Feed the room, not the fear of running out.

Plan on about 350 g (12 oz) of finished bone-in meat per adult, 280 g (10 oz) boneless, or 180 g (6 oz) of sausage. But you do not buy finished meat — you buy raw. Bone-in cuts lose roughly 30% to bone and another 25% to cooking, so multiply by 1.45; boneless loses about 22%, so multiply by 1.28; sausages only lose about 12%, so multiply by 1.12. For 20 adults on boneless steak that is 20 × 280 g × 1.28 ≈ 7.2 kg, or just under 16 lb, before the 10% buffer. Children under 12 count as half an adult.

When burgers are the main event, plan 2 per adult and 1.5 per child. If they share the grill with steak, sausage or hot dogs, drop to 1.5 and 1. If burgers are the only food at a long party, go to 2.5 and 2. Then add 15% — that buffer covers late guests, the person who eats three, and the patty that falls through the grate. At a US quarter-pound patty (113 g raw), every 4 burgers is about 1 lb of ground beef.

Count slices, not pies. An adult eats 3 slices when pizza is the meal, 2 when there are appetizers and sides, and 4 when pizza is the only thing served; children eat 1–2. Add 10%, divide by the slice count of the size you are ordering (a 14-inch large is 8 slices), and always round up. For 20 adults and 6 kids as a main course that is 60 + 12 = 72 slices, ×1.10 = 79.2, ÷8 = 10 pies.

About 250 g of dough per 12-inch pie, and a 12-inch pie feeds roughly three people. Dough weight is not flour weight: at 60% hydration with 2% salt, 3% oil and 1% instant yeast, the divisor is 1.66, so a 250 g ball comes from about 151 g of flour, 90 g of water, 3 g of salt, 4.5 g of oil and 1.5 g of yeast. Neapolitan style skips the oil and drops to a 1.63 divisor.

Five to six per adult as a main course (use 5.5 for planning) and 3 per child. As a snack or appetizer alongside other food, 2.5 and 1.5. If empanadas are the only food, 7 and 4. Add a 12% buffer and round up to whole dozens, because that is the unit they are sold in. Order at least two or three fillings — a meat-only order strands every vegetarian at the table.

The standard adult portion is 80 g (2.8 oz) of dry pasta. Drop to 60 g (2.1 oz) for a first course or a light plate, and go to 120 g (4.2 oz) for a hungry table. Children take about 40 g. Dry pasta absorbs water and comes out at roughly 2.3× its dry weight, so 80 g dry is about 184 g on the plate. A 1 lb (454 g) box therefore covers about five standard adult portions.

About 70 g (2.5 oz) of raw long-grain rice per person for a side. Rice roughly triples in weight, so that lands as about 210 g cooked on the plate. Water goes in at 2 mL per gram of raw rice, which is the familiar 2 cups of water to 1 cup of rice. For a main dish rather than a side, plan closer to 100 g raw per person.

Eleven pieces per adult for a full meal, 6 for an appetizer, and 18 for a broad tasting spread; children eat 7, 4 and 10 respectively. Add 10% and round up, because restaurants sell whole rolls of 8 pieces — you cannot order half a roll. Eleven pieces is about a roll and a third, so for four adults you are ordering roughly 6 rolls.

The catering rule is guests × hours × a weather factor: 0.5 kg per guest per hour indoors or in cool weather, 0.75 kg outdoors in mild weather, and 1.0 kg on a hot day in the sun. Add half a kilo per guest if you are also icing a food cooler, then add 20% for melt. For 26 guests over 4 hours in mild weather with a food cooler: (26 × 4 × 0.75 + 13) × 1.20 ≈ 109 kg, which is about 12 standard 20-lb bags.

Because you buy raw weight and serve cooked weight, and those are two different numbers. Water loss during cooking runs 20–25% for most cuts, and bone-in cuts carry roughly 30% inedible bone on top of that. A rack that weighs 10 lb at the butcher puts about 5 lb of meat on the plate. The calculator works backwards from the portion you actually want to serve, which is why the shopping number looks generous.

For meat, pasta, rice and sushi, yes — half is the catering standard and it holds up well for kids twelve and under. Pizza and burgers are the exceptions: kids eat pizza by the slice at close to two-thirds of an adult rate, and burgers at about three-quarters, because a slice and a patty are fixed-size units they will finish. Teenagers should be counted as adults, and in some cases as hungry adults.

Dry goods — pasta, rice, flour — whenever you like. Meat no more than a day or two ahead, held below 40 °F (4 °C). Sushi and anything with raw fish, the same day, no more than two hours before serving. And ice the morning of the event: bags stored in a home freezer fuse into a single unusable block within about a day, which is the single most common party-supply mistake.