Onions are a staple in nearly every cuisine, yet their fiber contribution is often overlooked when planning meals. This calculator converts any serving size — from grams and ounces to cups, single onions, and more — into a precise fiber estimate. Just choose the onion variety and preparation method to account for the small differences between raw, cooked, and sautéed onions.
Built for home cooks, meal planners, and anyone tracking fiber for digestive health or weight management, the calculator uses USDA-informed values for common varieties including yellow, red, white, sweet, green onions, and shallots. It then expresses the result in grams plus a percentage of the FDA’s 28-gram daily fiber target, with a step-by-step breakdown to show exactly how the estimate was derived.
Use it to plan a high-fiber dinner, check a recipe’s nutrient contributions, or log onions in your diet app with more confidence.
🧅 Serving Amount
⚙️ Onion Details
🧾 Step-by-step breakdown
How the Fiber Estimate Is Calculated
This tool uses a simple proportion: the weight of your onion serving, the fiber density per 100 g for that onion type and preparation, and a standard daily fiber target. The general formula is:
Serving weight is derived from the amount and unit you select. For example, 1 cup of chopped raw onion is treated as about 160 g, while 1 medium onion is treated as about 110 g.
The fiber values are based on USDA FoodData Central entries and common nutrient databases, harmonized so you get one consistent number for each combination. Because individual onions vary in size and moisture, the result is a useful estimate, not a laboratory measurement. For the most precise tracking, weigh your onions on a kitchen scale and use the grams option.
Quick Start Guide
- Enter the amount of onion in the first field and pick the matching unit. If you are using a recipe that calls for chopped onion, choose cups; if you are tracking by food scale, choose grams.
- Pick the onion type that is closest to what you are eating. Yellow, red, white, sweet, green onion, and shallot options are included.
- Choose the preparation method. Raw onions are the reference state; cooked and sautéed onions have slightly lower fiber density per 100 g because they absorb moisture or lose water during cooking.
- Click Calculate Fiber to see total fiber, fiber per 100 g, serving weight, and the percentage of the 28 g daily value.
Reference: Fiber Content by Onion Type
Values are grams of fiber per 100 grams of edible portion. Use them as a quick reference when planning meals or adjusting recipes.
| Onion Type | Raw | Cooked / Boiled | Sautéed |
|---|---|---|---|
| Yellow onion | 1.7 | 1.4 | 1.5 |
| Red onion | 1.5 | 1.3 | 1.4 |
| White onion | 1.5 | 1.3 | 1.4 |
| Sweet onion | 1.2 | 1.0 | 1.1 |
| Green onion / scallion | 2.6 | 2.2 | 2.3 |
| Shallot | 3.2 | 2.6 | 2.9 |
These are averaged values from USDA FoodData Central and reputable food-composition databases. Individual onion sizes and storage conditions can shift the numbers by roughly 0.1–0.3 g per 100 g.
Worked Examples
Tips for Accurate Fiber Tracking
- Weigh onions raw before cooking whenever possible. Cooking removes water, so cooked weight is not directly comparable to raw weight.
- If you are counting net carbs, fiber is typically subtracted from total carbohydrates. Use this calculator to get the fiber portion, then subtract it from total carbs. A net carbs calculator can help with the full calculation.
- For recipes that use multiple onions, calculate once per ingredient type, or weigh the total chopped batch and use the grams input.
- Green onions and shallots are more fiber-dense than common bulb onions. If you substitute them in a recipe, your fiber estimate changes noticeably.
Frequently Asked Questions
Does cooking onions change their fiber content?
Cooking does not destroy fiber, but it changes the water content. Since boiled onions absorb water and sautéed onions lose some moisture, the fiber per 100 g can go down or up slightly, while the total fiber in the same raw-equivalent portion remains nearly identical. The calculator accounts for the per-100-g density shift so you can log cooked servings directly.
Are shallots and green onions really onions?
Yes. Shallots are a botanical variety of onion, and green onions (scallions) are immature onions harvested before the bulb fully forms. Both are slightly higher in fiber than mature bulb onions, which is why they have separate options in the type menu.
Why does my diet app show a different fiber value?
Nutrient databases vary in how they define a serving and which food entry they use. The values here are centralized estimates intended to be practical and consistent. If you need lab-exact numbers for medical purposes, consult a registered dietitian and use the specific food label or USDA FoodData Central entry. The onion fiber per 100g calculator can also give a quick per-100-gram reference.
What is the 28 g daily fiber target based on?
The FDA uses 28 g of fiber per day as the Daily Value on Nutrition Facts labels for a 2,000-calorie diet. Individual needs vary, so this percentage is a general benchmark rather than a personal recommendation. See the FDA’s Daily Value guidance at fda.gov for details.