Order enough pizza. Not enough to eat pizza for three days.

Every kid birthday reaches the same moment: an adult in a parking lot doing anxious fractions. Twelve adults, eight kids, and three toddlers do not eat anything alike, and the standard three-slices-a-person rule prices them as though they do.

Kait and I ordered badly enough times, in both directions, that I started writing the numbers down. The calculator splits the crowd, applies defaults we tuned party by party, and shows every assumption it is making.