The easiest way is to use a standard atmospheric table like the one here: http://modelweb.gsfc.nasa.gov/atmos/us_standard.html
http://hypertextbook.com/facts/2000/RachelChu.shtmlIf you want to get down and dirty with density calculations, look around for NIST's publication NBSIR 77-1278 The Air Density Equation and the Transfer of the Mass Unit.