How to measure cycle seat hight.

Source.

1. In-seam measuring: stand erect. insert a book approximately 5 cm width in the groin (in between the thighs) touching the groin. measure the distance from the closed end of the book touching the groin to the floor.
2. Center to top measurement: Multiply the figure in step1 by .88 (Point 88).
   

This tells you the exact Seat height.

This method was first adopted by an American cyclist Greg Le Mond.