Onions add flavor to nearly every savory dish, and they also contribute a small but useful amount of dietary fiber. Because portions vary so much — a sliced onion on a burger, half a cup in a stir-fry, or a whole onion in a pot of soup — it helps to know exactly what you are getting. This onion fiber per 100g calculator converts your portion into a standard per-100g figure and tells you the total fiber in the amount you actually plan to eat.
Pick the onion form that matches your meal (raw yellow, red, white, cooked, or dehydrated), enter the amount in grams, ounces, cups, or whole onions, and hit calculate. The result shows fiber per 100 g, the total fiber in your serving, and the percentage of the recommended daily fiber target it covers. Whether you are tracking macros, planning a high-fiber diet, or just curious about onions, this tool gives you a quick, practical answer.
How to Use This Tool
Select the onion form that matches what you are preparing. If you are eating a raw onion in a salad or on a burger, choose the matching raw option. If the onion has been boiled or sautéed, choose cooked. For dried onion used in soups or seasoning blends, choose dehydrated flakes. Then enter your portion — the tool accepts grams, ounces, cups of chopped onion, or whole onions.
If you know only the approximate size, use the quick-fill buttons: 100 g, 1 cup chopped, 1 medium onion, or 150 g. The calculator converts your portion to grams, applies the fiber-per-100g value for that onion form, and shows total fiber. Keep the “Show % of daily fiber target” checkbox enabled if you want to see what fraction of the recommended 28 g daily value your serving supplies.
Tip: Weigh the peeled, trimmed onion because that is the part you actually eat. Onion skins and root ends are removed and should not be counted.
Fiber Values at a Glance
The numbers below are based on typical USDA food composition values for common onion products. They are planning estimates, not lab results for a specific onion; growing conditions, variety, and water content can shift the values slightly.
| Onion form | Fiber per 100 g | Typical use |
|---|---|---|
| Raw yellow onion | 1.7 g | Everyday cooking, soups, stir-fries |
| Raw red onion | 1.9 g | Salads, salsas, pickled toppings |
| Raw white onion | 1.7 g | Stews, Mexican dishes, raw salsas |
| Cooked/boiled onion | 1.6 g | Drained, measured after cooking |
| Dehydrated onion flakes | 9.9 g | Dry seasoning, instant soups |
For more detailed data, browse the USDA FoodData Central database. If you are comparing fiber in other root vegetables, the turnip fiber per 100g calculator uses the same per-100g approach.
How the Calculation Works
Each onion form has a fiber density expressed as grams of fiber per 100 g of edible portion. The calculator converts your portion to grams, then uses a simple proportion:
Total fiber = (fiber per 100 g ÷ 100) × serving weight in grams
For volume and count conversions, the tool uses average values: 1 cup of chopped raw onion weighs about 160 g, a medium onion about 110 g, and a large onion about 150 g. These are handy shortcuts for everyday cooking, but a digital scale is more accurate because onion size and chop density vary. The math behind the tool is the same one used in most nutrition tracking apps: standardized per-100g values multiplied by the actual amount consumed.
Worked Examples
Example 1 — One medium raw yellow onion. A medium onion weighs about 110 g. Using 1.7 g fiber per 100 g: 110 × 1.7 ÷ 100 = 1.87 g fiber. That covers about 6.7% of the 28 g daily value.
Example 2 — One cup of raw red onion. One cup of chopped red onion is about 160 g. At 1.9 g per 100 g: 160 × 1.9 ÷ 100 = 3.04 g fiber, or roughly 10.9% of the daily value.
Example 3 — Dehydrated flakes in a soup. If you stir 20 g of dehydrated onion flakes into a batch of soup: 20 × 9.9 ÷ 100 = 1.98 g fiber. The same weight of fresh onion would give far less fiber because most of the weight is water.
Tips for Accurate Tracking
- Weigh the edible portion. Remove the skin, root, and any bruised outer layers before weighing — those parts are usually not eaten.
- Match the state of the onion. If you measure after cooking, use the cooked value; if before cooking, use the raw value. Cooking changes water content, so fiber per gram changes even though the total fiber stays roughly the same.
- Be consistent with cup measurements. Finely diced onions pack more tightly than chunky slices, so a cup can vary from about 130 g to 180 g. Grams remove that guesswork.
- Drink enough water. Fiber absorbs water in the digestive tract. If you increase onion intake along with other high-fiber foods, make sure your fluid intake goes up too.
- Watch for FODMAPs. Onions are rich in fructans, which can cause bloating in people with IBS. Fiber per 100 g is still accurate, but tolerance can vary.
Note: Dehydrated onion is much more concentrated than fresh. A small handful can contain several grams of fiber and also a significant amount of sodium if it is a seasoned blend. Weigh dried onion before adding it to a recipe.
Frequently Asked Questions
Does cooking destroy onion fiber? No. Heat softens the cell walls and can allow some soluble fiber to leach into cooking water, but the total fiber in the onion is mostly retained. Because boiled onion absorbs some water, a 100 g portion of cooked onion contains slightly less fiber than 100 g of raw onion — 1.6 g versus 1.7 g for yellow onions.
Which onion has the most fiber? Among the common raw bulb onions, red onion is a little higher at about 1.9 g per 100 g, compared with 1.7 g for yellow and white. Dehydrated flakes are much higher (about 9.9 g per 100 g) simply because the water has been removed, so the fiber is more concentrated by weight.
How much fiber is in one whole onion? A medium onion of about 110 g provides roughly 1.9 g of fiber. A large 150 g onion provides about 2.6 g if it is a yellow variety, or slightly more if it is red.
Is onion fiber the same as the fiber in supplements? No. Onions contain a mix of insoluble fiber and soluble fructans, including inulin. Inulin is often extracted commercially for prebiotic supplements, but whole onions also provide water, antioxidants, and other phytonutrients. For most people, getting fiber from whole foods is the better choice.
If you are also tracking carbohydrates, the broccoli carbs per 100g calculator uses a similar per-100g format.