Onion Calories & Nutrition Calculator

Onion is one of the richest common dietary sources of quercetin, a flavonoid present at approximately 10–25mg per 100g in raw onion (varying by variety and growing conditions). Red onions contain 2–3x more quercetin than yellow onions, and yellow onions contain significantly more than white onions. The outermost 2–3 layers of any onion concentrate the highest quercetin levels ^[1][3].

Quercetin is not captured in USDA's standard nutrient panel, making it invisible in typical nutrition databases. However, research consistently identifies onion as the single largest quercetin contributor in most diets worldwide. The compound is heat-stable relative to other flavonoids — sautéing retains approximately 75–80% of quercetin, while boiling in water leaches more (use the cooking water in soups to retain it).

For food journaling, standard calorie trackers won't show quercetin. If tracking phytonutrient diversity, note onion as a unique quercetin source. Peeling away too many outer layers removes the most quercetin-rich tissue. Use the outer rings in cooking rather than discarding them.

Raw onion has 4.24g sugar per 100g — higher than tomatoes (2.6g), bell peppers (2.4g), carrots (4.7g), and significantly higher than leafy greens (<1g). The sugar is predominantly glucose and fructose in roughly equal proportions. This natural sugar content is why caramelized onions (cooked slowly until sugars undergo Maillard browning) taste intensely sweet ^[1].

Cooking increases the perceived sweetness even more: boiled onion has 4.73g sugar per 100g (an 11% concentration effect from water loss), and caramelized onion can reach 8–10g sugar per 100g as water evaporates and sugars concentrate. A tablespoon of caramelized onion contributes approximately 10–12 kcal — nearly all from sugar.

For food journaling, raw onion's sugar content is nutritionally modest in typical salad quantities (20–30g). But when onion is the primary ingredient — like in a French onion soup (using 300g+ onion) or onion-heavy gravy — the sugar contribution becomes substantial: 300g onion = 12.7g sugar.

Boiled onion has 44 kcal per 100g — a 10% increase over raw onion's 40 kcal. This is unusual (most vegetables lose calories when boiled) and occurs because onion's water content drops slightly (89.1% → 87.9%) while carbohydrate concentration increases. Potassium also rises from 146mg to 166mg (14% increase) — another reversal of the typical boiling pattern ^[1][2].

Vitamin C follows the expected pattern: 7.4mg raw drops to 5.2mg cooked (a 30% loss). Folate drops from 19mcg to 15mcg (21% loss). Fiber decreases from 1.7g to 1.4g. So cooking onion concentrates energy and some minerals while depleting heat-sensitive vitamins.

For food journaling, the calorie difference is small (4 kcal/100g) but directionally unusual. If precision matters, use 40 kcal for raw onion (salads, garnishes) and 44 kcal for cooked onion (sautéed, in gravies). For sautéed onion, the added oil typically contributes far more calories than the onion itself.

A standard Indian curry gravy starts with 1–2 medium onions (110–220g), contributing 44–88 kcal before oil, tomatoes, or spices are added. Most home cooks and food journals track oil and protein but overlook onion — treating it as calorie-free seasoning. Across 3 meals a day with onion-based cooking, untracked onion can add 60–150 kcal daily ^[1].

The onion contribution per dish: raw salad garnish (20–30g: 8–12 kcal, negligible), tadka/tempering (30–50g: 12–20 kcal, minor), curry base (100–150g: 40–60 kcal, meaningful), onion-heavy dishes like do pyaza or French onion soup (200–300g: 80–120 kcal, significant).

For food journaling, onion falls in the gray zone — too high in calories to ignore completely (unlike cucumber or lettuce at 15 kcal/100g) but too low to obsess over. The practical rule: track onion when using more than 100g per dish. Below 100g, the onion contributes 40 kcal or less — within the rounding error of most food logs.

All onion varieties — red, yellow, and white — have virtually identical macronutrient profiles: 40 kcal, 9.3g carbs, 1.1g protein, 0.1g fat per 100g. USDA does not maintain separate entries for each variety because the calorie, protein, carb, and fat values are statistically indistinguishable. The differences are entirely in phytonutrients ^[1][3].

Red onions are richest in quercetin (20–25mg/100g) and anthocyanins (the pigments responsible for their color). Yellow onions have moderate quercetin (10–15mg/100g) and are the most commonly used cooking variety worldwide. White onions have the least quercetin (<10mg/100g) but the mildest flavor, preferred for raw applications.

For food journaling, use the same calorie and macronutrient values regardless of onion color. If phytonutrient tracking matters, note the variety: red for maximum flavonoid content, yellow for balanced flavor and phytonutrients, white for mild raw applications. The calorie counter does not need to distinguish between them.