Torr (torr)

What Is Torr?

A torr is a unit used to measure pressure. It is most common in vacuum science, physics, and some engineering fields. One torr is a small amount of pressure, useful for describing very thin air or near vacuum conditions.

The torr is based on the pressure of the Earths atmosphere at sea level. It cuts that standard atmospheric pressure into 760 equal parts. One of these parts is one torr.

Definition

The torr is defined in a precise mathematical way. Today it is tied to the pascal, which is the official SI unit for pressure.

• 1 standard atmosphere is defined as exactly 101 325 pascals.
• 1 torr is defined as exactly 1 divided by 760 of a standard atmosphere.

From this definition we get:

• 1 torr = 101 325 ÷ 760 pascals ≈ 133.322 pascals.
• 1 pascal ≈ 0.0075006 torr.
• 1 atmosphere = 760 torr.

Because the value is exact by definition, scientists can convert between torr, pascals, and atmospheres with high accuracy.

History / Origin

The torr is named after the Italian scientist Evangelista Torricelli. In the 1600s he studied how air pressure works using a mercury barometer. His work helped show that air has weight and presses on us from all sides.

Early barometers used the height of a mercury column to show air pressure. A common pressure reading was about 760 millimetres of mercury at sea level. Over time, people started using this height as a way to measure pressure.

Later, scientists created the unit called “torr” to honor Torricelli and to give a clear, fixed size to this type of pressure measure. At first, the torr was considered almost the same as 1 millimetre of mercury. Later definitions became more exact and tied the torr directly to the standard atmosphere and the pascal.

Symbol & Abbreviation

The usual symbol and way of writing the unit are:

• Name: torr
• Symbol: Torr (capital T, rest lower case) when used as a symbol with numbers
• Written in text: “torr” as a word

The torr is not an SI unit. The official SI unit for pressure is the pascal (Pa). However, the torr is accepted for use with SI in some technical fields, especially vacuum technology and surface science.

Current Use Around the World

Today, the torr is mostly used in scientific and technical areas where very low pressures must be measured or controlled. Examples include:

• Vacuum systems such as vacuum pumps and vacuum chambers.
• Thin film and semiconductor manufacturing where gas pressures must be very low and very stable.
• Surface science and materials research where experiments take place in high vacuum or ultra high vacuum.
• Some physics experiments involving gases, plasmas, or particle beams.

In everyday life, most people do not see pressure written in torr. Weather reports usually use hectopascals or millibars. Tire pressure is often in psi or kilopascals. Blood pressure usually uses millimetres of mercury (mmHg), which is very close but not exactly the same as torr.

Even though the pascal is the official SI unit, many laboratories and engineers still prefer torr for vacuum work because it is familiar and easy to relate to traditional vacuum ranges.

Example Conversions

Below are some simple example conversions using the exact relationship 1 torr = 133.322 pascals and 1 atmosphere = 760 torr.

From torr to pascals

• 1 torr ≈ 133.322 Pa
• 10 torr ≈ 1 333.22 Pa
• 100 torr ≈ 13 332.2 Pa
• 760 torr = 101 325 Pa (by definition this equals 1 atmosphere)

From pascals to torr

• 1 Pa ≈ 0.0075006 torr
• 100 Pa ≈ 0.75 torr
• 1 000 Pa ≈ 7.50 torr
• 10 000 Pa ≈ 75.0 torr

From torr to other common pressure units

• 1 torr ≈ 0.001333 bar
• 1 torr ≈ 0.001316 atmosphere (since 1 atm = 760 torr)
• 1 torr ≈ 0.01934 psi (pounds per square inch)
• 1 torr ≈ 0.999997 mmHg at 0 °C by modern definitions

Sample step by step conversion

Example Convert 50 torr to pascals.

1. Write the factor: 1 torr = 133.322 Pa.
2. Multiply: 50 × 133.322 = 6 666.1 Pa.
3. So, 50 torr ≈ 6.66 × 10³ Pa.

Related Units

The torr is closely related to several other pressure units.

• Pascal (Pa) The SI unit for pressure. 1 torr ≈ 133.322 Pa.
• Atmosphere (atm) A unit based on average sea level air pressure. 1 atm = 760 torr.
• Bar A metric unit often used in industry and weather. 1 bar = 100 000 Pa ≈ 750.06 torr.
• Millimetre of mercury (mmHg) A traditional unit based on the height of a mercury column. Very close to the torr. 1 mmHg ≈ 1.00000 torr by modern definitions, but not exactly.
• Pounds per square inch (psi) A common pressure unit in the United States. 1 psi ≈ 51.7 torr.

In modern science, the pascal is preferred. However, torr and mmHg remain common where traditional or very low pressure ranges are important.

FAQs

Is torr an SI unit?

No. Torr is not an SI unit. The official SI unit for pressure is the pascal (Pa). The torr is a non SI unit that is still allowed in some scientific and technical work, especially in vacuum science.

Is torr the same as millimetres of mercury (mmHg)?

Torr and mmHg are almost the same but not exactly. Historically 1 torr was treated as equal to 1 mmHg. Modern definitions tie the torr exactly to the atmosphere and pascal. This makes a very tiny difference compared to mmHg. For most everyday uses, they are close enough to treat as equal, but in high precision work the small difference matters.

Why do vacuum scientists still use torr?

Vacuum scientists still use torr because it has been used for a long time, the numbers are convenient for typical vacuum ranges, and many instruments and tables are already written in torr. Switching fully to pascals would require changing lots of existing equipment and habits.

How do I quickly convert between torr and pascals in my head?

You can use simple rounded factors.

• To go from torr to pascals: multiply by about 133.
• To go from pascals to torr: divide by about 133.

These rounded numbers are good enough for rough estimates. For accurate work, use 133.322 as the factor.

Where might I see pressure written in torr?

You are most likely to see torr used in:

• Research papers about vacuum systems or surface science.
• Technical manuals for vacuum pumps, leak detectors, and coating systems.
• Settings and displays on scientific vacuum gauges.

For normal daily life, such as weather reports or car tire pressure, other units are used instead of torr.

What range of pressures is usually measured in torr?

Torr is especially handy for pressures lower than normal air pressure, such as:

• Rough vacuum around 1 to 10^-1 torr.
• High vacuum down to about 10^-7 torr.
• Ultra high vacuum below about 10^-7 torr.

For pressures around or above normal air pressure, pascals, bars, or atmospheres are often more common.