Bar to PSIG Converter

The Bar to PSIG Converter handles a pressure-reference problem, not just a unit-size problem. Bar and psi can both be absolute or gauge units, but PSIG is explicitly gauge pressure: pounds per square inch above the surrounding atmosphere. The calculator therefore asks whether your input bar value is absolute bar or gauge bar. That choice changes the formula. Absolute bar must be converted to psia and then shifted down by an atmospheric offset. Gauge bar has already removed atmosphere, so it converts directly to psig.

This distinction is essential for tires, compressors, pressure regulators, refrigeration service, vacuum systems, and process equipment. A tire label in bar usually means gauge pressure. A thermodynamic table in bar often means absolute pressure. A vacuum gauge might report a negative gauge pressure or a small absolute pressure. The calculator derives psi per bar from the exact pound, inch, and standard-gravity definitions. For absolute input it uses the standard atmosphere, 101,325 Pa, as the gauge reference; local atmospheric pressure can differ.

Gauge pressure versus absolute pressure

Absolute pressure starts at a perfect vacuum. Gauge pressure starts at local atmospheric pressure and reads zero when the system is open to the air. PSIA means pounds per square inch absolute. PSIG means pounds per square inch gauge. Bar absolute may be written as bara or bar(a), while gauge bar may be written as barg or bar(g).

At standard atmosphere, the offset is about 14.7 psi. An absolute pressure of one standard atmosphere is therefore about 14.7 psia but 0 psig. Because one bar is slightly below one standard atmosphere, 1 bar absolute converts to a small negative psig value. That does not mean negative absolute pressure; it means the pressure is slightly below the atmospheric reference used by the calculator.

Formula

The calculation first calculates psia from the bar input:

$\text{psia} = \text{bar} \times 14.5037737730209$

If the basis is absolute bar, it subtracts the atmospheric offset:

$\text{psig} = \left(\text{bar}_{\text{absolute}} \times 14.5037737730209\right) - 14.6959$

If the basis is gauge bar, it does not subtract anything:

$\text{psig} = \text{bar}_{\text{gauge}} \times 14.5037737730209$

The calculator also displays the gauge pressure in bar. For absolute inputs, it subtracts the standard atmospheric pressure expressed in bar. For gauge inputs, the gauge bar row is the same as the input.

Bar to PSIG example

With the default input of 10 bar and absolute bar selected, the calculation gives:

$\text{psia} = 10 \times 14.5037737730209 = 145.037737730$

Then it subtracts the standard atmospheric offset:

$\text{psig} = 145.037737730 - 14.6959 = 130.341788955$

Rounded like the primary result, the display shows 130.3418 psig. The item rows show 145.037737730 psia before gauge offset, 14.695949 psi atmospheric offset, and about 8.98675 bar(g) for the gauge-pressure equivalent. If you switch the basis to gauge bar while keeping 10 bar, the calculator skips the offset and displays 145.0377 psig.

Reference table

Domains and examples

Automotive tire pressures are a classic gauge-pressure case. A tire specified at 2.4 bar is normally 2.4 bar(g), so the psig result is about 34.8091 psig. Subtracting 14.6959 would be wrong because the gauge reading already starts at atmosphere. Compressors and pneumatic tools behave the same way: shop gauges usually read psig or bar(g).

Vacuum and process work often use absolute pressure. A chamber at 0.2 bar absolute converts to 2.9008 psia and then to about -11.7951 psig. That negative gauge value is a vacuum relative to atmosphere, not an impossible negative absolute pressure. For vacuum units near mercury columns, compare Torr to atm conversion. For ordinary unit-size conversions without gauge offsets, use bar to psi, kPa converter, or the broader pressure converter.

Precision and pitfalls

The most dangerous mistake is choosing the wrong basis. A 5 bar(g) compressor setting is 72.5189 psig. A 5 bar absolute process condition is 57.8229 psig after the offset. Those are different by exactly the atmospheric offset used here. The second mistake is assuming local atmosphere is always 14.695949 psi. It is a standard value; field calibration at elevation may need local barometric pressure. The third mistake is using “psi” without the suffix. When safety relief valves, tires, and pressure vessels are involved, write psig or psia whenever the reference matters.

Sources

• NIST, Guide for the Use of the International System of Units — Appendix B conversion factors and pressure-unit SI usage guidance.
• NIST, Standard atmosphere constant — standard atmosphere value 101,325 Pa used for the gauge offset.
• BIPM, The International System of Units brochure — official SI framework for pressure units.