Estimate popcorn for a gathering.
Popcorn expands enormously, which is why buying kernels by weight for a party feels wrong until you do the arithmetic. One cup of un-popped kernels yields roughly 32 cups of popped corn, and a cup of kernels weighs about seven ounces. Twenty guests at one and a half servings of four cups each need 120 cups popped — under four cups of kernels, or about a pound and three quarters. That is the whole point: a couple of pounds of kernels feeds a crowd, whereas the equivalent in pre-popped bags fills a car. Butter at a quarter ounce per cup of popcorn is a moderate coating; cinema-style is closer to half.
Popcorn Party
Popped cups = guests x servings x cups per serving; kernel cups = popped cups / yield; kernel lb = kernel cups x 0.44; butter oz = popped cups x butter per cup
About 70 cups popped, which is roughly 17 four-cup servings. Two pounds comfortably feeds a party of twenty.
Pop it no more than an hour ahead, keep it in open bowls rather than sealed containers so steam escapes, and add butter and salt just before serving rather than in advance.