The BD Ultra-Fine Original Pen Needle is 29 gauge 12.7mm (1/2"). It's identical to the needle on the BD Ultra-Fine insulin syringe, the best selling insulin syringe in the world.
The BD Ultra-Fine III Mini pen needle is 31 gauge, 5 mm (3/16"). It's the shortest pen needle in the world, and no pen needle is thinner.
The BD Ultra-Fine III Short Pen Needle is 31 gauge, 8 mm (5/16"). No pen needle is thinner. It is BD's best-selling pen needle.