Biology on a Budget: What size test tubes fit into a standard test tube rack?

 While I was scouring websites for deals it occurred to me to ask what size is a standard test tube or a standard test tube rack?  The standard test tube rack has a 25 mm opening, which fits 18-20 mm test tubes.  (BTW the pegs on the rack is for drying inverted test tubes.). Six test tubes, one rack, and one brush is fine for one student taking Bio and Chem.  I use both plastic (polypropylene) and wooden racks.  (The plastic does melts a little if the test tube is hot.). Even the cardboard test tube racks from eScience kits are fine for most labs.  Be sure the glass test tubes are borosilicate ( Pyrex or Kimax).