Atmospheric Pressure

Atmospheric pressure at sea level = 14.7 psi
For each foot of water depth, pressure increases 0.445 psi
Pressure doubles every 33 feet
Example:
Calculate pressure at a depth of 66’
66’ x 0.445 psi / ft = 29.37 + 14.7 = 44.1 = 3 atmospheres
For each 1,000 feet in altitude, pressure decreases 1 foot of head or 0.433 psi, so at 2,000 feet above sea level, pressure would be 14.7 – 2 x 0.433 = 13.83

Barometric pressure
Average at sea level: 29.92 inches = 1013.2 millibars (range 950 mb – 1035 mb)
Inches of mercury x 33.86 = Millibars
Millibar ÷ 33.86 = Inches of mercury