The top of a standard adult graduation cap (the square mortarboard) is usually about 9–9.4 inches (around 23–24 cm) on each side, so roughly a 9.4 × 9.4 inch square.

Quick Scoop: Typical Dimensions

  • Top board (mortarboard): About 9.4 inches (24 cm) per side for most high school and college caps.
  • Child sizes: Often a bit smaller, around 8–9 inches (20–23 cm) per side.
  • Doctoral or larger styles: Can go up to about 10 inches (25.4 cm) per side, or 10–12 inches (25–30 cm) in diameter for round tam-style caps.
  • Cap depth (the part on your head): Usually about 3–4 inches (7.6–10 cm) deep.
  • Typical tassel length: Around 9 inches (23 cm) long.

Think of the standard graduation cap as a slightly smaller-than-normal sheet of printer paper, but square instead of rectangular, sitting flat on your head. TL;DR:
Most graduation caps for teens and adults are about 9–9.4 inches (24 cm) square on top, with kids’ caps a bit smaller and doctoral caps a bit larger.

Information gathered from public forums or data available on the internet and portrayed here.