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Pressure altitude is the altitude read on an altimeter set to the standard datum of 1013.25 hPa (29.92 inHg). Each hectopascal below standard raises pressure altitude by roughly 27 ft, so a low pressure day puts the aircraft at a higher pressure altitude than the field elevation suggests.
Pressure altitude
PA = elevation + (1013.25 - QNH) x 27 ft per hPa
Aviation results are planning estimates only. Always verify take-off, landing, fuel, weight and balance figures against the certified Aircraft Flight Manual, current performance charts and applicable aviation regulations before flight.
Near sea level the atmosphere loses about 1 hPa every 27 ft, so the altimeter's aneroid scale converts pressure setting changes at roughly that rate.
Only when the altimeter setting is exactly 1013.25 hPa (29.92 inHg), the standard sea-level pressure.