How These Estimates Are Built
Pizza ordering is a slice-count problem disguised as a pizza-count problem. The reliable method works in slices, not pies: estimate slices per person by appetite, multiply by headcount, then divide by slices per pizza and round up. The classic party-planning rule of roughly three slices per adult is a decent anchor, adjusted up for heavy eaters or a long event and down when there are sides. Rounding up is not generosity — a large pizza cut into eight means the last person either gets a slice or does not, and fractional pizzas cannot be ordered.
Size scales by area, not by diameter — and this is where money is lost. An 18-inch pizza is not twice a 12-inch; it is 2.25 times the food, because area scales with the square of the radius. π × 9² is 254 square inches against π × 6² at 113. This is the single most useful thing on this page: two mediums almost always lose to one large on both quantity and price, and the intuition that "two twelves beat one eighteen" is simply wrong.
Protein needs scale with body weight and training load. The tool applies grams per kilogram of body weight, adjusted by activity. The sedentary baseline sits near 0.8 g/kg — a minimum to prevent deficiency rather than a target for anyone active. People training seriously generally land somewhere in the 1.6–2.2 g/kg range, where the research on muscle protein synthesis clusters, and past which additional intake shows sharply diminishing returns.
Two honest caveats. First, higher is not automatically better: beyond roughly 2.2 g/kg the evidence for further benefit thins out considerably. Second, these are population guidelines. Kidney disease, pregnancy and certain medical conditions change protein requirements meaningfully, and those are conversations for a clinician rather than a calculator.