Originally Posted by f4rrest

How to measure:

Close the toilet lid.

Put towel on the lid.

Put a sheet of aluminum foil on the towel.

Wear only your briefs.

Sit down on the foil, not moving around. Lean forward about the same angle you'd ride the bike.

Stand up.

Notice the two dents left in the foil from your ischial tuberosities. Mark their centers with a marker.

Measure the distance between the marks.