If one end of a runway is numbered 09, what is the designation for the opposite end?

When one end of a runway is numbered 09, it indicates that the runway is oriented 90 degrees from magnetic north (090 degrees). Runway numbers are based on the magnetic azimuth of the runway's direction, rounded to the nearest 10 degrees. The opposite end of a runway is designated by adding 180 degrees to the runway number.

Therefore, when you add 180 degrees to 090 degrees, you obtain 270 degrees. This corresponds to the opposite end of the runway being numbered 27. So, the designation for the opposite end of a runway numbered 09 is indeed 27.