About

In thermodynamics and fluid mechanics, distinguishing between Absolute Pressure and Gauge Pressure is vital. Absolute pressure references a perfect vacuum (zero pressure), whereas Gauge pressure references the ambient atmospheric pressure. Most pressure gauges on equipment (like tire pumps or boilers) read zero when open to the air; this is Gauge Pressure. However, scientific calculations often require Absolute Pressure. This tool converts between the two by adjusting for atmospheric pressure, which defaults to standard sea level (14.7 PSI or 101.325 kPa) but can be customized for high-altitude engineering.

Formulas

The relationship is linear but dependent on the local atmospheric pressure ($P_{atm}$).

P_gauge = P_abs − P_atm

If the result is negative, it indicates a vacuum (pressure below atmospheric). In SI units, $P_{atm}$ is standardly 101,325 Pa.

Reference Data

Condition	Absolute Pressure ($P_{abs}$)	Gauge Pressure ($P_{g}$)	Notes
Perfect Vacuum	0 psi	-14.7 psi	Maximum theoretical vacuum
Atmospheric (Sea Level)	14.7 psi	0 psi	Open air
Car Tire (Typical)	46.7 psi	32 psi	Gauge reads 32
Espresso Extraction	10 bar	9 bar	1 bar approx atm
HVAC System (High)	264.7 psi	250 psi	Refrigerant Line
Deep Space	≈ 0 Pa	-101.3 kPa	Near absolute zero
Denver, CO (Altitude)	12.1 psi	0 psi	Reads 0 at altitude

Frequently Asked Questions

PSIA stands for Pounds per Square Inch Absolute, and PSIG stands for Pounds per Square Inch Gauge. PSIA includes the weight of the atmosphere, while PSIG ignores it. If your tire gauge says 30 PSI, that is PSIG. The PSIA would be 30 + 14.7 = 44.7 PSI.

Atmospheric pressure decreases as altitude increases. At sea level, it is 14.7 PSI. In a high city like Denver (5000 ft), it drops to about 12.1 PSI. A gauge calibrated at sea level might read differently at altitude if it relies on a sealed reference chamber, though most mechanical gauges vent to the ambient air and self-correct.

Yes. Negative gauge pressure indicates a partial vacuum (pressure lower than the surrounding atmosphere). This is common in suction lines, condensers, and medical vacuum systems.