In other words a K18 meter ski jump means that it is 18 meters from the take off to the point where the hill begins to flatten out of the K point begins. A K90 meter jump means that it is 90 meters from the end of the take off to where the hill begins to flatten out or K point.
A competitor's distance is measured between the takeoff and the point where the feet came in full contact with the landing slope (for abnormal landings, touchpoint of one foot, or another body part is considered). Jumps are measured with accuracy of 0.5 meters for all competitions.