What is a regular hexagon?
A regular hexagon has six equal sides. Every interior angle equals 120 degrees. Its vertices lie on one circle. Its sides touch another inner circle. These symmetries make many calculations especially simple.
Joining the center to every vertex creates six equilateral triangles. Each triangle has the hexagon’s side length. This explains why the circumradius equals the side length.
Apothem and circumradius
The apothem runs from the center perpendicular to a side. It is also the inradius. The circumradius runs from the center to a vertex. Their values are different.
The apothem equals √3 divided by two times the side. The circumradius simply equals the side.
Diagonals and widths
A regular hexagon has short and long diagonals. A short diagonal skips one vertex. It equals √3 times the side. A long diagonal connects opposite vertices. It equals twice the side.
Across flats matches the short diagonal. Across corners matches the long diagonal.