A cup and an ounce measure different things. A cup is a measure of volume, while an ounce is a measure of weight. Generally speaking, dry spaghetti (or most any dry pasta) will double in size when you cook it. I'd estimate that if you cook 2 ounces of dry spaghetti, you will get roughly one cup cooked spaghetti.
Permanent increase in dry weight accompanied by permanent increase in size and volume.
A serving size of spaghetti is considered to be 1/4 to 1/2 cup. Since you can not measure spaghetti in a measuring cup let's switch to ounces. 1/4 of a cup of dry spaghetti is 2 ounces, once cooked it measures about 4.5 ounces. So to get enough cooked spaghetti for 1 serving per person, for 20 people you need 20 about 20 ounces of dry spaghetti, or 1.25 lbs. 1.25 lbs of dry spaghetti will cook into approximately 2.8 lbs of cooked spaghetti, which is 45 ounces. 45 ounces which is roughly 5.6 cups, and 5.6 cups divided into 1/4th cup servings of cooked spaghetti gives you about 22 servings. If you feel people are going to want more than 1 serving then cook up another half pound of spaghetti to be safe.
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Spaghetti noodles are sold by the pound.
Four ounces of dry spaghetti will make two cups when cooked.
That is approximately 4 pounds dry.
Spaghetti is originally hard and contracted. When heat is supplied to any object, most of them expand. so, when heat is supplied to spaghetti, it expands. However, it is important to note that expansion due to increase in temperature (Thermal expansion) is a very small. So, spaghetti expands only by about an inch.
The increase in body size without change in shape can mean a few things. This could just be a sign of growth.
The English spelling is spaghetti (which can refer to a specific size or generically to various pasta products).
Spaghetti
Any dry bean when cooked will swell,which increases in size.