BBQ Grill Size Calculator
Size the grill by what goes on it at once, which is the meat weight rather than the guest count.
The formula
total = servings per person * guests
People eat less at a buffet
The quantities caterers use are lower than a host's instinct, and the reason is choice: faced with eight dishes people take a little of each and eat less in total than they would from a plated meal with two. Catering for the appetite you imagine rather than the one that turns up is the standard way to end a party with half the food left.
Bone-in weighs more for the same meal
A boneless cut is about 200 g a head as a main course; the same meal on the bone is 350, because a third or more of what you buy is not edible. Ham and turkey are quoted higher again. Buying bone-in weight against a boneless figure is the single most common catering error.
Drinks front-load
People drink roughly twice as much in the first hour as in later ones, so the rate that matters is the opening one. Two drinks in the first hour and one an hour after is the usual planning assumption, and running out early is far more noticeable than a few bottles left over.
Recipes are starting points
Ovens run hot or cold by 20 degrees, flour absorbs water differently by season and brand, and the same joint of meat cooks at a different rate depending on its shape rather than its weight. Everything here is a planning figure that gets you close enough to start, and the thermometer, the scales and your own notes from last time are what finish the job.