You cannot determine foil thickness from its weight per area, because of the significant variation in the density of electrodeposited [ED] copper.
But people don't care. They do it regardless. (oz/ft2)||(mils)||(mm) 1/2||0.7||17.8 1||1.4||35.6 2||2.8||71.1 3||4.2||106.7
The thickness of 1oz Cu foil may start out at 1.4 thou but it ends up at about 1.3 thou due to processing steps such as deburring and microetch. So in the real world, the boards that were fabricated using 1 oz foil should have a FINISHED Cu foil thickness of 1.3 thou after conventional processing.
To complicate this further, it's very uncommon to see 1 oz copper at 1.3 thou in a finished board. Almost everyone uses foil that is at the thin end of the spec. Why? Because copper is sold by weight and thinner foil weighs and costs less. IPC 4562 says that ED foils can be 10% below the listed nominal (nominal for 1 oz is 34.3 um or 1.35 thou). Foil more commonly comes in at 1.30 thou or less. Processing does reduce this down to 1.2 or thinner.
reply »