The only thread which one can measure sensibly with a caliper is a monofilament thread. Martin's suggestion is the easiest and least technologically demanding. However, if you have a sensitive balance, and enough thread to waste (or patience to rewind!), one can use the denier of the thread to compare sizes. Of course one needs to know the denier. Only UNI and one (to my knowledge) other company gives this info. Some others don't because it would reveal their chicanery in the use of the X/0 "system."
Cheers, Paul http://www.galesendpress.com -- Paul Marriner Outdoor Writing & Photography. Member OWAA & OWC. Author of Stillwater Fly Fishing: Tools & Tactics (Print [NEW] & CD), Modern Atlantic Salmon Flies, Miramichi River Journal, Ausable River Journal, and Atlantic Salmon.
