What Gauge or thickness of steel or Iron was used to make plate armor by our Medieval predecessors? Obviously they couldn't just go of to a store and buy sheet metal sold in precise thicknesses, I would imaging that the thickness varied seeing as though they would have had to hammer out their own "sheet metal", with out the precision of modern steel factories.

Interestingly enough, on the Spotlight: The 14th Century Bascinet article adresses the weight of the Bacinet, which gives some insight into the thickness of the steel.

"From these few reported numbers it is obvious that the bascinet varied markedly in size, weight, and manufacture. If considering the weight of only the skull and the visor, here we see a variation from 1857g (4.125 lb) to 2890g (6.37lb). It must be emphasized that we are considering a very limited sample size here and the variation of weights is most likely much greater. As outlined above, the quality of the steel and heat-treatment of the various bascinets varied as well with location and time."

This is a lot lighter than I would of thought, Considering that some make Helm reproductions of 12 guage steel, easily up to 10,11,12 Lbs. not even including the Aventail.

Nick,
There are a scattering of threads that deal with this or related info:

Historical Helm Thickness and 18 gauge steel
Medieval armour weights

There are likely a number of others that the search function will point you to.

the simple answer to this is "they did'nt use guage or thickness" when making such objects.

the idea of uniform, even plate thickness is a wholly modern concept, due to the supply of uniform rolled plate from steel mills, whereas original armour would have come from individual billets of steel, hammered out into plates and formed to shape, so a breastplate could conceivably change from 10ga steel at the median ridge, to 20ga at the edges underneath the armpit, moving metal to the places where most was needed, and removing metal from where it was excessive.

Thanks Chad. I definitely expected this to have been brought up before.

I was lucky enough some years ago to get to wear an original Landsknecht armour, so made for a foot soldier and of munitions grade. It felt to be about the weight of a water soaked overcoat when it was on, so not that heavy overall. Most reenactment armours are heavier than original field armours, but are close to the weight of original tourney or jousting armours, which is not a surprise as really they perform the same function.