How many calories in
onion?
A medium onion is 44 calories — slightly higher than other vegetables because of the natural sugars that caramelize when sautéed. Caramelized onions concentrate the calories but the cooked weight matters; it's still mostly a flavor base with minor calorie impact.
Common serving sizes.
Calories scale with portion. Here's how a typical onion serving translates in your tracker:
What 40 calories means.
Very low calorie — at 40 kcal per 100g, onion adds volume to a meal without meaningfully denting a calorie deficit.
Onion vs. similar foods.
How 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 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 onion?
Onion has approximately 40 calories per 100g. A typical serving — 1 medium (110g) — comes in at 44 calories.
How much protein is in onion?
1.1g of protein per 100g. That puts onion at roughly 2.8g of protein per 100 calories — a useful lens when comparing to other vegetables sources.
How do I log 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 onion good for weight loss?
It depends on your overall calorie target. Onion is naturally low-calorie at 40 kcal per 100g, so it's easy to fit into a deficit.