It really depends on a few factors. How lean is your beef? If you have really good beef (80/20 or better) you can figure to get 5 or 6 good size burgers out of each pound. Another factor is how many of these people eat burgers and how many of them EAT burgers. Many women eat 1, most men eat 2. Kids vary greatly, some don't eat burgers (or at least not on a bun) but many kids eat hot dogs. You may want to have some of those for kids and people who prefer dogs over beef.
So, let us say you have 50 men and 50 women. Each couple will average 3 burgers. If we get 6 burgers out of every pound then 1 pound will feed 2 couples (4 people). For a hundred people you will need 25 lbs. to feed 50 couples. If children are involved then figure the average couple has 1.5 kids each. That is a good many kids, 75 to be exact. You can say 2/3 of those kids would prefer a hotdog. So for 25 kids you need another 25lbs to be safe. With regard to the hotdogs you need 13 (8 dog) packs (104 dogs) to be safe.
If you were counting kids in the 100 people then this is way to much. But you can probably get a better idea this way.
I would think it depends on how much hamburger you want each person to have. If you want to serve each person a quarter pounder (4 oz) then you would need 25 pounds of hamburger. For a half pound (8oz) per person you need 50 pounds.
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