In this article
- Do onions have fiber? The numbers by size and form
- Myth vs fact: cooking onions destroys their fiber
- How onions compare with other vegetables with fiber
- What else a medium onion brings
- Onions, fiber and a sensitive gut
- One pot of soup, and where the fiber really comes from
- One easy change to make tonight
- Onion fiber questions people ask
- Sources
To reach the 28 gram Daily Value for fiber from onions alone, you’d have to eat about 15 medium onions. That number says a lot about where onions belong in a high fiber diet.
So do onions have fiber? They do, about 1.9 grams in a medium raw onion, according to USDA. Onions are a flavor base that adds a little fiber to nearly every savory dish, rather than a main source on their own.
Do onions have fiber? The numbers by size and form
Mayo Clinic defines dietary fiber as the parts of plant foods your body can’t digest or absorb. Onions are plants, so they carry some, mixed into a bulb that is about 89% water by weight.
USDA FoodData Central and the FDA’s raw vegetable label give these figures:
| Onion | Amount | Fiber | Calories |
|---|---|---|---|
| Raw onion | 100 g | 1.7 g | 40 |
| Raw onion, medium | 110 g | 1.9 g | 44 |
| Raw onion, FDA label serving | 1 medium (148 g) | 3 g (12% DV) | 45 |
| Boiled onion, medium | 94 g | 1.3 g | 41 |
| Boiled onions, chopped | 1 cup | 2.9 g | 92 |
| Green onions (scallions), chopped | 1 cup (100 g) | 2.6 g | 32 |
The FDA serving is a larger onion than USDA’s medium, which is why its label shows 3 grams. Either way, a whole onion lands somewhere between about 7% and 12% of the Daily Value.
Most recipes don’t use a whole onion per person, either. Half a medium onion split across two servings gives each plate less than half a gram.
How much onion actually lands on your plate
Onion portions are usually smaller than they look in a recipe. A quarter of a medium onion, about 28 grams, is a generous amount for one burger or one bowl of chili, and it brings roughly half a gram of fiber.
That’s why onion counts tend to vanish when you log a meal. The fiber is real, but it rarely reaches a full gram per serving unless onions are a main ingredient, as in an onion soup or a tray of roasted onion wedges.
Fiber in onions next to green onions
Gram for gram, green onions carry more fiber than bulb onions: 2.6 grams per 100 grams against 1.7, with fewer calories. A cup of chopped scallions weighs about 100 grams, so it’s an easy amount to scatter over soups, eggs and grain bowls through a week.
They’re also easy to use raw. Slice them thin over tacos, fold them into a bean salad or stir them into a pot of rice at the last minute, so the bright green tops keep their color and crunch.
Green onions also bring a lot of vitamin K. That same cup has 207 micrograms, about 173% of the Daily Value, plus 19 milligrams of vitamin C, roughly 21%.
Myth vs fact: cooking onions destroys their fiber
Myth: Once onions go soft in the pan, most of their fiber is gone.
Fact: USDA lists boiled onions at 1.4 grams of fiber per 100 grams, close to the 1.7 grams in raw. The bigger change is in volume, since soft cooked onion packs into a cup at about 210 grams, which is why the Dietary Guidelines list a cooked cup at 2.9 grams.
So if a recipe calls for a big pile of onions cooked down until jammy, you still eat nearly all the fiber that went into the pot. You just eat it in a much smaller heap.
USDA’s own portion sizes show this shrinkage. Its medium raw onion weighs 110 grams, while its medium boiled onion weighs 94 grams, because cooking draws water out of the layers.
The same logic applies to slow, golden caramelized onions. As water cooks off, the fiber that’s left gets packed into fewer grams, so a spoonful of caramelized onion carries more fiber than a spoonful of raw.
How onions compare with other vegetables with fiber
Measured by the cup, cooked onions sit near the bottom of the vegetable list for fiber and near the top for calories. Here’s how they compare in the Dietary Guidelines’ standard portions:
| Vegetable | Portion | Fiber | Calories |
|---|---|---|---|
| Green peas | 1 cup | 8.8 g | 134 |
| Broccoli, cooked | 1 cup | 5.2 g | 54 |
| Cauliflower | 1 cup | 4.9 g | 34 |
| Carrots, cooked | 1 cup | 4.8 g | 54 |
| Spinach, cooked | 1 cup | 4.3 g | 41 |
| Mushrooms | 1 cup | 3.4 g | 44 |
| Red bell pepper, raw | 1 cup | 3.1 g | 39 |
| Onions, cooked | 1 cup | 2.9 g | 92 |
| Asparagus, cooked | 1 cup | 2.9 g | 32 |
Part of that calorie count comes from natural sugar. USDA lists about 4.2 grams of sugar per 100 grams of raw onion, and the FDA’s medium onion carries 9 grams.
Stretching the onion base with more vegetables
Many savory recipes start the same way, with a chopped onion softening in a pan. That first step is the easiest place to add fiber, because whatever you cook alongside the onion blends into the base of the dish.
Add a cup of mushrooms and a cup of chopped red bell pepper to one medium onion, and the base of the meal jumps from about 1.9 grams to 8.4 grams of fiber. The dish tastes much the same, just fuller and more colorful.
The useful takeaway is about pairing. Onions make nearly every vegetable on that list taste better, so they help you eat more of the higher fiber ones.
Not sure how much fiber your own day should include? Our fiber calculator gives you a target based on a few quick details.
