Calories in Onion

Q: How much sugar does onion have?

1 slice of onion (15 g) has 0.87 g of sugar, and 100 g has 5.8 g.

Q: How many calories in each size of onion?

It depends on the size: slice (14 g) about 5 kcal; small (70 g) about 27 kcal; medium (110 g) about 42 kcal; large (150 g) about 57 kcal.

Q: How many calories in raw and cooked onion?

It depends on how it is prepared. Per 100 g: raw 38 kcal, cooked, fat added 73 kcal, cooked, no added fat 47 kcal. This page shows onion (raw).

Q: Does onion contain flavonoids?

Yes. 100 g of onion has 26 mg of flavonoids, mostly flavonols.

Q: Is onion low in calories?

Yes. 100 g of onion has only 38 kcal.

Q: Will eating onion help me lose weight?

It can be part of a diet that loses weight: what counts is the total calories you eat in a day, not any single food. 1 slice of onion (15 g) has about 6 kcal. The calorie calculator works out how many you need.

100 g of onion has 38 kcal, and 1 slice of onion (15 g) has about 6 kcal.

38 kcalper 100 g

6 kcal in 1 slice (15 g)

Very low density · 0.38 kcal per g

Protein 0.9g 9% of the calories
Carbs 8.5g 89% of the calories
Fat 0.1g 2% of the calories
Fiber 1.7g

Per 100 g. Figures from USDA FoodData Central. Averages across measured samples; a few percent either way is normal. Updated 2026-10-04.

100 g is about 9 minutes of walking, or 3 minutes of running, for a 70 kg body - context, not a debt to work off.

Onion nutrition facts

A dash means nobody measured it - not that there is none of it.

Per serving: 1 slice (15 g)
Nutrient	Per 100 g	Per serving
Energy	38kcal	6kcal
Fat	0.08g	0.012g
Saturated Fat	0.04g	0.006g
Unsaturated Fat	0.04g	0.006g
Carbs	8.5g	1.3g
Sugars	5.8g	0.87g
Fiber	1.7g	0.26g
Protein	0.86g	0.13g
Salt	0g	0g
Water	90.3g	13.5g

Alcohol	0g	0g
Caffeine	0mg	0mg

Monounsaturated	0.013g	0.002g
Polyunsaturated	0.017g	0.003g
Omega-3	0.004g	0.001g
ALA	0.004g	0.001g
DHA	0g	0g
EPA	0g	0g
Omega-6	0.013g	0.002g
AA	0g	0g
LA	0.013g	0.002g
Cholesterol	0mg	0mg

Vitamin B1 (Thiamine)	0.046mg	0.007mg
Vitamin B2 (Riboflavin)	0.027mg	0.004mg
Vitamin B3 (Niacin)	0.12mg	0.017mg
Vitamin B6 (Pyridoxine)	0.12mg	0.018mg
Vitamin B12 (Cobalamin)	0µg	0µg
Folate (Vitamin B9)	19µg	2.9µg
Vitamin A	0µg	0µg
Vitamin C	8.2mg	1.2mg
Vitamin D	0IU	0IU
Vitamin E	0.02mg	0.003mg
Vitamin K	0.4µg	0.06µg
Choline	6.1mg	0.91mg

Calcium	17mg	2.5mg
Copper	0.041mg	0.006mg
Iron	0.24mg	0.036mg
Magnesium	9mg	1.3mg
Phosphorus	33mg	5mg
Potassium	171mg	25.6mg
Selenium	0µg	0µg
Zinc	0.17mg	0.026mg

Beta-carotene	1µg	0.15µg
Lycopene	0µg	0µg
Lutein + zeaxanthin	4µg	0.6µg
Flavonoids	26mg	3.9mg
Flavones	0.03mg	0.004mg
Flavonols	26mg	3.9mg

Read it as an averageIt is what a laboratory measured, averaged over samples - variety, ripeness and cut all move it, and a few percent either way is normal.

What stands out in onion

1 slice (15 g) is 6 kcal, about <1% of a 2,000 kcal day.

Medium in sugars5.8 g per 100 g (6% of the daily reference)
Low in calories38 kcal per 100 g
Low in fat0.08 g per 100 g
Low in saturated fat0.04 g per 100 g
Low in salt0 g per 100 g

Claims follow the EU nutrition claims rules and the "high" warnings the UK/EU front-of-pack labels; the 2,000 kcal day is the reference intake for an average adult.

Calories in different amounts of onion

15 g1 slice	6 kcal
30 g2 servings	11 kcal
50 g	19 kcal
100 g	38 kcal
200 g	76 kcal

Any amount g 6 kcal

Foods with similar calories

Other published foods within reach of 38 kcal per 100 g, so the number has something to sit against.

Broccoli 7 kcal less 31kcal / 100 g

Spinach 11 kcal less 27kcal / 100 g

Green beans 7 kcal less 31kcal / 100 g

Tofu 23 kcal more 61kcal / 100 g

Carrots 7 kcal more 45kcal / 100 g

Cauliflower 15 kcal less 23kcal / 100 g

Questions about onion

1 slice of onion (15 g) has 0.87 g of sugar, and 100 g has 5.8 g.

It depends on the size: slice (14 g) about 5 kcal; small (70 g) about 27 kcal; medium (110 g) about 42 kcal; large (150 g) about 57 kcal.

It depends on how it is prepared. Per 100 g: raw 38 kcal, cooked, fat added 73 kcal, cooked, no added fat 47 kcal. This page shows onion (raw).

Yes. 100 g of onion has 26 mg of flavonoids, mostly flavonols.

Yes. 100 g of onion has only 38 kcal.

It can be part of a diet that loses weight: what counts is the total calories you eat in a day, not any single food. 1 slice of onion (15 g) has about 6 kcal. The calorie calculator works out how many you need.

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