How many calories are in an onion?
A medium onion — about 110 g — is roughly 44 kcal. Raw onion runs about 40 kcal per 100 g. Hardly anyone eats it raw in quantity, which makes this one of the few foods where the honest answer is a question about the pan rather than the vegetable.
Per portion
| Portion | Calories | Protein | Carbs | Fat |
|---|---|---|---|---|
| 1 medium onion, raw (110 g) | ~44 kcal | 1.2 g | 10 g | 0.1 g |
| 100 g raw, chopped | ~40 kcal | 1.1 g | 9.3 g | 0.1 g |
| 100 g boiled or steamed | ~44 kcal | 1.4 g | 10 g | 0.2 g |
| 2 onions sautéed in 1 tbsp oil | ~210 kcal | 2.4 g | 20 g | 14 g |
| 100 g caramelised (from ~250 g raw, 1 tbsp oil) | ~220 kcal | 2.8 g | 23 g | 14 g |
| 20 g birista / fried onion garnish | ~90 kcal | 0.6 g | 6 g | 7 g |
| 1 onion bhaji (60 g) | ~170 kcal | 3 g | 14 g | 11 g |
| 100 g battered onion rings | ~370 kcal | 5 g | 40 g | 21 g |
| 50 g spring onions | ~16 kcal | 0.9 g | 3.7 g | 0.1 g |
What changes the number
Oil absorption, and nothing else comes close. Onion is about 89 per cent water. As that water drives off in a hot pan, oil moves into the space it leaves. Thin slices fried to a deep brown birista can take up 8–10 g of oil per 100 g of raw onion, which is why the finished garnish runs 400–500 kcal per 100 g — most of it fat, not onion.
The base, which is where most onions in the world go. Two onions softened in three tablespoons of oil, usually with tomato following them into the pan, is about 90 kcal of onion and 360 kcal of oil. Across four servings that is roughly 110 kcal a plate, of which 20 is the vegetable. Cutting the oil to one tablespoon saves about 240 kcal from the pot without changing a single thing anyone tastes, which makes it the best-value edit in most deficit cooking.
Boiling and steaming add nothing. Onion in a dal, a soup or a stew that was never fried is simply the raw figure. Deep-frying is the other end: bhajis and rings are batter and oil with onion inside them.
Variety, barely. Red, white and yellow onions sit within a few kilocalories. Shallots are a little higher, near 70 kcal per 100 g; spring onions are lower, near 32. Size does more than type: a “medium” onion runs 90–150 g, so one onion is 36–60 kcal.
Where it fits in a deficit
Onion is a flavour tool, not a volume food. At 40 kcal and 1.7 g of fibre per 100 g it is too dense to eat for bulk the way you would eat broccoli, and too light to matter if you do. It will never be the reason a day goes over.
The leverage is entirely in the medium. The same dish can land at 350 kcal or 600 depending on how generous the base was, and that gap is the difference between holding a calorie deficit and quietly missing it. Onions themselves belong in the free column alongside the rest of the high-volume staples.
One honest note that is not about calories: onions are high in fructans, and a proportion of people bloat badly on them. Spring onion greens are usually better tolerated.
Caramelising does not add sugar
It converts what was already there. Raw onion holds about 4.2 g of sugar per 100 g, much of it locked up in fructans — chains of fructose that taste of nothing. Long slow cooking breaks those chains into simpler sugars and drives off water, so the sweetness concentrates into a smaller mass. Browning adds flavour compounds, not energy.
So the carbohydrate in the pan does not rise. Sweetness is not a measure of sugar content, and the calories in a jar of caramelised onions arrived with the oil, the butter, and in many recipes a spoon of added sugar or balsamic that the label will list separately.
Figures are typical values and vary by variety, size and preparation. General information, not medical advice.