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 AI estimates the portion and macros in a few seconds. Check it, adjust anything that looks off, and log it.
Download on theApp StoreQuick answers.
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 a few seconds. 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.