The US announces to the world in the differences from ICAO section of our Aeronautical Information Publication that we use inches of mercury. Then in our Aeronautical Information Manual and in the weather Advisory Circular 00-45G it is stated that the altimeter setting will be given as a 4 digit number, tens, units, tenths and hundredths of inches of mercury. The decimal point is not coded. What you'll hear on the ATIS or from ATC is "Altimeter Two Niner Niner Two" or "Altimeter Three Zero One Four." Always 4 digits preceded by "Altimeter" but no "Point" or "Decimal" in the middle.