About the Pressure Converter

The pressure converter translates between nine units of pressure: pascal (the SI unit), kilopascal, megapascal, bar, millibar, atmosphere, pound per square inch, millimetre of mercury, and torr. All conversions are expressed in pascals as the pivot unit, with the factors derived from the standard definitions.

The pascal is the SI derived unit of pressure, defined as one newton per square metre. It is named after Blaise Pascal, the 17th-century French mathematician and physicist whose experiments demonstrated that atmospheric pressure decreases with altitude. Per the SI Brochure 9th edition, the pascal is derived from the SI base units of kilogram, metre, and second.

The atmosphere (atm) was originally defined as the pressure exerted by a 760-millimetre column of mercury at sea level, which is approximately 101,325 pascals. Since 1954, the standard atmosphere has been defined as exactly 101,325 Pa. The bar is defined as exactly 100,000 pascals (close to one atmosphere), and the millibar is exactly 100 pascals.

The pound per square inch (psi) is the imperial unit of pressure, defined as one pound-force per square inch. One psi equals approximately 6894.76 pascals. The millimetre of mercury (mmHg) and torr are nearly identical units based on the height of a mercury column; the torr is defined as exactly 1/760 of a standard atmosphere, or 133.322 Pa.

How It Works

The converter uses the two-step method through pascals:

base_in_pa = value * factor[from]
result = base_in_pa / factor[to]

The factor table (in Pa) is:

Pa  = 1 Pa          kPa = 1,000 Pa        MPa = 1,000,000 Pa
bar = 100,000 Pa     mbar = 100 Pa          atm = 101,325 Pa
psi = 6,894.76 Pa    mmHg = 133.322 Pa       torr = 133.322 Pa

The metric factors are exact powers of 1000 (kPa, MPa) or 100,000 (bar). The standard atmosphere is exactly 101,325 Pa by the 1954 definition. The psi is approximately 6894.76 Pa, derived from the imperial definitions of the pound-force and the inch.

The mmHg and torr are nearly identical but defined slightly differently. The mmHg is the pressure exerted by a 1-millimetre column of mercury under standard gravity, which depends on the density of mercury (approximately 13,595.1 kg/m³). The torr was redefined in 1954 as exactly 1/760 of a standard atmosphere, or 133.322 Pa, removing the dependence on mercury density.

The two-step method through pascals keeps the code simple and matches the SI convention. The pivot unit also makes the magnitude of the input obvious: atmospheric pressure is about 101,325 Pa, while tyre pressure is typically 30 to 35 psi (about 200,000 to 240,000 Pa).

Worked Examples

Converting 1 atmosphere to psi: the input in Pa is 101,325 Pa, and the result is 101,325 / 6894.76 = 14.696 psi. This is the standard atmospheric pressure at sea level, often approximated as 14.7 psi.

Converting 32 psi to kilopascals: the input in Pa is 32 * 6894.76 = 220,632 Pa, and the result is 220.632 kPa. This is the typical tyre pressure for a passenger car. Converting to bar gives 2.206 bar.

Converting 760 mmHg to atmospheres: the input in Pa is 760 * 133.322 = 101,325 Pa, and the result is 1 atmosphere exactly. This is the historical definition of one standard atmosphere, which is why the torr is defined as 1/760 of an atmosphere.

When to Use This Tool

Use the pressure converter when you need to:

Convert tyre pressure between psi and bar or kilopascals for imported vehicles.
Translate blood pressure readings from mmHg to kPa for international medical records.
Convert atmospheric pressure from millibars to pascals for meteorology.
Translate industrial pressure ratings from MPa to psi for engineering specifications.
Convert diving depths from atmospheres to bar for scuba equipment.
Translate vacuum measurements from torr to pascals for laboratory equipment.
Verify that a pressure gauge reading matches the specified range in different units.

Limitations & Disclaimer

This converter uses exact conversion factors defined in SI and the 1954 standard atmosphere definition. It does not handle gauge pressure (which requires subtracting atmospheric pressure), the technical atmosphere (at, 1 kgf/cm²), inches of mercury (inHg), or inches of water (inH2O). The mmHg and torr are treated as identical; in practice they differ by about 0.000015 percent. See our disclaimer for full details.

Frequently Asked Questions

What is the difference between mmHg and torr?

The mmHg is the pressure exerted by a 1-millimetre column of mercury, which depends slightly on the density of mercury and the local gravity. The torr was redefined in 1954 as exactly 1/760 of a standard atmosphere, or 133.322 Pa. In practice, the two are nearly identical (the difference is about 0.000015 percent), but the torr is the preferred unit in vacuum physics because it does not depend on mercury.

Why is atmospheric pressure 101,325 Pa?

By the 1954 definition of the standard atmosphere, which was set to match the historical 760 mmHg value at standard gravity. The choice of 101,325 (rather than a rounder number) preserves backward compatibility with the mercury-column measurement: 760 mm * 13,595.1 kg/m³ * 9.80665 m/s² equals 101,325 Pa to within the precision of the measurements.

What is the difference between gauge pressure and absolute pressure?

Gauge pressure is measured relative to atmospheric pressure: a flat tyre reads 0 psi gauge but 14.7 psi absolute. Absolute pressure is measured relative to vacuum. To convert gauge pressure to absolute, add the local atmospheric pressure (about 101,325 Pa). This converter handles absolute pressure values; add or subtract atmospheric pressure manually if needed.

Why is the bar approximately equal to one atmosphere?

Because the bar was originally intended as a metric replacement for the atmosphere, defined as exactly 100,000 pascals. The difference between the bar and the atmosphere is about 1.3 percent (1 bar = 0.9869 atm). The bar is widely used in meteorology (millibars) and in European tyre pressure gauges.

What does psi stand for?

Pound per square inch. It is the imperial unit of pressure, defined as one pound-force per square inch. One psi equals approximately 6894.76 pascals. Tyre pressure in the US and UK is typically measured in psi; the recommended range for passenger cars is 30 to 35 psi.

Is the pascal a large or small unit?

A small one. One pascal is roughly the pressure exerted by a one-newton force spread over one square metre, which is about the weight of an apple distributed over a small table. Atmospheric pressure is about 101,325 pascals, which is why the kilopascal and megapascal are more practical for everyday use.

Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team