Inches of Mercury to PSI Converter

Converting mercury column measurements (in Hg) to pressure units (psi) for barometric pressure reporting, vacuum gauge calibration, and absolute pressure instrumentation.

Weather station barometers in the US display in in Hg (standard sea-level pressure is 29.92 in Hg, with a typical continental range of 28.5 to 31.0 in Hg corresponding to 985 to 1,070 millibars) while the same instruments elsewhere read in mmHg, millibars, or hPa — converting to psi is rarely required for weather reporting but appears in aerospace and high-altitude engineering, where atmospheric pressure in psi decreases with altitude per the ISA model (sea level 14.696 psi, 5,000 ft 12.23 psi or 24.89 in Hg, 10,000 ft 10.11 psi or 20.58 in Hg, 25,000 ft 5.46 psi or 11.10 in Hg).

Aircraft altimeters use the in Hg to barometric pressure relationship directly, and pilots reading US-certified altimeters adjust the Kollsman window to the local altimeter setting in in Hg — the conversion to psi is used in engine performance calculations and cabin pressurization specifications.

Mercury barometers and manometers in laboratory and process measurement remain in use as primary standards for low-pressure calibration, even though mercury has been phased out of many applications due to toxicity concerns — the National Institute of Standards and Technology (NIST) and other national laboratories use mercury manometers as the primary realization of pressure in the 0.1 Pa to 100 kPa range, with the in Hg to psi conversion being fundamental to the calibration certificate.

Vacuum pump capacity and ultimate vacuum specifications for laboratory and process use are quoted in in Hg (for 'rough' pumps down to about 29.9 in Hg), mmHg or torr (for 'medium' vacuum down to 10⁻³ torr), and microns (10⁻³ torr) for high vacuum — the conversion between in Hg and psi is most commonly needed at the boundary between atmospheric (where psi is natural) and vacuum (where in Hg is natural).

Engine manifold vacuum gauges for automotive diagnosis read in in Hg (idle 18–22, cruise 15–18, deceleration up to 24+) and converting to psi below atmospheric is necessary to size vacuum-operated accessories such as brake boosters (which multiply vacuum by diaphragm area to produce pushrod force), HVAC mode doors, and EGR valve actuators.

HVAC altitude compensation for combustion appliances requires adjusting gas orifice pressure and combustion air supply based on local atmospheric pressure (in Hg or psi from the barometer), with derating per ANSI Z21 for elevations above 2,000 ft where atmospheric pressure drops below about 27.5 in Hg.

Medical suction (aspiration) systems in hospitals and dental offices are calibrated in in Hg or mmHg, with typical therapeutic ranges from 0 to -29.5 in Hg (0 to -14.5 psi below atmospheric) for general suction and 0 to -9.5 in Hg for neonatal and delicate procedures — the conversion to psi is needed when sizing vacuum pumps and verifying regulator settings against gauge readings.