29.92" HG or 1013.2 millibars or 101.3kpa at 15°C.
A column of air one square inch in cross-section, measured from sea level to the top of the atmosphere, would weigh approximately 14.7lbs.
"From high to low, look out below." This means that flying from a high pressure system to a low pressure system, you will be at a lower altitude than your altimeter reads if you never adjusted the pressure setting. The same applies to flying from warm air to cold air.
- Atmospheric Pressure - Wikipedia