Chose to climb a route listed as 12m in the guide book using a 30m rope. We didn't take into account the amount of rope used up by the knots and the anchor clearing process at the top of the route. The route was climbed successfully but during the lower off it became clear that there wasn't enough rope left to fully lower to the ground. Was about 4m short. There was a knot in the end of the rope so no injury occured but the participant had to clip off to a bolt and re-tie the knot to allow a full lower off.