Does anyone have a good calculation for calculating atmospheric at a given elevation in metric units? I have a formula for the English calculation for this but it does not hold true by simply changing to metric.

I don't undestand - if you have a formula that you like in english units, why can't you convert it to metric? If you do the conversion properly the metric version would be just as accurate as the english version.

Not sure what I'm doing wrong;

English
Atmospheric Pressure (PSIA) = Standard Press (PSIA) - [Air density (lb/ft3) * (460+Standard Temp)/(460+Site Temp)*Site Elevation (ft) / 144]

Metric
Atmospheric Pressure (bar) = Standard Press (bar) - [Air density (kg/m3) * (273.15+Standard Temp)/(273.15+Site Temp)*Site Elevation (m)]

The English works out fine, but the metric doesn't?

Two problems with the units on the right hand side of the equation:

1. You need to multiply air density by g ( the force of gravity) to get a weight per cubic meter. This gets the right hand term after you multiply be elevation into units of Nt/m^2.
2. A bar is equal to 100 K-Newton per meter squared. So you then need to divide by 100,000 (much as you divided by 144 in the english units version).

One other thing: I really don't understand your equation - where did you get it from? Specifically your equation says that air pressure can go negative if the elevation is a big enough number, or if the temp is low enough.

I'm not sure where that equation came from. I did notice that at higher altitudes, the equation is incorrect.

I was able to find another equation which I think is correct;

Atmospheric Pressure (in Hg) = Std Pressure (in Hg) + [0.00000001357*Elevation (ft)^2] - [0.001071*Elevation (ft)]