A raw medium-sized onion (about 150 grams) contains roughly 60–70 calories. These calories come almost entirely from natural carbohydrates, including sugars and fibre. Onions are low in fat, contain minimal protein, and have a relatively high water content, making them a low-calorie food that adds flavour and depth to meals without significantly increasing calorie count.

A small onion (around 70–80g) has about 30–35 calories, while a large onion (up to 200g) can have 80–90 calories.

Calories by Onion Type

The calorie count doesn't vary much between onion varieties, but here's a rough idea per 100g:

  • Brown/yellow onions: ~40 calories
  • Red onions: ~40–45 calories
  • White onions: ~40 calories
  • Shallots: ~70–75 calories (denser and smaller)
  • Spring onions (scallions): ~30 calories per 100g (or around 5 calories per stalk)

Red onions may contain slightly more sugar, but the difference in calories is minimal and unlikely to affect your diet unless you're eating large quantities.

Cooked vs Raw: Do Calories Change?

The calorie count of onions doesn't change during cooking, but what you cook them with makes a big difference. Onions sautéed in olive oil or butter will absorb fat, adding extra calories. A tablespoon of oil, for example, adds about 120 calories, which could turn a low-calorie dish into a much heavier one.

Caramelising onions takes time and often uses butter, oil, or sugar all of which significantly raise calorie content. If you're calorie-conscious, dry frying or lightly steaming onions is a better option.

Nutritional Value of Onions

Onions are more than just low-calorie flavour boosters. They’re rich in antioxidants, particularly quercetin, which may help reduce inflammation and support heart health. They also contain vitamin C, B6, potassium, and fibre, especially in raw form. Even though the calories are low, onions add nutritional value to meals in meaningful ways.

Their natural prebiotic fibres also support gut health, feeding beneficial bacteria in the digestive system.

Do Onions Affect Blood Sugar?

Onions have a low glycaemic index (GI), usually around 10–15, meaning they have little impact on blood sugar when eaten in moderate portions. Even though they contain natural sugars, the fibre and water content help slow digestion, making them safe for people managing diabetes or insulin sensitivity.

If you're watching carbs, onions are still a smart choice. One medium onion contains roughly 14–15 grams of total carbs, but only around 6–7g of net carbs, making them suitable for most low-carb or keto-friendly diets when used sparingly.

Caramelised Onions Can Be a Hidden Calorie Trap

While raw onions are very low in calories, caramelised onions often used in burgers, tarts, or as toppings can carry two to three times the calories due to added oil, butter, or sugar. For example, one medium onion sautéed in a tablespoon of olive oil instantly jumps from 60–70 calories to over 180–200. If sugar is added to help with caramelisation, that adds even more.

This doesn’t mean you need to avoid them, just be mindful. Caramelised onions can be delicious, but they’re not calorie-free garnish.

Onions Shrink When Cooked — Calorie Density Increases

When onions are sautéed or roasted, they lose water and shrink in size. This means the same weight of cooked onion will look like a smaller portion but still contains the same number of calories. This is important for portion control: a spoonful of cooked onion contains more calories than a spoonful of raw onion simply because it’s more compact and concentrated.

In recipes, especially when mixing raw and cooked ingredients, remember that volume ≠ calories when water content changes.

Pickled Onions Are Still Low in Calories — But Watch the Sugar

Pickled onions are another way onions show up on the plate in salads, sandwiches, or cheese boards. These are still very low in calories (roughly 10–20 calories per 30g serving), but if the pickling brine contains sugar, it can add a few grams of carbs. That said, most commercial pickled onions are still light and not a major calorie source, unless heavily sweetened.

Always check the label if you’re tracking sugar intake or managing carbs tightly.

Onions as a Base Ingredient: Small Calories, Big Impact

In many recipes soups, sauces, stir-fries and onions are used as a base, sautéed with garlic and herbs. Even though you might only use half an onion (30–40 calories), they bring major flavour, allowing you to reduce or eliminate higher-calorie flavour boosters like heavy sauces, extra cheese, or creamy dressings.

This makes onions a smart choice for flavourful, lower-calorie meals, especially when used with other aromatics like celery, carrot, ginger, or chilli.

Onions Work Well in Low-Calorie Swaps

You can use onions to bulk out or replace higher-calorie ingredients. Finely chopped onion mixed with lean mince can stretch out burgers or meatballs with fewer calories. Roasted onion rings can be a lighter side dish than chips. Even red onion in salads can replace croutons or cheese for texture and flavour without the added fat.

If you’re building low-calorie meals or doing recipe swaps, onions are an easy way to add volume and taste for almost no extra energy.

Summary

A medium onion contains about 60–70 calories, while a small one contains around 30–35. Onions are low in fat and rich in nutrients, offering flavour and fibre without pushing your calorie count too high. While cooking methods can change the total energy content of a dish, the onion itself remains a low-calorie, nutrient-rich ingredient that supports flavourful, healthy eating.

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