Engineering Guide

Pressure Units: A Practitioner's Guide for Engineers — PSI, Bar, kPa, MPa & More

Published June 12, 2026 · by Industrial Unit Converter Editorial Team

Why pressure unit fluency matters

Pressure is the most fragmented measurement in engineering. A single project can encounter seven different units — psi on the compressor nameplate, bar on the European valve data sheet, kPa on the building automation system, inH₂O on the duct static sensor, mmHg on the vacuum pump spec, and MPa on the hydraulic power unit. Converting between these units incorrectly is a direct path to equipment damage, safety incidents, and commissioning delays.

Consider a hydraulic system: a US supplier rates the pump at 3,000 psi. A Japanese valve spec says "maximum 21 MPa." Are they compatible? (21 MPa × 145 = 3,045 psi — yes, just barely.) An engineer who doesn't know the conversion either over-specs the valve (adding cost) or under-specs it (blowing the seal).

This guide covers every pressure unit an engineer encounters daily, when to use each, the gauge-vs-absolute distinction, vacuum ranges, and the conversions worth memorizing.

The pressure units landscape: who uses what, and why

Unit	System	Typical Domain	1 unit =
psi (lb/in²)	US imperial	Pneumatics, hydraulics, gas cylinders, tire pressure	The baseline: 14.7 psi = 1 atm
bar	Metric (non-SI)	European process, pneumatics, hydraulics, scuba	1 bar ≈ 14.5 psi ≈ 100 kPa
kPa (kilopascal)	SI	Building HVAC, low-pressure gas, medical, weather	1 kPa = 0.145 psi
MPa (megapascal)	SI	High-pressure hydraulics, structural, materials	1 MPa = 145 psi = 10 bar
atm (atmosphere)	Scientific	Chemistry, thermodynamics, reference standard	1 atm = 14.696 psi = 1.01325 bar
inH₂O (inches of water)	US imperial	HVAC duct static, cleanroom pressurization	1 inH₂O = 0.0361 psi
inHg (inches of mercury)	US imperial	Vacuum systems, barometers, engine manifold	1 inHg = 0.4912 psi
mmHg / torr	Scientific/medical	Blood pressure, vacuum physics, lab instruments	760 mmHg = 1 atm; 1 torr ≈ 1 mmHg
kg/cm² (technical atmosphere)	Metric (legacy)	Japanese/Korean industrial equipment	1 kg/cm² = 14.22 psi = 0.981 bar

Why different industries chose different units

psi dominates the US because the first industrial revolution's steam engines and hydraulic presses were English-invented and specified in pounds per square inch.
bar dominates Europe because it's close to atmospheric pressure (1 bar ≈ 0.987 atm) and maps naturally to the metric decimal system: 1 bar = 100,000 Pa = 100 kPa — easy to scale.
kPa is used in HVAC because duct pressures are low (<1 psi). Measuring a 0.5 inH₂O duct static in psi (0.018 psi) requires four decimal places — impractical. kPa (0.125 kPa) is easier and SI-compliant.
MPa is used in hydraulics because hydraulic pressures (10-40 MPa) are unwieldy in psi (1,450-5,800 psi) for international specifications. 35 MPa is easier on a nameplate than 5,076 psi.
mmHg / torr survived in vacuum science because mercury manometers were the only reliable vacuum gauges for 200 years, and the U-tube manometer reads directly in mm of mercury column height.

