


If I understand your question correctly, you want to know how to calculate the atmospheric pressure in pounds per square inch given the altitude. Below is a discussion of how the Atmospheric Properties Calculator works that I hope meets your needs. The following equations are based on material you should be able to find in any good basic aerodynamics textbook in a chapter covering atmospheric properties. All units are English, but the methodolgy can easily be adapted for use in the Metric system.
between 36,089 ft and 65,617 ft
Delta = 0.223361 * exp( (36089h)/20806 )
between 65,617 ft and 104,987 ft
Delta = (0.988626 + h/652600)^(34.16319)
between 104,987 ft and 154,199 ft
Delta = (0.898309 + h/181373)^(12.20114)
between 154,199 ft and 167,323 ft
Delta = 0.00109456 * exp( (h  154200)/25992 )
between 167,323 ft and 232,940 ft
Delta = (0.838263  h/577922)^(12.20114)
convert  into  multiply by 

psf  psi  1 / 144 
psf  atm  1 / 2116.224 
psf  kPa  0.0478927 
between 36,089 ft and 65,617 ft
Theta = 0.751865
Sigma = 0.297076 * exp( (36089  h)/20806 )
between 65,617 ft and 104,987 ft
Theta = 0.682457 + h/945374
Sigma = (0.978261 + h/659515)^(35.16319)
between 104,987 ft and 154,199 ft
Theta = 0.482561 + h/337634
Sigma = (0.857003 + h/190115)^(13.20114)
between 154,199 ft and 167,323 ft
Theta = 0.939268
Sigma = 0.00116533 * exp( (h  154200)/25992 )
between 167,323 ft and 232,940 ft
Theta = 1.434843  h/337634
Sigma = (0.79899  h/606330)^(11.20114)
convert  into  conversion 

Rankine  Fahrenheit  subtract 460 
Rankine  Kelvin  divide by 1.8 
Kelvin  Celsius  subtract 273 
convert  into  multiply by 

sl/ft³  lb_{m}/ft³  32.174 
sl/ft³  kg/m³  536.523 
convert  into  multiply by 

ft/sec  mph  0.6818 
ft/sec  m/sec  0.3048 
ft/sec  km/h  1.097 
ft/sec  knots  0.5921 
sl/ft/sec  kg/m/sec  47.88 
ft²/sec  m²/sec  0.0929 
Also, the aforementioned Atmospheric Properties Calculator uses
these equations to compute the above quantities automatically given nothing more than the altitude in English or
Metric units. The calculator can also be used to find a number of other properties when provided with a velocity
and a reference length of the body under study.
 answer by Jeff Scott, 21 October 2001
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