[fdelre]

1 inchHg = 25.4 mmhg
18 inchHg = 457,2 mmHg

In Il2 FM this could be emulated using:

Normal max rating no boost 48.4 = 1229 mm Hg = 1.62 atm (100)

Take off rating 54.3 inchHg = 1380 mmHg = 1.81 atm (110)

Military rating 66.4 inchHg = 1686 mmHg = 2,22 atm (110+MW)

conversions have been rounded