Gauge vs absolute: the distinction that prevents disasters

The concept

Gauge pressure (psig, barg): Zero = local atmospheric pressure. A flat tire reads 0 psig.
Absolute pressure (psia, bara): Zero = perfect vacuum. A flat tire reads ~14.7 psia (at sea level).

psia = psig + atmospheric pressure

At sea level: psia = psig + 14.7 At 5,000 ft (Denver): psia = psig + 12.2 At 10,000 ft: psia = psig + 10.1

When gauge vs absolute matters

Application	Use Gauge or Absolute?	Why
Tire pressure	Gauge	You care about the pressure difference keeping the tire inflated
Compressor discharge	Gauge	Pipe stress and tool performance depend on differential pressure
Gas law calculations (PV = nRT)	Absolute	The ideal gas law requires absolute pressure (P = 0 at vacuum)
Compressor FAD/SCFM ratings	Absolute	Mass flow depends on absolute pressure ratio
Vacuum furnace	Absolute	0.1 torr absolute is meaningless as a gauge number
Altitude correction	Absolute	Atmospheric pressure changes with elevation, gauge zero doesn't
Leak testing (pressure decay)	Absolute	Must track absolute pressure change, not gauge

The most common gauge-vs-absolute error: Using psig in the ideal gas law. If you calculate V₂ = (P₁ × V₁) / P₂ with P₁ = 100 psig instead of 114.7 psia, you're off by 14.7% — enough to mis-spec a receiver tank by a full size.

Worked example: altitude effect on a pressure vessel

A pressure vessel is rated for 150 psig at sea level and tested to 150% of design (225 psig). The same vessel in Denver at 5,280 ft:

Sea level absolute pressure at 225 psig: 225 + 14.7 = 239.7 psia
At Denver (12.2 psia atmosphere), the gauge equivalent: 239.7 − 12.2 = 227.5 psig

The 150 psig rating is a gauge rating — no de-rating needed because the vessel wall sees absolute pressure difference. The safety valve setpoint (in psig) also stays the same. What changes is the SCFM capacity of the compressor feeding the vessel, which drops by the atmospheric pressure ratio. See our SCFM/ACFM/ICFM Guide for the full treatment.

Use our PSIG to PSIA Calculator for gauge-to-absolute conversions with altitude correction.

The pascal: the SI unit of pressure and its measurement

The pascal (Pa) is the SI-derived unit of pressure, defined as one newton per square meter: 1 Pa = 1 N/m². Because 1 Pa is an extraordinarily small pressure (atmospheric pressure is ~101,325 Pa), engineers typically work in kilopascals (kPa = 1,000 Pa) or megapascals (MPa = 1,000,000 Pa).

How pressure is measured

Pressure measurement instruments fall into three categories:

Instrument Type	Principle	Typical Range	Accuracy	Common Use
Manometer (U-tube)	Fluid column height: P = ρ × g × h	0.1-100 inH₂O or mmHg	±1-2%	HVAC duct static, lab calibration
Bourdon tube gauge	Elastic deformation of curved tube under pressure	0-10,000+ psi	±1-3%	Industrial process, compressors, hydraulics
Piezoresistive transducer	Semiconductor strain gauge, electrical output	0-10,000 psi	±0.1-0.5%	Building automation, SCADA, data acquisition
Capacitance diaphragm	Diaphragm deflection changes capacitance	0.001-1,000 psi	±0.1-1%	Cleanroom, pharmaceutical, low-pressure control
Deadweight tester	Pressure balanced against calibrated weights	10-10,000 psi	±0.01-0.05%	Primary calibration standard

The SI pressure unit hierarchy

The pascal unit system spans 15 orders of magnitude through metric prefixes:

Prefix	Symbol	Factor	Equivalent in psi	Engineering Context
nanopascal	nPa	10⁻⁹	1.45 × 10⁻¹³	Acoustic pressure at threshold of hearing
micropascal	μPa	10⁻⁶	1.45 × 10⁻¹⁰	Underwater acoustics, sonar
millipascal	mPa	10⁻³	1.45 × 10⁻⁷	Ultra-high vacuum measurement
pascal	Pa	1	1.45 × 10⁻⁴	SI base unit
hectopascal	hPa	10²	0.0145	Weather maps, barometry (1 hPa = 1 mbar)
kilopascal	kPa	10³	0.145	HVAC, building systems, low-pressure gas
megapascal	MPa	10⁶	145	Hydraulics, structural engineering, materials
gigapascal	GPa	10⁹	145,038	Diamond anvil cells, material science

Key relationship for daily engineering work: 1 kPa = 0.145 psi. This means typical HVAC duct pressures of 125 Pa = 0.125 kPa = 0.018 psi — unwieldy in psi, natural in pascals.

