sautéed onions are mapped separately, and cooking fat must also be counted.
How are the portion calories calculated?
Each portion is scaled from 44 kcal per 100 g. Named portion weights come from the selected source record.
KEEP EXPLORING
Related food guides
SOURCES & METHOD
Where these numbers come from
Onions, red, raw, per 100 g edible portion. Numeric nutrition and named-portion claims use the selected local USDA record. Preparation notes cover the common USDA forms of this food. The nutrient record does not establish storage duration or food-safety limits; use package directions and current local guidance where relevant.
Open USDA FoodData Central Foundation record 790577 ↗Page reviewed 26 August 2026. We show the food state, gram basis and source record so readers can check the estimate instead of treating one number as universally exact.