Air Pressure

How are air pressure readings adjusted to what they would be at sea level?

I know that air pressure needs to be adjusted to what it would be at sea level to compare pressure systems but how do you do it. Is there a formula. I think there is one but it seems weird and it doesn’t make sense. Can someone tell me how this is done and why it works.

Here is the link to a pressure calculator

http://www.srh.noaa.gov/epz/?n=wxcalc_stationpressure

And here is the link to the formula

http://www.srh.noaa.gov/images/epz/wxcalc/stationPressure.pdf

And here is the link to the compleate weather calculator

http://www.srh.noaa.gov/epz/?n=wxcalc

Air Pressure