300
g
1.88
cups
120
kcal
90
mg
300
g
1.88
cups
120
kcal
90
mg
The Onion Calculator helps you determine how many onions you need for a recipe, convert onion quantities between whole onions and cup measurements, and understand the nutritional content of onions. Onions are among the most widely used vegetables in global cooking, forming the flavor base of countless dishes from French onion soup to Indian curries to Mexican salsas.
Onions (Allium cepa) vary significantly in size, and recipe measurements are not always consistent. Some recipes call for a specific number of onions while others specify cups of chopped or sliced onion. This can be confusing when you are at the grocery store or in the middle of cooking. As a general guideline: a small onion weighs approximately 100 grams, a medium onion about 150 grams, and a large onion about 220 grams.
When chopped, one medium onion (150 grams) yields approximately 1 cup of chopped onion (about 160 grams per cup due to denser packing). Sliced onions pack less densely, with approximately 115 grams per cup. Understanding these conversions helps you buy exactly the right amount and prepare just what a recipe needs.
Nutritionally, onions are remarkably beneficial despite being low in calories — only about 40 calories per 100 grams. They are rich in vitamin C, folate, potassium, and B vitamins. Most notably, onions are one of the richest dietary sources of quercetin, a potent flavonoid antioxidant with anti-inflammatory, antiviral, and potential anti-cancer properties. Red onions generally contain more quercetin and anthocyanins than yellow or white onions.
The characteristic sharp flavor and tear-inducing effect of raw onions come from volatile sulfur compounds similar to those in garlic. When onions are cooked, these compounds break down and transform into sweet-tasting compounds through the Maillard reaction, which explains why cooked onions taste dramatically sweeter than raw ones. Slow-cooking onions for 30-45 minutes creates deeply caramelized onions with concentrated sweetness.
The calculator uses the following standard conversions:
Weight per onion: Small = 100g, Medium = 150g, Large = 220g
Volume conversions: 1 cup chopped onion = 160g; 1 cup sliced onion = 115g
Calories: 40 kcal per 100g = 0.40 kcal/g
Quercetin: approximately 30 mg per 100g of raw onion (based on USDA nutrient data for yellow onion)
The cups output helps translate between whole onions and the volume measurements typically used in American recipes. Quercetin content is highest in raw, red onions and decreases with cooking. For maximum quercetin intake, eat some of your onions raw or lightly cooked. The calorie count confirms that onions are a very low-calorie way to add flavor and nutrition to dishes.
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Two medium onions chopped yield approximately 1.9 cups of chopped onion — nearly the 2 cups needed, suggesting you may want to add a little extra from a third onion.
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Four large sliced onions for French onion soup weigh 880 grams and will reduce dramatically to about 1 cup of caramelized onions after slow-cooking.
A medium onion weighs approximately 140-160 grams (about 5-6 oz), with 150 grams being a good average. Small onions weigh about 85-110 grams, and large onions 200-250 grams. Jumbo or extra-large cooking onions can weigh 300 grams or more. Weight varies considerably by variety — pearl onions are much smaller, while Spanish onions are typically large and mild.
One medium onion chopped yields approximately 1 cup of chopped onion (about 160 grams). This is a widely used rule of thumb in American cooking. However, the exact amount depends on how finely or roughly you chop, the onion's moisture content, and how firmly you pack the cup. A large onion yields about 1.5 cups and a small onion about 2/3 cup.
Several techniques reduce lachrymation when cutting onions. Chill onions in the refrigerator or freezer for 20-30 minutes before cutting — cold temperatures slow the release of volatile compounds. Cut with a very sharp knife to minimize cell damage. Work under a running exhaust fan or near an open window. Wear goggles for large quantities. Cut near a lit candle — the flame's combustion consumes some of the volatile compounds before they reach your eyes.
Yellow onions are the most versatile and widely used, with a balanced sweet-savory flavor that intensifies and sweetens when cooked. White onions are sharper and more pungent raw, with a cleaner, slightly more acidic flavor — preferred in Mexican cuisine. Red onions are milder and slightly sweet raw, with a beautiful purple color and the highest quercetin content among onion varieties. They are commonly used raw in salads and pickled. Sweet onions (Vidalia, Walla Walla) are high in sugar and low in pyruvic acid, making them very mild and ideal for eating raw.
Onions are rich in quercetin, a powerful flavonoid antioxidant linked to anti-inflammatory effects, reduced risk of heart disease, and potential cancer prevention. They also contain organosulfur compounds similar to those in garlic. Regular onion consumption has been associated in epidemiological studies with reduced risk of cardiovascular disease, certain cancers (especially stomach and colorectal), and improved bone density in postmenopausal women. Their prebiotic fibers (especially fructooligosaccharides) also support gut microbiome health.
Yes, onions are a great food choice for people with diabetes. They have a low glycemic index (about 10-15) and low glycemic load, meaning they cause minimal blood sugar elevation. Research has shown that onion extract may have hypoglycemic effects, partly attributed to their quercetin and sulfur compounds. Raw onions appear to have stronger blood sugar-lowering effects than cooked. Onions can be freely incorporated into a diabetic diet as a flavor base for many dishes.
Raw onions contain volatile sulfur compounds (primarily propyl sulfoxide) that create the sharp, pungent flavor. When cooked, heat breaks down these compounds and promotes the Maillard reaction and caramelization, converting complex carbohydrates into simple sugars and creating hundreds of new flavor compounds. The longer onions cook at moderate heat (caramelizing for 30-45 minutes), the more their inherent sugars concentrate and develop deep, sweet, complex flavors.
Whole, uncut onions should be stored in a cool (50-60°F), well-ventilated, dark, and dry place — a pantry or cellar is ideal. They should not be refrigerated before cutting, as cold and humidity cause them to soften and mold faster. Do not store near potatoes, as both emit gases that accelerate each other's spoilage. Once cut, wrap tightly in plastic or store in an airtight container in the refrigerator and use within a week.
Scallions (green onions) are immature onions harvested before the bulb develops, with a mild flavor. Shallots are a related but distinct species with small, elongated bulbs, a complex onion-garlic flavor, and high sugar content — favored in French cooking. Yellow, white, and red onions are mature Allium cepa bulbs with fully developed flavor. Pearl onions are small, marble-sized onions from the same species, often used in stews and roasts.
Raw onion juice has traditional uses in many cultures for treating coughs, skin conditions, and hair loss. Some small studies support antimicrobial and anti-inflammatory effects of onion extract. Onion juice applied topically has shown promise in some trials for improving scar appearance and alopecia areata (patchy hair loss). However, most of this research is preliminary, and onion juice should not replace conventional medical treatment for any condition.
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