Units within a pascal — what it means

When someone asks "what are the units in a pascal?", they're asking about the fundamental SI base units that compose it. Breaking down 1 Pa = 1 N/m²:

1 N = 1 kg·m/s² (from F = ma)
Therefore: 1 Pa = 1 kg / (m·s²) in SI base units

This decomposition reveals why pressure connects to mass, length, and time — the three fundamental mechanical dimensions. Energy per unit volume (J/m³) has the same units as pressure (Pa), which is why the Bernoulli equation works: P + ½ρv² + ρgh = constant, where every term has units of pressure.

The essential conversion table

From	To	Multiply by	Quick memory reference
psi	bar	0.06895	14.5 psi ≈ 1 bar
bar	psi	14.5038
psi	kPa	6.895	100 psi ≈ 690 kPa
kPa	psi	0.145
psi	MPa	0.006895	1,000 psi ≈ 6.9 MPa
MPa	psi	145.038
bar	kPa	100	Definitional
bar	MPa	0.1	10 bar = 1 MPa
atm	psi	14.696	Standard atmosphere
atm	bar	1.01325	Standard atmosphere
inH₂O	psi	0.0361	27.7 inH₂O = 1 psi
inHg	psi	0.4912	2.036 inHg = 1 psi
mmHg (torr)	psi	0.01934	51.7 mmHg = 1 psi
kg/cm²	psi	14.223
Pa	psi	0.000145	6,895 Pa = 1 psi

Conversions worth memorizing (no calculator needed):

1 bar ≈ 14.5 psi — the most common industrial conversion
100 kPa = 1 bar ≈ 14.5 psi — metric round numbers
1 atm = 14.7 psi = 1.013 bar — the standard reference
1 MPa ≈ 145 psi — hydraulic systems
1 inH₂O = 0.036 psi — HVAC duct static
27.7 inH₂O = 1 psi — the inverse of #5

Vacuum pressure ranges

Vacuum is not one range — it spans 15 orders of magnitude from atmospheric pressure to deep space. Different units dominate each range.

Vacuum Range	Pressure (absolute)	Typical Units Used	Application
Rough vacuum	760 to 1 torr (atm to 1 mmHg)	inHg, torr, mbar	Vacuum packaging, pick-and-place, brake boosters
Medium vacuum	1 to 10⁻³ torr	torr, mbar	Vacuum drying, distillation, plasma cleaning
High vacuum	10⁻³ to 10⁻⁸ torr	torr, mbar	Electron microscopes, mass spectrometers, coating
Ultra-high vacuum	10⁻⁸ to 10⁻¹² torr	torr (log scale)	Semiconductor fabrication, particle accelerators, surface science

Key conversion for vacuum work:

29.92 inHg = 760 mmHg = 760 torr = 1 atm = 14.696 psi = 1.013 bar

InHg vs mmHg/torr convention by region:

US vacuum systems are rated in inHg (inches of mercury vacuum), typically 0-29.92 inHg
European and scientific vacuum systems use torr or mbar (1 mbar = 0.75 torr)

Use our PSI to Bar Calculator for general pressure conversions and PSIG to PSIA Calculator for absolute pressure.

Hydraulic pressure: the MPa domain

Hydraulic systems operate at pressures that make other units awkward. A 3,000 psi pump is 20.7 MPa — the MPa number is cleaner, which is why international hydraulic specifications use MPa almost exclusively.

