All Pressure Units

How to Convert Pressure Units?

Pressure conversion enables you to translate force per unit area measurements between different systems. Pressure is defined as force applied perpendicular to a surface divided by the area over which that force acts. Different industries and regions use various pressure units, making conversion essential for international collaboration and technical work.

The key conversion factors are:
- 1 bar = 100,000 Pa = 14.504 psi
- 1 atm = 101,325 Pa = 1.01325 bar = 14.696 psi
- 1 psi = 6,894.76 Pa = 0.0689 bar
- 1 mmHg = 133.322 Pa = 1 Torr

Frequently Asked Questions

What units of pressure does this converter handle?

This converter handles the SI pascal (Pa) and its standard prefixed forms (kPa, MPa, GPa), the bar family (millibar, bar), the atmosphere (atm), the torr (mmHg), pounds per square inch (psi), and millimeters of water (mmH2O). 1 Pa equals 1 N/m^2 - one newton of force distributed over one square meter. The pascal is the SI derived unit, very small for everyday use (atmospheric pressure is about 101,325 Pa), so kPa, bar, and atm are far more common in practice. The converter does not include vacuum-specific units like millitorr below the torr range, nor the obsolete 'technical atmosphere' (at = 98.0665 kPa, distinct from atm).

What are the exact conversion factors between common pressure units?

1 bar = exactly 100,000 Pa = 100 kPa (defined as exactly 10^5 Pa). 1 atmosphere (atm) = exactly 101,325 Pa (defined since 1954). 1 psi = exactly 6894.757293168361 Pa (derived from 1 lbf = 4.4482216152605 N exactly and 1 inch = 25.4 mm exactly). 1 torr = exactly 101325/760 Pa = approximately 133.3224 Pa. 1 mmHg is approximately equal to but not exactly identical to 1 torr; ISO defines 1 mmHg = exactly 133.322387415 Pa (using mercury density 13595.1 kg/m^3 and g = 9.80665 m/s^2). For practical purposes treat 1 mmHg = 1 torr to 5-figure precision.

When should I use Pa, kPa, bar, atm, or psi?

Use Pa for SI-strict scientific work and small pressure differences (room ventilation pressures are 10 to 100 Pa). Use kPa for atmospheric pressure (101.325 kPa standard), weather (sea-level varies 95 to 105 kPa), and blood pressure where SI-preferred. Use bar for industrial process pressures, tire pressures in metric countries (2.2 bar is typical), and SCUBA gas (compressed cylinders 200 to 300 bar). Use atm in chemistry and physics textbooks. Use psi for tire pressures in the US (32 psi typical), industrial pneumatics (80 to 120 psi), hydraulic equipment (1500 to 5000 psi), and water-supply specs. Use mmHg or torr exclusively for blood pressure (120/80 mmHg) and vacuum-system specs.

How precise are the conversions and what about gauge vs absolute?

The conversions use the exact defining factors above and 64-bit float arithmetic, giving 15+ significant digits internally. However, the gauge-vs-absolute distinction is critical and NOT handled mathematically here: 'psig' = pressure above atmospheric (gauge), 'psia' = pressure above absolute zero. A tire at 32 psig actually has 46.7 psia at sea level. The converter assumes your number is one consistent type; you must add or subtract atmospheric pressure (101.325 kPa = 14.696 psi) to switch between them. For meteorological pressures, always absolute. For tire pressures and pressure vessels, almost always gauge. Misreading this is responsible for many engineering errors.

What are common pressure gotchas?

First, gauge vs absolute (above). Second, US 'inHg' (inches of mercury) vs mmHg vs torr - all mercury-column units but with different temperature and gravity assumptions. Third, the 'feet of water column' for hydraulic head (1 ft H2O at 4 C = 2989.07 Pa). Fourth, hPa (hectopascal) used in weather forecasts is identical to millibar (1 hPa = 1 mbar = 100 Pa) - they renamed mbar to hPa around 1985 with no numeric change. Fifth, tire pressure 'cold' vs 'hot' differs by 10 to 15% from heating. Sixth, pressure is gauge in everyday speech ('the pump puts out 60 psi') but absolute in thermodynamic equations (PV = nRT requires absolute P).

