That could depend on several different factors, such as altitude etc...
I do believe that at about sea level its -14.7 psi.
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27.7 "h2o
Inches of mercury x 0.4912 = psi
Inches of water x 0.03613 equals pounds per square inch (psi).
10" Water Column equals about .4 Pounds per Square Inch or 0.36127291827412783psi exactly.
About 2 1 pound per square inch = 2.0362... inches of mercury