What Is Millimeter of Mercury (mmHg)?
Millimeter of mercury, written as mmHg, is a unit that measures pressure. It shows how strong a gas or liquid presses on something. Doctors use it very often to measure blood pressure.
Definition
Imagine a thin glass tube standing in a dish of liquid mercury. Air pushes on the mercury in the dish and makes the mercury rise inside the tube. The height of the mercury column shows the pressure of the air.
One millimeter of mercury is the pressure made by a mercury column that is exactly 1 millimeter high at normal gravity at sea level. In science terms,
- 1 mmHg is about 133.3 pascals (Pa)
- 1 mmHg is about 0.0013158 standard atmospheres (atm)
- 1 mmHg is about 0.1333 kilopascals (kPa)
Even though the pascal is the official SI unit for pressure, mmHg is still very common in medicine and some labs.
History / Origin
The idea of using mercury to measure pressure started in the 1600s. An Italian scientist named Evangelista Torricelli built one of the first mercury barometers in 1643.
He filled a long glass tube with mercury, closed at one end, then flipped it into a dish of mercury. Some mercury stayed up in the tube. The height of this mercury column showed the pressure of the air pushing on the dish.
At sea level, the column was about 760 millimeters high. This led people to measure pressure using the height of mercury in millimeters. That is how the unit millimeter of mercury or mmHg was born.
Later, scientists defined a standard atmosphere as the pressure made by a 760 millimeter high column of mercury. From this, they linked mmHg to other pressure units like pascals and atmospheres.
Symbol & Abbreviation
The usual symbol for millimeter of mercury is:
- mmHg (lowercase mm and capital H and g)
This symbol has two parts:
- mm means millimeter, which is one thousandth of a meter
- Hg is the chemical symbol for mercury, a heavy silver colored liquid metal
Related symbol you may see:
- Torr is a very close unit. By definition, 1 Torr is exactly 1 over 760 of a standard atmosphere. In practice, 1 Torr and 1 mmHg are almost the same, and many people treat them as equal.
Current Use Around the World
Today, millimeter of mercury is not an official SI unit, but it is still used a lot in real life.
Medicine
- Blood pressure is almost always given in mmHg, for example 120 over 80 mmHg.
- Eye pressure in glaucoma tests is often shown in mmHg.
- Some gas pressures in hospitals, like in ventilators and anesthesia machines, may be written in mmHg.
Science and technology
- Vacuum systems in labs may use mmHg or Torr to show how low the pressure is.
- Some older weather reports and barometers still use millimeters of mercury to show air pressure.
Regions and standards
- Most countries now use pascals or kilopascals in science and engineering.
- However, doctors and nurses worldwide still use mmHg for blood pressure because it is familiar and easy to compare with older records.
Example Conversions
Here are some simple examples to show how mmHg relates to other pressure units.
Basic rules
- 1 mmHg ≈ 133.3 Pa
- 1 mmHg ≈ 0.1333 kPa
- 1 atm = 760 mmHg
From mmHg to kilopascals kPa
- 120 mmHg × 0.1333 ≈ 16.0 kPa
- 80 mmHg × 0.1333 ≈ 10.7 kPa
- 760 mmHg × 0.1333 ≈ 101.3 kPa
From mmHg to atmospheres atm
- 760 mmHg = 1 atm
- 380 mmHg = 0.5 atm
- 1520 mmHg = 2 atm
From atmospheres atm to mmHg
- 1 atm × 760 = 760 mmHg
- 0.25 atm × 760 = 190 mmHg
- 1.5 atm × 760 = 1140 mmHg
From mmHg to Torr
- 1 mmHg ≈ 1 Torr
- 10 mmHg ≈ 10 Torr
- 760 mmHg ≈ 760 Torr
Related Units
Millimeter of mercury is one of many pressure units. Here are some related ones and how they compare.
- Pascal Pa The main SI unit for pressure. 1 Pa is 1 newton of force on 1 square meter. 1 mmHg ≈ 133.3 Pa.
- Kilopascal kPa A common unit in science and weather. 1 kPa is 1000 Pa. 1 mmHg ≈ 0.1333 kPa.
- Atmosphere atm A unit based on average air pressure at sea level. 1 atm = 760 mmHg ≈ 101.3 kPa.
- Bar Another pressure unit used in industry and weather. 1 bar = 100 kPa ≈ 750.1 mmHg.
- Torr Very close to mmHg for most uses. 1 atm = 760 Torr by definition, so 1 Torr is almost the same as 1 mmHg.
FAQs
What does mmHg stand for?
mmHg stands for millimeter of mercury. Millimeter is a length unit, and mercury is a heavy liquid metal. Together they describe pressure as the height of a mercury column.
Why is blood pressure measured in mmHg?
Old blood pressure tools used mercury columns. Doctors got used to reading the height in millimeters of mercury. Even with new digital machines today, the unit mmHg stayed the same, so older and newer readings are easy to compare.
Is mmHg an SI unit?
No. The SI unit for pressure is the pascal Pa. But mmHg is allowed for use with SI in some fields, and it is still widely accepted in medicine and some lab work.
What does 120 over 80 mmHg mean?
120 over 80 mmHg is a typical adult blood pressure reading. The first number 120 is the pressure when the heart pumps. The second number 80 is the pressure when the heart rests between beats.
Is 1 mmHg exactly the same as 1 Torr?
By strict science rules, they are defined in slightly different ways, but the difference is extremely small. For almost all everyday and medical uses, you can treat 1 mmHg as equal to 1 Torr.
Why does this unit mention mercury if we do not use mercury much now?
Many countries now avoid mercury because it is poisonous. Modern tools use other methods to measure pressure. But the name and unit mmHg stayed, because it is very common and easy to understand for health workers.
How can I quickly change mmHg to kPa in my head?
A simple shortcut is to divide the mmHg value by 7.5. For example, 150 mmHg divided by 7.5 is about 20, so 150 mmHg is about 20 kPa.
When should I use mmHg and when should I use Pa or kPa?
Use mmHg mainly for medical readings like blood pressure or eye pressure, and for some lab vacuums. Use Pa or kPa for school physics, engineering, and most scientific reports.