Hydraulic Class	Typical Pressure (psi)	Typical Pressure (MPa)	Application
Low pressure	<1,000	<7	Lubrication, cooling
Medium pressure	1,000-2,000	7-14	Machine tools, mobile equipment
High pressure	2,000-5,000	14-35	Industrial presses, injection molding
Ultra-high pressure	5,000-10,000+	35-70+	Water jet cutting, hydrostatic testing

The hydraulic conversion you'll use most: MPa × 145 = psi. For quick mental math, 1 MPa ≈ 150 psi (the 3% error is acceptable for feasibility checks).

Use our MPa to PSI Calculator for precise conversions.

Frequently asked questions

Q: Why doesn't the US switch to bar like Europe?

The installed base of US industrial equipment is specified in psi — compressors, valves, fittings, gauges, safety devices, and standards (ASME, API, NFPA) all use psi. The cost of dual-labeling and retraining would be enormous, and the risk of conversion error during the transition period is considered greater than the cost of maintaining the dual-unit system. In practice, most US industrial engineers are bilingual in psi and bar. For SI conversions in daily work, see our HVAC Engineer's Conversion Cheat Sheet.

Q: Is 1 bar exactly 100 kPa?

Yes — 1 bar = 100,000 Pa = 100 kPa, by definition in SI. Confusingly, 1 atm = 101,325 Pa = 101.325 kPa, so 1 bar is approximately but not exactly 1 atmosphere. The 1.3% difference matters in precise gas law calculations but is negligible for most industrial pressure spec comparisons.

Q: What's the difference between mmHg and torr?

For all practical engineering purposes, they're identical (1 mmHg ≈ 1 torr, difference <0.000015%). The torr was defined as exactly 1/760 of a standard atmosphere (1 atm = 760 torr), while 1 mmHg is the pressure exerted by a 1 mm column of mercury at 0°C under standard gravity. The definitions differ by about 0.000015%, which is completely negligible at any measurable pressure.

Q: How do I convert between -30 inHg vacuum and absolute pressure?

Vacuum in inHg is typically expressed as "inches of mercury vacuum" — the difference between atmospheric pressure and the measured absolute pressure. A "30 inHg vacuum" gauge at sea level means:

P_absolute = 29.92 − 30.00 = negative — wait, that can't happen. A vacuum gauge reading of -30 inHg means the gauge reads 30 inHg below atmospheric. So P_absolute = 29.92 − 30 = -0.08 inHg absolute — impossible. In practice, "30 inHg vacuum" means the gauge is pegged at its maximum and the actual absolute pressure is somewhere between 0 and ~1 torr. Any reading above 29 inHg on a mechanical gauge should be treated as an approximation.

Q: When should I use kg/cm² instead of bar?

Almost never in new designs. kg/cm² (also called "technical atmosphere" or "at") is a legacy unit still found on Japanese, Korean, and some older European equipment. 1 kg/cm² = 0.981 bar — the 2% difference has caused countless specification errors. When you encounter kg/cm² on legacy equipment, convert to bar or MPa for all new calculations and specify replacement equipment in SI units.

Key takeaways

Concept	Rule
Gauge vs absolute	Gauge reads zero at atmosphere; absolute reads zero at vacuum. Know which your formula requires.
Gas law	Always use absolute pressure (P_abs = P_gauge + P_atm). Using psig directly gives errors of 12-18%.
Altitude	Atmospheric pressure drops ~0.5 psi per 1,000 ft. Correct absolute pressure for altitude.
HVAC	Use inH₂O or Pa for duct pressures (<1 psi). Using psi requires too many decimal places.
Hydraulics	Use MPa for international specs; 1 MPa ≈ 145 psi. Know the domains: bar for European pneumatics, kPa for building systems, MPa for hydraulics, psi for US industrial.
Vacuum	Use inHg (US) or torr/mbar (SI). Know the ranges: rough (760-1 torr), medium (1-10⁻³), high (10⁻³-10⁻⁸), ultra-high (<10⁻⁸).
kg/cm²	Legacy unit — convert to bar or MPa for all new work. 1 kg/cm² = 0.981 bar.