Onions are keto-friendly in small amounts. Raw onion has about 7.6g net carbs per 100g, but you rarely eat that much at once. A typical 1/4-cup (40g) of chopped onion is only ~3g net carbs, and a tablespoon is under 1g. Use onion as a flavoring rather than a main ingredient and it fits a keto day easily.

What kind of onion is lowest in carbs?

Green onions (scallions) and chives are the lowest-carb options, with about 4.7g net carbs per 100g and almost nothing in the small amounts you actually use. Leeks and shallots are higher, and sweet onions like Vidalia carry the most sugar.

Are caramelized onions keto?

Caramelized onions concentrate the natural sugars as the water cooks off, so a small pile can pack 8-12g net carbs from a much smaller starting volume. Keep portions to a tablespoon or two, and never add sugar while cooking.

Is Onions Keto? Carbs, Net Carbs & Verdict

Q: How many carbs are in onions?

Per 100g, raw onion has about 9.3g total carbs, 1.7g fiber, and 7.6g net carbs. A 1/4-cup of chopped onion (40g) has ~3g net carbs; a tablespoon (10g) has ~0.8g; a whole medium onion (~110g) has ~8.4g net carbs.

Are onions keto? Yes, in small amounts—raw onion has ~7.6g net carbs per 100g. See per-serving carbs, smart portions, and lower-carb swaps.

Yes, onions are keto-friendly—as long as you treat them as a flavoring, not a vegetable serving.

Raw onion has about 7.6g net carbs per 100g (9.3g total carbs minus 1.7g fiber), which sounds high for keto. But you almost never eat 100g of onion in one sitting. A realistic 1/4-cup of chopped onion (40g) is only ~3g net carbs, and a single tablespoon is under 1g. With a keto daily limit of 20-50g net carbs, onions fit comfortably when used for flavor. The verdict: keto-friendly in small portions, easy to overdo if you make onions the star of the dish or pile on sweet or caramelized varieties.

The trap with onions isn't the carb count per gram—it's the volume. A whole medium onion in one bowl of soup can quietly cost you 8g+ net carbs. Here's how the numbers break down by how you actually use them.

How many carbs are in onions?

Per 100g of raw mature onion (about the size of a small onion):

Total carbs: ~9.3g
Fiber: ~1.7g
Net carbs: ~7.6g
Sugar: ~4.2g
Protein: ~1.1g
Fat: ~0.1g

Most of those carbs are simple sugars, which is why onions taste mildly sweet and turn jammy when cooked. The fiber is modest, so the net-carb count stays close to the total. That makes portion size the single most important factor on keto.

How much onion can you eat on keto?

The honest answer: as much as you'd use as a seasoning, not as a side dish. Onion shines when it's diced into eggs, scattered over a salad, or sautéed into a base for meat and vegetables. In those roles you're using a tablespoon to a quarter-cup—a few grams of net carbs at most.

Where it adds up is when onion becomes a main ingredient: a French onion soup, a heaping plate of fajita onions, or onion rings (which add a carb-heavy breaded coating on top). At that point you can blow past 15g net carbs from onions alone.

Onion serving	Amount	Net carbs
1 tablespoon, chopped	~10g	~0.8g
2 tablespoons, chopped	~20g	~1.5g
1/4 cup, chopped	~40g	~3g
1/2 cup, chopped	~80g	~6g
Whole medium onion	~110g	~8.4g
100g raw onion	100g	~7.6g

A good rule: keep onion to 1/4 cup or less per meal and you'll rarely notice it on your macros.

Which onions are lowest in carbs?

Not all onions are equal. The greener and more pungent, the lower the sugar; the sweeter the variety, the higher the carbs.

Onion type	Net carbs per 100g	Best use on keto
Green onion / scallion	~4.7g	Best—garnish freely
Chives	~1.8g	Best—near-zero in real portions
White / yellow onion	~7.6g	Good—use 1/4 cup or less
Red onion	~7.3g	Good—small amounts raw
Shallot	~14g	Limit—use sparingly
Leek	~11g	Limit—small portions
Sweet onion (Vidalia)	~7-8g	Limit—highest sugar feel

Green onions and chives are the standout keto wins: you can sprinkle them on almost anything and the carbs stay negligible. Shallots and leeks, despite their reputation as "gourmet" alternatives, are actually higher in net carbs—use them with restraint.

Best low-carb alternatives to onions

When you want onion flavor without the carb creep, reach for these:

Swap	Net carbs	Why it works
Green onion tops	~0g in a garnish	Onion flavor, fraction of the sugar
Chives	~0g in a garnish	Bright, oniony, almost carb-free
Onion powder	~0.4g per 1/2 tsp	Big flavor, tiny portion
Garlic (1 clove)	~1g	Savory depth, different but compatible
Leek greens (small amount)	~1g	Mellow onion note for soups

Onion powder is the quiet hero here—half a teaspoon delivers a full hit of onion flavor for almost no carbs, which is why it's a staple in keto spice rubs and dressings.

Are cooked or caramelized onions still keto?

Cooking onions doesn't add carbs, but it changes the volume. As onions sauté, water cooks off and the pile shrinks, so the same net carbs end up concentrated in a smaller spoonful. Caramelizing takes this further: a generous mound of caramelized onions can hide 8-12g net carbs because it started as a full onion or two. Enjoy them, but measure—a tablespoon or two as a topping, not a heap. And never add sugar to the pan, which some recipes call for to speed up browning.

The bottom line

Onions are keto-friendly when used the way most recipes intend: as an aromatic, not a vegetable serving. A tablespoon to a quarter-cup of chopped onion costs you 1-3g net carbs—easy to fit in a 20-50g day. Lean on green onions, chives, and onion powder for nearly free flavor, and watch out for sweet varieties, caramelized piles, and onion-heavy dishes like soups and rings.

The smartest move is to log it so the small amounts don't sneak up across a day of cooking. Open CarbMeNot, search "onion," pick your exact portion, and you'll see your net carbs update in real time—no guessing whether that quarter-cup just used up your budget.