What is the difference between pressure, stress, and pressure head?

Pressure is force per area on a fluid or gas, isotropic (same in all directions for a static fluid). Stress (also force per area) applies to solids and is a tensor with shear and normal components. Pressure head is the hydrostatic equivalent height of fluid column: head = pressure / (rho * g). So 1 atm = 10.33 m of water column = 760 mm of mercury column. This is why blood-pressure cuffs and barometers historically used liquid columns: they directly read pressure as height. Pumps are often rated in head (meters) rather than pressure (Pa) because head is independent of fluid density. The converter treats all inputs as fluid pressure; head conversions need additional fluid-density data.

How is the pascal defined in the modern SI system?

The pascal is an SI derived unit: 1 Pa = 1 N/m^2 = 1 kg/(m*s^2). After the 2019 SI redefinition, all base units are fixed via universal constants: kilogram (Planck constant h), meter (speed of light c), second (caesium frequency). The pascal therefore inherits the exactness of these constants. Primary pressure realization uses mercury manometers traceable to NIST's barometric standards, piston gauges (calibrated by lifted-mass and area), and modern force-balanced quartz transducers. Below 1 Pa (high vacuum), pressure is realized by orifice-flow and spinning-rotor gauges. Above 1 GPa (ultra-high pressure), diamond-anvil cells and fixed-point ruby fluorescence shifts provide secondary standards. BIPM and the CCM coordinate global pressure metrology.

What are pressure edge cases at extreme scales?

Very low pressures: interplanetary space is ~10^-12 Pa, ultra-high-vacuum lab chambers reach 10^-9 Pa, intergalactic gas is 10^-17 Pa. Very high pressures: ocean trench bottoms reach 110 MPa (Mariana Trench at 11 km depth); diamond-anvil cells hit 600 GPa in labs; the Earth's inner core is ~360 GPa; the Sun's core is ~25 PPa (25 x 10^15 Pa); neutron stars reach 10^33 Pa. The converter handles arbitrary numeric magnitudes, but at extremes the gas/fluid model changes: ideal gas fails above ~10 MPa, water becomes compressible at hundreds of MPa, and solids enter the plasma state at PPa scales. The unit math stays the same; the underlying physics shifts dramatically.

Units

Pascal (Pa)

Pascal is the SI unit of pressure, named after French mathematician Blaise Pascal. One pascal equals one newton per square meter (N/m2). While pascals are the scientific standard, they represent very small pressures, so kilopascals (kPa) or megapascals (MPa) are often used in engineering and industrial applications.

Bar

Bar is a metric unit of pressure equal to 100,000 pascals. It is widely used in meteorology, oceanography, and industrial applications, particularly in Europe. One bar is approximately equal to atmospheric pressure at sea level (0.987 atm). The millibar (mbar) is commonly used for weather forecasting and barometric pressure readings.

PSI (Pounds per Square Inch)

PSI is the standard pressure unit in the United States and is widely used in automotive, aerospace, and industrial applications. It measures the force in pounds applied to one square inch of surface area. Tire pressures, hydraulic systems, and compressed gas cylinders commonly use psi measurements.

Atmosphere (atm)

Standard atmosphere is a unit of pressure defined as 101,325 Pa. It represents the average atmospheric pressure at sea level on Earth. This unit is commonly used in chemistry, physics, and diving applications to describe ambient pressure conditions. One atm equals approximately 14.7 psi or 1.013 bar.

mmHg (Millimeters of Mercury)

Millimeters of mercury is a pressure unit based on the height of a mercury column in a manometer. It is primarily used in medicine for blood pressure measurements (sphygmomanometer readings) and in meteorology for barometric pressure. Normal blood pressure is typically expressed as 120/80 mmHg. One mmHg equals exactly one Torr.

Common Pressure Conversions