The internal diameter is 18mm (0.70" ~ or 45/64" to be exact!), with a full slide playing surface of 60mm (2.3/8") to cover all six strings, something those traditional Coricidin bottles never offered - we also drill a tiny breather-hole into the roof of the slide to stop the other complaint regularly heard from pill-bottle slide players ~ stuffiness inside the slide!

 

 

  ......."full and liquid sounding....a perfect gap-free finger fit, definitely the best of it's type we've encountered".......