Every host makes one of two mistakes: a fridge still full at midnight, or a buffet scraped clean while guests are still arriving. The gap between them isn't luck — caterers plan from a small set of per-person rules that work for 10 guests or 200. This guide lays out those rules: appetizer counts by party length, protein ounces by format, the drink formula, how kids change the math, and how much cushion to add.
Start with one question: is there a meal coming, or are the appetizers the meal? The answer sets the count.
For 30 adults and 10 kids at a three-hour open house, that's (30 × 15) + (10 × 0.5 × 15) = 525 pieces, or 578 with a 10% cushion — call it 500-600 and buy in that neighborhood. Quantity beats variety here: guests eat more pieces when there are fewer choices, so six platters of three items outperform twelve platters of twelve.
| Format | Protein per adult | Pounds for 20 adults |
|---|---|---|
| Sit-down dinner | 8 oz | 10 lb |
| BBQ / cookout | 10 oz | 12.5 lb |
| Wedding reception (plated) | 7 oz | 8.75 lb |
| Buffet with two mains | 8 oz total | 10 lb |
Two adjustments matter. Bone-in meat — ribs, bone-in chicken, brisket with a fat cap — yields roughly half its purchase weight in eaten meat, so double the pounds. And raw ground beef and sausages lose about 25% to cooking shrinkage: buy 13 pounds raw to serve 10 cooked pounds. The calculator handles the served-weight math; when you shop, convert to raw at the butcher counter.
Plan 8 ounces of total sides per adult at a dinner — two or three dishes totaling half a pound each guest. BBQ runs 10 ounces because slaw, beans, and potato salad do real feeding work next to smoked meat. In pounds for 20 adults: 10 pounds of finished sides for dinner, 12.5 for BBQ. A practical split for three sides: 40% of the weight in the starchy dish (potatoes, pasta, rice), 40% in the vegetable or salad, 20% in the rich one (mac and cheese, deviled-egg equivalent).
Enter adults, kids, hours, and party type — get appetizer pieces, main and side pounds, desserts, and drinks with a cushion.
Party Food Calculator →One serving per guest when dessert is plated (cake at a wedding); 1.5 when it's a served-but-casual course; 2 when there's a dessert table people graze — the graduation-party standard. Kids count as half a serving at plated events and a full serving where cake is the main event they came for. A half-sheet cake cuts into 40-48 party servings; two of them cover a 75-dessert order for 40 guests with the kitchen extra you'll want anyway.
The formula caterers use: 2 servings in the first hour, 1 per additional hour, per adult. A three-hour party = 4 servings each; four hours = 5. In 12-ounce terms, 30 adults over three hours is 120 servings, or about 11 gallons; with the kid half-portions and the 10% cushion you're at roughly 154 servings (14.4 gallons) for the 30-adult, 10-kid graduation party.
Splitting that between alcohol and soft drinks? The wedding alcohol calculator does the beer/wine/spirits breakdown with bottle counts — the same 60/40 alcohol-to-soft ratio it uses for receptions works for backyard parties.
Count children under 12 as half an adult for everything except cake at their own party. In practice: a 9-year-old eats a full slice of pizza but half the grazing adults do, drinks less, and takes one dessert instead of two. Toddlers barely register. The half-portion rule exists so you don't buy 40 adult shares for a guest list that's half elementary-schoolers.
Ten percent, always. It covers the cousin who arrives with two friends, second helpings of the thing everyone actually liked, and the tray that goes down at the buffet table. Leftovers keep; mid-party shortages don't. The only place to skip the cushion is per-plate catered service, where the caterer's own count is the contract — and even then, order two extra plates.
Duration drives two numbers: appetizer pieces (3 more per hour past two) and drinks (1 more serving per hour). Timing drives everything else. A party that overlaps a mealtime — noon, 6 pm — is a meal party: buy mains. A 2-4 pm open house sits between meals: guests graze, so heavy appetizers are the play. An 8 pm start after dinner? Wine, dessert, and one good cheese board, and don't let anyone talk you into 400 meatballs.
12 pieces per guest when appetizers are the whole menu — a cocktail party standard. If a meal follows, drop to 6 pieces per person. For parties longer than two hours, add about 3 pieces per additional hour.
A full meal runs about 1.5 pounds of total food per adult: 8 ounces of protein, 8 ounces of sides, and 6-8 ounces of appetizers and bread. BBQ formats push toward 1.75 pounds because 10 ounces of meat per person is the norm when burgers and sausage are the event.
For a 3-hour grazing party (no seated meal): roughly 750-800 appetizer pieces, 100 dessert servings, and 200 twelve-ounce drinks (about 19 gallons). For a plated dinner: 25 pounds of protein, 25 pounds of sides, 300 appetizer pieces, and 50 desserts. Add 10% for safety in either case.
10 ounces per adult of cooked-weight meat — about 20 pounds raw for 24 adults once you account for cooking shrinkage. Buy burgers, hot dogs, and chicken in roughly equal thirds and the variety self-balances at the grill.
Plan for 100% of yes-RSVPs and skip a no-show discount — the standard 10% cushion already absorbs the difference between realistic eating and your purchase. No-show discounts only make sense for catered, per-plate events where the caterer charges by the head.