How many calories are in an onion?

A medium onion (approximately 150g) contains about 60 kcal. Per 100g, onions provide roughly 40 kcal. This is very low — onions are mostly water (around 89%). Since onions are typically used as a flavour base rather than eaten as a main ingredient, their caloric contribution to a dish is usually negligible. Red, brown, and white onions have virtually identical calorie counts.

What is the difference between red, brown, and white onions?

Nutritionally, all onion varieties are very similar at around 39-41 kcal per 100g with comparable sugar, fibre, and protein content. The differences are in flavour and best use: brown onions are the all-purpose cooking onion with a balanced flavour that sweetens when cooked; red onions are slightly milder and best raw in salads or sandwiches; white onions have a sharper, cleaner bite suited to Mexican and Asian dishes; spring onions are the mildest and used raw as a garnish or in stir-fries.

Are onions high in sugar?

Onions contain about 5-6g of natural sugars per 100g, which sounds moderate but is rarely a concern in practice. Most recipes use half an onion or less, spread across multiple servings. The sugars are what caramelise when onions are slowly cooked, producing that sweet, golden flavour. With a glycaemic index of only 10, onions have virtually no meaningful impact on blood sugar levels.

Glucose: Very LowNOVA 1

Nutritional Information for Onions

Onions are a low-calorie kitchen staple at around 40 kcal per 100g, used as a flavour base in countless dishes. They have a very low glycaemic index of approximately 10 and minimal impact on blood sugar. Onions come in several varieties — brown (most common, all-purpose), red (slightly milder, good raw in salads), white (sharper, crisp), and spring onions (mild, eaten raw) — all with very similar nutritional profiles. All raw onions are NOVA 1. The sugars in onions (about 5-6g per 100g) are naturally occurring and caramelise when cooked, but the amounts used in typical recipes are too small to cause glucose concern.

Nutrition per 100g

Nutrient	Per 100g	Per serving
Energy	40 kcal / 168 kJ	60 kcal
Protein	1.1g	1.7g
Carbohydrates	8g	12g
of which sugars	5.9g	—
Fat	0.1g	0.2g
of which saturated	0g	—
Fibre	1.2g	—
Salt	0.01g	—

Serving size: 1 medium onion (~150g) · Source: Tesco product page, October 2025

Glucose impact

Glycemic Index

Very Low

Glucose risk

Onions have a very low glycaemic index of approximately 10 and contain only 8g of carbohydrates per 100g. In practical cooking, a typical recipe might use half a medium onion (75g), contributing just 6g of carbs spread across multiple servings. Even eating a whole raw onion would produce minimal blood sugar response. The fibre content and low carb density make onions one of the safest vegetables from a glucose perspective.

Processing level

NOVA 1 (unprocessed)

Whole raw onions of any variety are NOVA 1 — unprocessed single-ingredient vegetables. Pre-sliced onions (like Tesco Sliced Red Onion) remain NOVA 1 as slicing is minimal physical processing with no additives. Pickled onions move to NOVA 3 (processed) due to vinegar and salt. Onion-based products like stuffing, onion rings, or onion bhajis are NOVA 3-4 depending on their ingredient lists.

Onions at Tesco

Source: Tesco product page, October 2025

Tesco Brown Onions 1Kg

£0.99*NOVA 1 (unprocessed)

Tesco Red Onions Loose

£0.20*NOVA 1 (unprocessed)

Tesco Red Onions 3Pack Minimum

£0.95*NOVA 1 (unprocessed)

Tesco Sliced Red Onion 180G

£0.90*NOVA 1 (unprocessed)

Tesco Finest Wild Sage & Buttered Onion Stuffing 225G

3 UPF markers

£3.30*NOVA 4 (ultra-processed)

*Prices were correct at the time of collection and may have since changed. Check Tesco.com for current pricing.

What to watch for when shopping

Raw whole onions are always NOVA 1 regardless of variety. Pre-sliced onions are also NOVA 1 but cost more per kg for the convenience. The danger zone is onion-based prepared foods: stuffings, onion rings, onion bhajis, and caramelised onion chutneys often contain modified starch, sugar, and flavourings pushing them to NOVA 3-4. Budget tip: Tesco Brown Onions 1Kg at £0.99 is the best value — brown onions are the most versatile variety and store well for weeks in a cool, dark place.

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