&fmt=18 Share definition Flag A magic code used on YouTube to make videos appear slightly sharper when added to the end of the video url. I've been watching YouTube videos with the "&fmt=18" code; now it hurts to watch them without. &fmt=18 by Sinkbot June 2, 2008 0 0 Get merch Get the &fmt=18 mug. Buy Now — $32.95 See more merch