An altimeter is really nothing but a highly sensitive pressure measuring device. It reads in ft (or meters depending on altimeter) because it is simply converting a pressure reading to what would be the height in standard atmosphere that would have that pressure. That reading is further adjusted by the altimeter setting in the Kollsman window. If it is set to 29.92 inHg or 1013.25 HPa there is no adjustment and the height displayed is exactly that of standard atmosphere at the pressure the altimeter is sensing. This height is called pressure altitude.

Example:

I have an extra altimeter that I took from an aircraft and want to use that to measure the actual pressure (absolute atmospheric pressure) of the location where I am at. I set the Kollsman window on the altimeter to 29.92 inHg. The altimeter then reads 5420 ft which in this case is the pressure altitude of my location. What is the absolute atmospheric pressure in inHg of where I am at?

Begin by going to the 2nd calculator for pressure altitude and highlight the pressure altitude field, since that is what I have.

Enter 5420 and read the result which is 24.51 inHg (a) for the absolute atmospheric pressure. You also get the value in HPa or mbars (b) if you'd like.