What else a medium onion brings
Onions aren’t a nutrient heavyweight, but they add small amounts of several vitamins and minerals. Based on USDA values and FDA Daily Values, a medium raw onion (110 g) gives you about:
- 8 mg of vitamin C, about 9% of the Daily Value
- 0.13 mg of vitamin B6, about 8%
- 21 µg of folate, about 5%
- 161 mg of potassium, about 3%
- 1.2 grams of protein and 10.3 grams of carbohydrate
Mayo Clinic suggests about 25 grams of fiber a day for women 50 and younger and 38 grams for men in that group, then 21 and 30 grams after 50. A medium onion covers about 8% of the 25 gram target and 5% of the 38 gram one.
The FDA’s larger serving puts vitamin C at 20% of the Daily Value. Since onions show up in so many meals, those small amounts add up over a week.
Onions, fiber and a sensitive gut
Mayo Clinic notes that fiber helps keep bowel movements regular by adding bulk and softening stool. It also suggests raising fiber gradually and drinking plenty of water so your digestive system can adjust.
Onions bring something extra to the conversation. Monash University’s FODMAP team notes that onions are high in fructans, a group of fermentable carbohydrates that some people with irritable bowel syndrome react poorly to.
Fructans are water soluble but not fat soluble, according to Monash. That’s why lifting onion pieces out of a soup or stock doesn’t remove them, while onion infused oil can be a low FODMAP way to keep the flavor.
If you suspect onions bother your stomach, onion infused oil is worth trying in their place. For a step by step approach to testing trigger foods, our gut health guides walk you through it.
One pot of soup, and where the fiber really comes from
Say you start a pot of lentil soup the classic way, softening two medium onions before adding everything else. Here is roughly how the fiber stacks up in a batch that makes four bowls:
| Ingredient | Amount | Fiber |
|---|---|---|
| Onions, raw | 2 medium (220 g) | 3.7 g |
| Lentils, cooked | 2 cups | 31.2 g |
| Carrots, cooked | 1 cup | 4.8 g |
| Whole pot | 4 bowls | 39.7 g |
Each bowl comes out near 9.9 grams, about 35% of the Daily Value. The onions contribute under 1 gram of that per bowl, while the lentils do most of the heavy lifting.
That’s the pattern with onions in general. They make the high fiber ingredients taste good enough to eat in generous amounts, which is a bigger contribution than their own grams.
Looking for soups, stir fries and bean dishes that start with a pan of onions? Our recipe eBooks have plenty of high-fiber meals to cook this week.
One easy change to make tonight
Next time you chop an onion for dinner, chop a few green onions too and scatter a full cup over the finished plates. That adds about 2.6 grams of fiber across the meal for 32 calories, along with a big dose of vitamin K, without changing the recipe at all.
Do that on four dinners a week and you’ve added more than 10 grams of fiber to your week from a garnish alone. It’s a small habit, but it’s one that needs no new recipes and almost no extra prep.
Then let the onion do its real job: carrying flavor into beans, lentils, peas and broccoli. Those are the foods that turn a 1.9 gram onion into a 10 gram bowl.
Onion fiber questions people ask
How much fiber is in one onion?
A medium raw onion of about 110 grams has roughly 1.9 grams of fiber, according to USDA. The FDA’s larger 148 gram serving lists 3 grams, or 12% of the Daily Value.
Are onions high in fiber?
No. Onions are a low to moderate source, with 1.7 grams of fiber per 100 grams raw, so it would take about 15 medium onions to reach the 28 gram Daily Value.
Do cooked onions have less fiber than raw onions?
Only slightly. USDA lists boiled onions at 1.4 grams per 100 grams compared with 1.7 grams raw, and a cup of cooked onions holds about 2.9 grams because it packs more onion into the cup.
Do green onions have more fiber than regular onions?
Yes, per gram. Green onions have about 2.6 grams of fiber per 100 grams compared with 1.7 grams in raw bulb onions, and they have fewer calories.
Why do onions upset some people’s stomachs?
Onions are high in fructans, a type of fermentable carbohydrate that some people with IBS react poorly to. Onion infused oil can keep the flavor with a low FODMAP profile, since fructans don’t dissolve into fat.
How many calories are in an onion?
A medium raw onion of about 110 grams has around 44 calories. Raw onion has about 40 calories per 100 grams.
Can onions help you reach your daily fiber goal?
A little. A whole onion adds about 2 to 3 grams, so pair onions with beans, lentils, whole grains and higher fiber vegetables to reach 25 to 38 grams a day.
Sources
- U.S. Department of Agriculture, Agricultural Research Service. “Onions, raw (170000); Onions, cooked, boiled, drained, without salt (170001); Onions, spring or scallions, raw (170005).” FoodData Central, SR Legacy. 2019. fdc.nal.usda.gov
- U.S. Food and Drug Administration. “Nutrition Information for Raw Vegetables.” 2017. www.fda.gov
- U.S. Department of Agriculture and U.S. Department of Health and Human Services. “Food Sources of Dietary Fiber.” Dietary Guidelines for Americans, 2020 to 2025. dietaryguidelines.gov
- Monash University FODMAP. “All about onion, garlic and infused oils on the low FODMAP diet.” 2023. www.monashfodmap.com
- Mayo Clinic Staff. “Dietary fiber: Essential for a healthy diet.” Mayo Clinic, 2025. www.mayoclinic.org
- U.S. Food and Drug Administration. “Daily Value on the Nutrition and Supplement Facts Labels.” 2024. www.fda.gov
