The **key difference** between torr and mmHg is that **the value of one torr can be obtained by dividing 101325 from 760, whereas the value of one mmHg is given exactly as 133.3224. **

Both torr and mmHg are units of measurement of pressure. In the past, it was considered that both these units have the same value for a particular amount of pressure, but now scientists have found that they have a slight difference in their values, that is about a 0.000015% difference.

### CONTENTS

1. Overview and Key Difference

2. What is Torr

3. What is mmHg

4. Side by Side Comparison – Torr vs mmHg in Tabular Form

5. Summary

## What is Torr?

Torr is a unit of measurement of pressure where one torr equals the value obtained by dividing 101325 from 760. It is derived based on an absolute scale (begins with zero and goes forward only in one direction). The value of one torr can be obtained from multiplying 1/760 from a standard atmospheric pressure unit. Therefore, this term is equal exactly to the value obtained by dividing one standard atmospheric pressure value (101325 Pa) from 760. This unit was named after the scientist Evangelista Torricelli, who discovered the theory behind the barometer.

Although scientists earlier thought that the value of one torr is similar to one unit of mmHg, the value obtained from dividing 101325 from 760 is not exactly similar to 1 mmHg; there is a very slight difference about 0.000015%. Moreover, torr is not a part of the international system of units (SI units). In order to express the values of very small pressure amounts, the prefix milli- is used often (millitorr). Then, 1millitorr=0.001torr. Followings are some conversion factors of torr.

- 1 Torr = 0.999 mmHg
- 1 mmHg = 1.000 0004 Torr
- 1 Pa = 7.5 x 10
^{-3}Torr - 1 bar = 75.06 Torr
- 1 atm = 760 Torr

## What is mmHg?

mmHg is a unit of measurement of pressure where one torr equals the value of one mmHg is equal to 133.3224. The name of this unit is “millimeter Mercury”. It is a manometric unit of pressure. It is equal to the extra pressure generated by a column of mercury with 1 mm height.

According to current definitions, the value of mmHg differs by 0.000015%. There are several uses of mmHg in the field of medicine, such as determining blood pressure, cerebrospinal fluid pressure, central venous pressure, intramuscular pressure, etc.

## What is the Difference Between Torr and mmHg?

Although the values of pressure measured from these two units are considered similar, they actually have a slight difference – about 0.000015%. The key difference between torr and mmHg is that the value of one torr can be obtained by dividing 101325 from 760, whereas the value of one mmHg is given exactly as 133.3224. While 1 torr equals 0.999 mmHg, I mmHg equals 1.0000004 torr.

Moreover, torr is used to express the pressure of a thermodynamic system while mmHg is important in expressing the blood pressure, cerebrospinal fluid pressure, central venous pressure, intramuscular pressure, etc.

Below is a side by side comparison of both units to discern the difference between torr and mmHg.

## Summary – Torr vs mmHg

Both torr and mmHg are units of measurement of pressure. Although the values of pressure measured from these two units are considered similar, they actually have a slight difference. The key difference between torr and mmHg is that the value of one torr can be obtained by dividing 101325 from 760 whereas the value of one mmHg is given exactly as 133.3224.

##### Reference:

1. “Torr.” Torr – an Overview | ScienceDirect Topics, Available here.

2. “Torr.” Wikipedia, Wikimedia Foundation, 25 Apr. 2020, Available here.

3. “Millimetre of Mercury.” Wikipedia, Wikimedia Foundation, 8 Mar. 2020, Available here.

##### Image Courtesy:

1. “Modern Aneroid Barometer” By Own work, Public Domain) via Commons Wikimedia

2. “Sphygmomanometer, automatic blood pressure monitor. Oscillometric measurements, millimeter of mercury (mmHg) unit” By Albarubescens – Own work (CC BY-SA 4.0) via Commons Wikimedia

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