Onions add flavor to nearly every savory dish, but their carbohydrate content is easy to overlook. This Onion Carbs Calculator converts any serving — grams, ounces, chopped cups, or whole medium/large onions — into total carbs, net carbs, fiber, sugars, and calories using USDA reference values for raw, cooked, and dried onion.
Whether you are following a keto or low-carb meal plan, tracking macros, or managing blood sugar, this tool gives you a reliable estimate in seconds. Choose the onion form that matches how you eat it — raw, cooked/boiled, or dried flakes — enter your portion, and press calculate. A full nutrient breakdown appears instantly, so you can log the right numbers without second-guessing.
Use the quick-fill presets for common servings like half a cup or one medium onion, and keep the net-carbs toggle on if you subtract fiber when counting carbs.
🧅 Calculate Carbs in Onions
How to Use This Onion Carbs Calculator
Start by choosing the onion form that matches your meal — raw, cooked/boiled, or dried flakes — then enter the portion you actually ate. The calculator accepts grams, ounces, chopped cups, or whole onions. Whole-onion weights are based on average market sizes: a medium onion weighs about 110 g and a large onion about 150 g. If you track net carbs, leave the toggle on; the big summary number will show net carbs, and the detailed grid below shows total carbs, fiber, and sugars separately.
For the most accurate logging, weigh raw onion before cooking. A recipe that calls for “one medium onion” can vary by 30–40 g depending on the grower and season.
Calculation Method and Data Source
The tool uses reference values from the USDA FoodData Central for raw onion, cooked/boiled onion, and dried onion flakes. The core formula is simple:
Nutrient in your serving = (portion weight in grams ÷ 100) × nutrient value per 100 g of that onion form
Because a cup of chopped onion is a volume measure, the calculator converts it to grams using an average density of 160 g per metric cup. One ounce equals 28.35 g. Values for the three forms used in this tool are:
| Onion form | Carbs (g/100 g) | Fiber (g/100 g) | Sugars (g/100 g) | Energy (kcal/100 g) |
|---|---|---|---|---|
| Raw onion | 9.34 | 1.7 | 4.24 | 40 |
| Cooked/boiled onion | 9.0 | 1.5 | 3.0 | 44 |
| Dried onion flakes | 77.6 | 9.2 | 42.4 | 349 |
Net carbs are calculated by subtracting fiber from total carbohydrates. In low-carb and keto meal plans, fiber is often excluded because it is not absorbed as glucose, although responses vary from person to person.
Worked Examples
Example 1 — One medium raw onion (110 g). Portion in grams = 110. Total carbs = (110 ÷ 100) × 9.34 = 10.3 g. Fiber = (110 ÷ 100) × 1.7 = 1.9 g. Net carbs = 10.3 − 1.9 = 8.4 g. Sugars ≈ 4.7 g and energy ≈ 44 kcal.
Example 2 — Half a cup of chopped raw onion (80 g). Total carbs = (80 ÷ 100) × 9.34 = 7.5 g. Fiber = 1.4 g. Net carbs = 7.5 − 1.4 = 6.1 g.
Example 3 — 10 g of dried onion flakes. Total carbs = (10 ÷ 100) × 77.6 = 7.8 g. Fiber = 0.9 g. Net carbs = 7.8 − 0.9 = 6.9 g. Notice that a small weight of dried flakes contains nearly as many carbs as an entire medium onion — dehydrated onion is concentrated, so portion control matters.
Reference Table: Carbs in Common Raw Onion Portions
| Portion | Weight (g) | Total carbs (g) | Net carbs (g) | Fiber (g) |
|---|---|---|---|---|
| 1 tbsp chopped | 10 | 0.9 | 0.8 | 0.2 |
| ¼ cup chopped | 40 | 3.7 | 3.1 | 0.7 |
| ½ cup chopped | 80 | 7.5 | 6.1 | 1.4 |
| 1 cup chopped | 160 | 14.9 | 12.2 | 2.7 |
| Small onion (~2¼″ diameter) | 70 | 6.5 | 5.3 | 1.2 |
| Medium onion (2½″ diameter) | 110 | 10.3 | 8.4 | 1.9 |
| Large onion (3″ diameter) | 150 | 14.0 | 11.5 | 2.6 |
Practical Tips for Carb Counters
- Weigh before cooking. Cooking onions drives off water, so a cooked portion weighs less but still contains the same carbohydrates as the raw onion you started with. If you enter a cooked weight, use the cooked row; if you enter raw weight, use raw values.
- Watch dried and powdered forms. Dried onion flakes pack 77.6 g of carbs per 100 g. Onion powder is even more concentrated, so weigh it carefully when adding to rubs and spice mixes.
- Net carbs are a guide, not a rule. Some people — especially those with diabetes or IBS — may respond differently to fiber and onion FODMAPs. Track blood glucose or work with a dietitian to find what works for you.
- Use a food scale if your plan is strict. A “large” onion might be 130 g in one market and 180 g in another, which can shift net carbs by 5 g or more.
- For other ingredients, check the USDA FoodData Central database and use this calculator alongside your regular macro tracker.
If you are planning a whole day around carbs, you can use the nutrition target calculator to set personal macro goals first.
Frequently Asked Questions
How many carbs are in half an onion? Half of a medium raw onion is about 55 g, so it contains roughly 5.1 g of total carbs and 4.2 g of net carbs. A generous half-cup serving is closer to 7.5 g total and 6.1 g net.
Are onions too high in carbs for keto? Onions are moderate, not keto-avoidance foods. One medium onion has about 8.4 g net carbs, so it can fit into a 20 g daily limit if you use a smaller portion — for example, ¼ cup provides only about 3 g net carbs. Caramelized onions are much lower in water but only contain whatever carbs you started with, so it is easy to eat more.
Does cooking change the carb count? The total carbohydrate mass does not change, but water loss makes the cooked onion’s carb density higher per gram. Boiling leaches a small amount of sugar into the water, which is why the cooked values are slightly lower in sugars than raw values. If you drained boiled onions, the per-100 g values in this tool are the right ones to use.
Why do net carbs differ from total carbs? Total carbohydrates include fiber, which is largely indigestible. Net carbs subtract fiber to estimate the carbs that affect blood sugar. This approach is popular in low-carb diets, but standard nutrition labels still report total carbs, so explain your method when comparing entries.