For the confused - prepare to be more so!
I think DVieau2 means 4-ply board, ie, standard matboard...
mils have nothing whatever to do with millimetres (mm), except the Latin derivation of the name (where deci means a tenth of, centi means one hundredth, milli one thousandth and micro equals one millionth).
A mil is a milli-inch ~ one thousandth of an inch, or if you prefer 1/1000th". I don't know of any symbol for it but to add to the confusion, some call it a micro-inch! (A bit like billions I suppose - the Imperial system holds that a billion is a million million, but the whole world now follows the American standard of a thousand million... now anyone can be a billionaire - under the old British system, there still wouldn't be any!) Back to the chase...
The equivalent Metric System mini-measure is microns or micrometres, or if you're American, I guess that's micrometers... but nobody ever accused the Yanks of knowing much about English! Ouch!
The symbol for micron is µm and one µm equals 1/millionth of a metre (m), or 1/1000th of a millimetre (mm), or 1/10000th of a centimetre (cm), or 1/25400th of an inch - ie, approx 1/25th of a mil - or if you prefer - 1mm = just on 40mil (ie 40 thousandths of an inch).
And for good measure - an inch is 25.4mm and a yard is 915mm. A metre is 3.2808 feet. There's 1000m in a kilometre (km) and there's about 1.6km to the mile. Enough already!
So, apart from showing that the US should stop being the last hold-out and get with the program in going metric asap, where does that leave us?
Well, standard 4-ply matboard is indeed about 1.5mm or 60mil thick. Jfeig's 0.055" is right on the button.
But sorry Bill - back to school! - 4mil (or "4 thou." if you're Australian) is 4/1000ths of an inch, or 0.004 inch, or 1/250th", but 1/32" is just over 31 thou. - 31.25mil to be precise... so 1/32" and 32 thou. are very different animals.
And after all that, I think I need a sleep!
Rod