How many calories in
red onion?
Per 100g, red onion brings 40 calories with 9.3g of carbs (most of which is fiber). Worth portioning if you treat it like a free veg; it has more density than spinach or cucumber.
Common serving sizes.
Calories scale with portion. Here's how a typical red onion serving translates in your tracker:
What 40 calories means.
Very low calorie — at 40 kcal per 100g, red onion adds volume to a meal without meaningfully denting a calorie deficit.
Red Onion vs. similar foods.
How red onion stacks up on calories and protein against foods people commonly compare it to:
Skip the lookup. Snap the plate.
Point your iPhone camera at red onion and Spotwell's on-device AI estimates the portion and macros in under a second — no manual entry, no barcode hunting.
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How many calories in red onion?
Red Onion has approximately 40 calories per 100g. A typical serving — 3 oz (85g) — comes in at 34 calories.
How much protein is in red onion?
1.1g of protein per 100g. That puts red onion at roughly 2.8g of protein per 100 calories — a useful lens when comparing to other vegetables sources.
How do I log red onion accurately?
The simplest method: weigh it on a kitchen scale, then enter grams in your tracker. With Spotwell, you can skip the scale — point the camera at the plate and the AI estimates the portion in under a second. Adjust the value if it looks off.
Is red onion good for weight loss?
It depends on your overall calorie target. Red Onion is naturally low-calorie at 40 kcal per 100g, so it's easy to fit into a deficit.