When it comes to barbecue, it doesn't seem to matter how much you estimate, it's almost guaranteed everyone will want more. Estimating how much people will eat is both an art and a science. You might consider 170grams/6 oz. an average serving size of meat, but that doesn't mean that's how much your guests will actually eat.
Find out how many different kinds of meat will be cooked. If there are chicken, pork and beef, it is likely that most guests will want to try all three, but will probably eat less of each than if there were just one type of meat offered.
Find out if there are other entrées being offered. If the barbecue is a potluck event, some people might cut down on their meat portion to try some of Grandma's baked ziti, for example. You should also take into consideration whether people will be served at the event, or if it will be a buffet where people will be free to take as much as they want. In general, figure about 600 grams/1.5 pounds of meat per person, especially if guests are serving themselves.
Calculate how many people will be attending the event. Count children under the age of 10 as half an adult serving. They are likely to eat more dessert and appetizers but less meat overall.
Write out your menu. Place an asterisk next to the items you think will be most popular, or items that are more expensive that you will serve less of overall.
Add 10 per cent to the number of entrées you need. For example, if you have 50 guests coming, add 5 entrées to this amount.
Picture your buffet plate. If you are having barbecued chicken, pulled pork and sausage, you might see a chicken leg, a pile of pulled pork and a half a sausage on a plate. This will help you decide what items will be the most popular. If everyone in 50 people eats a piece of chicken, you can estimate needing about 15 whole chickens.
Adjust your totals according to price and popularity. Recalculate the servings to make sure there is enough for everyone. Add side dishes to make up the bulk of the meal.