How do I convert bar to PSI?

Multiply bar by 14.5038. Example: 2.5 bar × 14.5038 = 36.26 psi. To convert PSI to bar, divide by 14.5038.

What is the difference between kPa and MPa?

1 MPa = 1,000 kPa. kPa is used for everyday pressures like tyre inflation (200–250 kPa) and blood pressure (10–16 kPa diastolic). MPa is used for higher pressures like concrete compressive strength (20–50 MPa) or hydraulic systems.

A torr is approximately equal to 1 mmHg (millimetre of mercury) and is used in vacuum technology and physiology. 1 torr = 133.322 Pa. Standard atmospheric pressure = 760 torr.

Is water pressure measured in PSI or bar in plumbing?

In North America, psi is standard. In Europe and most other countries, bar or kPa is used. Typical residential water pressure is 40–80 psi (2.76–5.52 bar or 276–552 kPa).

Common Pressure Units: PSI, Bar, kPa, atm, and MPa Explained

Pressure is force per unit area, and its units reflect the different industries and national standards that have evolved over centuries. This guide explains what each common pressure unit means, where it is used, and how to convert between them accurately.

Published March 19, 2026

Key takeaways

PSI (pounds per square inch) dominates in US industry, automotive, and plumbing.
Bar and millibar are widely used in meteorology and European engineering.
The pascal (Pa) is the SI unit; kPa and MPa are its practical multiples.
1 standard atmosphere = 101.325 kPa = 14.696 psi = 1.01325 bar exactly.
Gauge pressure (psig) is relative to ambient; absolute pressure (psia) is relative to a perfect vacuum.

The pascal: SI base unit of pressure

The pascal (Pa) is the SI unit of pressure, defined as one newton per square metre (N/m²). It is named after Blaise Pascal, who established the principle that pressure in a fluid is transmitted equally in all directions.

Because 1 Pa is very small (a US dollar bill exerts about 0.75 Pa lying flat), practical engineering uses kilopascals (kPa = 1,000 Pa) and megapascals (MPa = 1,000,000 Pa). Tyre pressure is typically 200–250 kPa; steel yield strength is measured in MPa; atmospheric pressure is ~101.3 kPa.

PSI — pounds per square inch

PSI (lbf/in²) is the dominant pressure unit in the United States and is still widely used in the UK for tyre pressure. It measures the force in pounds-force applied over one square inch of area.

Common reference values:

Car tyre pressure: 30–35 psi
Bicycle road tyre: 80–120 psi
Hydraulic system: 1,000–3,000 psi
Water main pressure: 40–80 psi
Atmospheric pressure at sea level: 14.696 psi

Bar and millibar

The bar is defined as exactly 100,000 Pa (100 kPa). It is not an SI unit but is accepted for use with SI and is widely used in meteorology, diving, and European industrial applications.

Atmospheric pressure at sea level is approximately 1.01325 bar, so 1 bar is close to — but slightly less than — 1 atm. Weather maps use hectopascals (hPa), which are numerically identical to millibar: standard sea-level pressure is 1013.25 hPa = 1013.25 mbar.

Standard atmosphere (atm)

Formula

1 atm = 101,325 Pa (exactly)
1 atm = 14.6959 psi
1 atm = 1.01325 bar
1 atm = 760 mmHg (torr)

The standard atmosphere (atm) is defined by its exact value in pascals and is used as a reference for reporting gas properties, altitude-pressure relationships, and diving depths. 1 atm of pressure is added for every ~10 m of seawater depth.

Gauge vs absolute pressure

Important

Absolute pressure (psia, kPa abs) is measured relative to a perfect vacuum (0 pressure). Gauge pressure (psig, kPa g) is measured relative to the local atmospheric pressure.

Gauge pressure = Absolute pressure − Atmospheric pressure

A car tyre at '32 psi' means 32 psig — the pressure above atmospheric. Its absolute pressure is 32 + 14.7 ≈ 46.7 psia. Gas laws (PV = nRT) require absolute pressure. Mixing up gauge and absolute is a common and potentially dangerous error.

Pressure unit conversion table

Formula

Quick reference — all values relative to 1 standard atmosphere:

 Unit     Value per 1 atm   Typical use
 Pa       101,325           SI base unit; very small values
 kPa      101.325           Tyre pressure (200–250 kPa), HVAC
 MPa      0.101325          Concrete strength, hydraulic systems
 bar      1.01325           Meteorology, European industry
 psi      14.6959           US/UK tyres, plumbing, hydraulics
 mmHg     760               Medical blood pressure, laboratory vacuum
 atm      1 (reference)     Diving, chemistry, altitude calculations

Conversions:
1 bar = 14.5038 psi = 100 kPa = 0.986923 atm
1 psi = 0.0689476 bar = 6.89476 kPa = 6,894.76 Pa
1 MPa = 145.038 psi = 10 bar

When to use each pressure unit

PSI: use when reading US or UK tyre pressure specifications, US plumbing codes, or any document from a North American engineering context. Most US pressure gauges read in psi.

Bar: use in European contexts, meteorology (weather maps use hPa = mbar), and scuba diving (tank pressure is typically 200–300 bar). Bar is intuitive because 1 bar ≈ 1 atm at sea level.

kPa and MPa: use for SI-standard engineering documents. Tyre pressure in kPa is the standard in most countries outside the US (e.g., 220 kPa is a typical car tyre). MPa is used for concrete mix design (28 MPa compressive strength), structural steel yield strength (250 MPa), and hydraulic system design.

atm: use in chemistry, gas law calculations, and diving depth tables. Also convenient for atmospheric science where sea-level pressure serves as a natural reference.