Onions are a kitchen staple, but if you're following a keto, low-carb, or diabetic meal plan, their carbohydrate content matters. This Onion Net Carbs Calculator helps you estimate the net carbs in any serving of onion — whether you're chopping a cup for a salad, caramelizing a large onion for soup, or measuring by weight. Net carbs are simply total carbohydrates minus dietary fiber, since fiber isn't digested like other carbs.
Choose your onion type, enter an amount in grams, ounces, cups, or whole onions, and the tool returns your net carbs, total carbs, and fiber. It's designed for anyone counting macros, managing blood sugar, or following a low-carb diet.
How to Use This Tool
Start by choosing the onion type you're working with. The default is a common yellow or white raw onion, but you can switch to red onion, sweet/Vidalia, green onion/scallion, or boiled onion. Then enter the amount in the most convenient unit: grams, ounces, cups of chopped onion, cups of sliced onion, or a whole small, medium, or large onion.
The calculator instantly converts your serving to grams, applies the correct nutrition values, and subtracts fiber from total carbs to show net carbs.
Formula Explained: Why Net Carbs Matter
The tool uses this simple formula:
Net Carbs = Total Carbohydrates − Dietary Fiber
Dietary fiber is a carbohydrate, but it passes through the body largely undigested. Because it doesn't raise blood sugar the way sugars and starches do, many low-carb diets exclude fiber from the carb count. Onions contribute a meaningful amount of fiber, so subtracting it can make a real difference in your daily totals.
Worked Examples
Example 1: Medium yellow onion. A medium raw yellow onion weighs about 110 g. Using yellow onion values (9.34 g carbs, 1.7 g fiber per 100 g):
Total carbs: 110 × 9.34 / 100 = 10.27 g
Fiber: 110 × 1.7 / 100 = 1.87 g
Net carbs: 10.27 − 1.87 = 8.40 g
Example 2: Half cup of chopped sweet onion. One cup of chopped raw onion weighs about 160 g, so half a cup is 80 g. Sweet onion contains 9.55 g carbs and 1.4 g fiber per 100 g.
Total carbs: 80 × 9.55 / 100 = 7.64 g
Fiber: 80 × 1.4 / 100 = 1.12 g
Net carbs: 7.64 − 1.12 = 6.52 g
These examples show how easy it is to under- or over-count if you rely on a generic onion serving size without checking your actual weight.
Onion Nutrition Reference Table
Per 100 grams of raw edible onion unless otherwise noted. Net carbs are calculated by subtracting fiber from total carbs.
| Onion Type | Total Carbs (g) | Fiber (g) | Net Carbs (g) |
|---|---|---|---|
| Yellow / white onion | 9.34 | 1.7 | 7.64 |
| Red onion | 9.15 | 1.7 | 7.45 |
| Sweet / Vidalia onion | 9.55 | 1.4 | 8.15 |
| Green onion / scallion | 7.34 | 2.6 | 4.74 |
| Cooked onion (boiled) | 8.32 | 1.5 | 6.82 |
Green onions are the clear winner for low-carb eaters. They deliver a similar fresh onion flavor with noticeably fewer net carbs per 100 g than mature bulb onions. Boiling also reduces the carbohydrate density slightly because onions absorb water during cooking.
Tips for Low-Carb Onion Use
- Use green onions more often. They have about 4.7 g net carbs per 100 g, making them a lighter choice for salads, stir-fries, and garnishes.
- Treat sweet onions as a treat. They taste milder and sweeter, but their net carbs are slightly higher than standard yellow onions.
- Buy whole onions and weigh them at home. Many nutrition databases list a medium onion as 110 g, but real onions range from 70 g to 150 g.
- Don't forget onions in marinades and sauces. Even a quarter-cup of chopped onion adds 2-3 g net carbs to a recipe.
- If you're on a strict keto plan, keep portions modest. An entire large onion can contain 10-12 g net carbs, which may be a large share of your daily carb budget.
Frequently Asked Questions
Are onions keto-friendly? Onions can fit into a low-carb or keto diet if you measure them. A small raw onion has about 5 g net carbs; a large one has about 10-11 g. Many keto recipes use small amounts of onion as a flavor base, and that's usually fine within a 20-50 g daily carb budget.
Does the onion type change the carb count? Yes, but not by a huge amount. Sweet onions have slightly more net carbs per 100 g than red or yellow onions, while green onions and scallions have the fewest. The choice matters most when you're eating a large portion or tracking very precisely.
Why does this calculator subtract fiber? Fiber is not absorbed as a source of usable glucose, so it is generally excluded from net carb calculations. The exact definition of net carbs can vary between diet plans, but subtracting fiber is the most common convention and the one used here.
Can I use this for caramelized onions? Caramelized onions are much lighter because their water content drops dramatically, so their carb density per gram is higher. This calculator is not intended for caramelized onions; weigh your onions raw before cooking and log that value.