# What is a Torr?

The torr (symbol: Torr) is a non-SI unit of pressure defined as 1/760 of an atmosphere. It was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644.

Torricelli attracted considerable attention when he demonstrated the first mercury barometer to the general public. He is credited with giving the first modern explanation of atmospheric pressure. Scientists at the time were familiar with small fluctuations in height that occurred in barometers. When these fluctuations were explained as a manifestation of changes in atmospheric pressure, the science of meteorology was born.

Over time, 760 millimeters of mercury came to be regarded as the “standard” atmospheric pressure. The unit of barometric pressure (one millimeter of mercury, also written as 1 mm Hg) was named in honor of Torricelli.

In 1954, the definition of atmosphere was revised by the 10e Conférence Générale des Poids et Mesures (10th CGPM)[2] to the currently accepted definition: one atmosphere is equal to 101325 pascals. The torr was then re-defined as 1/760 of one atmosphere. This change in the definition of “torr” has been a source of confusion ever since.

## SI units of pressure

The SI unit of pressure is the pascal (symbol: Pa), defined as one newton per square meter. Other units of pressure are defined in terms of SI units. These include:

• The bar (symbol: bar), defined as 105 Pa exactly.
• The atmosphere (symbol: atm), defined as 101,325 Pa exactly.
• The torr (symbol: Torr), defined as 1/760 atm exactly.

These four SI-related pressure units are used in different settings. For example, the bar is used in meteorology to report atmospheric pressures.[5] The torr, a more convenient unit for low pressures, is used in high-vacuum physics and engineering.

Pressure Units

pascal
(Pa)

bar
(bar)
technical atmosphere
(at)

atmosphere
(atm)

torr
(Torr)
pound-force per
square inch
(psi)
1 Pa 1 10-5 1.02*10-5 9.87*10-6 7.5*10-3 1.45*10-6
1 bar 1*105 1 1.02 0.987 750.06 14.5
1 at 9.8*104 0.98 1 0.968 735.56 14.223
1 atm 1.013*105 1.013 1.033 1 760 14.696
1 torr 133.32 1.33*10-3 1.36*10-3 1.316*10-3 1 1.934*10-2
1 psi 6.895*103 6.895*10-2 7.03*10-2 6.8*10-2 51.715 1

## Manometric units in medicine and physiology

In medicine, the mmHg (measured with a sphygmomanometer) is the gold standard for blood pressure measurement. In physiology, manometric units are used to measure Starling forces. Other applications include

• Intraocular pressure
• CSF pressure
• Intracranial pressure
• Intramuscular pressure (compartment syndrome)
• Central venous pressure
• Pulmonary artery catheterization
• Mechanical ventilation
• Pulmonary gas pressure
• Esophageal motility studies
• Venous ulcer compression regime

Manometric results in medicine are sometimes given in torr. This is usually incorrect, since the Torr and the mmHg are not the same thing. Pressures obtained with a manometer (or its transducer equivalent) should be reported in mmHg.