ON3P sizing = stand ski against a wall, drop a measure tape from tip to floor. Length of tape = listed length of ski.
Other companies measure base length (tape from tip to tip around, flat on the base of the ski, hence longer), some others just make it all up apparently. Get a meter stick and mark a wall at 171, 176, 181, 186cm - measure up, lean longer due to rocker and click 'order'.