For 20 adults you should order 8 large pizzas. This may seem like an arbitrary number, but there's actually a pizza ordering formula that can help you figure out the proper amount to order for any size group. When serving adults at a regular mealtime, go by what is known as the 3/8 pizza rule.

Medium-sized pizzas are about 12 inches in diameter and give you about eight slices. Large pizzas run about 14 inches in diameter and serve 10 slices.

Pizza Math Take the radius (half the diameter) of the pizza—4 inches for a small, 6 inches for a medium—and then multiply the answer by 3.14. That means your 8-inch baby pizza has a surface area of a little over 50 square inches, while the 12-inch medium pizza has a surface area of 113 square inches.

An 8-inch pizza is a medium-sized pizza that typically serves one to two people. It has a diameter of 8 inches and a total area of around 50 square inches. This makes it a perfect choice for those who want a personal pizza or are looking for a smaller portion.