What Is Millimeter of Water (mmH₂O)?
Millimeter of water, written as mmH₂O or mmH2O, is a unit that measures pressure. It tells you how strong a force is when water presses on a surface because of its height. One millimeter of water is the pressure made by a column of water that is 1 millimeter tall at standard gravity.
Definition
Pressure is how much force is pushed on an area. The millimeter of water unit is based on the weight of water.
Formal definition: One millimeter of water is the pressure at the bottom of a vertical column of pure water that is exactly 1 millimeter high, at standard gravity and a standard water density.
In modern scientific units:
- 1 mmH₂O is about 9.80665 pascals (Pa)
- 1 mmH₂O is about 0.00980665 kilopascals (kPa)
- 1 mmH₂O is about 0.0000980665 bar
- 1 mmH₂O is about 0.001422 pounds per square inch (psi)
This is a very small pressure compared with everyday air pressure around us.
History / Origin
Long ago, people used simple water columns to measure pressure. They filled a clear tube with water and watched how high the water rose or fell when pressure changed.
Instead of using complicated math, they simply said things like, the pressure is equal to a column of water that is this many millimeters high. From this practice, units like millimeter of water and meter of water were created.
Later, scientists created the pascal as the main pressure unit in the International System of Units. Still, millimeter of water stayed in use in some technical fields where water columns were easy to use and understand.
Symbol & Abbreviation
The most common symbol for millimeter of water is:
- mmH₂O or mmH2O
Other ways you might see it written include:
- mmAq or mmAq. from Latin aqua for water
- mmWC from water column
All of these point to the same basic idea, pressure caused by a column of water that is one millimeter high.
Current Use Around the World
Today, millimeter of water is not an official SI unit, but it is still used in some areas because it is easy to picture and measure.
Common uses include:
- HVAC and ventilation to describe small pressure differences in air ducts and filters
- Clean rooms and labs to control gentle air pressure between rooms
- Low pressure gas systems like some burner and boiler controls
- Water and liquid systems where old instruments still show readings in mmH₂O
However, in most modern engineering work, pressure is written in pascals, kilopascals, bar, or psi. Often, instruments and manuals show both mmH₂O and one of these larger units so that users can compare values easily.
Example Conversions
Here are some handy example conversions to help you understand the size of 1 millimeter of water.
From mmH₂O to other units
- 1 mmH₂O ≈ 9.81 Pa
- 10 mmH₂O ≈ 98.1 Pa
- 100 mmH₂O ≈ 981 Pa ≈ 0.981 kPa
- 1000 mmH₂O ≈ 9807 Pa ≈ 9.807 kPa ≈ 0.0981 bar
- 500 mmH₂O ≈ 4.90 kPa ≈ 0.0711 psi
From other units to mmH₂O
- 1 Pa ≈ 0.102 mmH₂O
- 1 kPa ≈ 102 mmH₂O
- 1 bar ≈ 10 197 mmH₂O
- 1 psi ≈ 703 mmH₂O
Simple comparison
- Normal air pressure at sea level is about 101 325 Pa.
- This equals roughly 10 330 mmH₂O.
- So normal air pressure is like having a water column around 10 meters high.
Related Units
Millimeter of water is one of many pressure units. Here are some related ones and how they compare.
- Pascal (Pa) main SI unit of pressure. 1 mmH₂O ≈ 9.81 Pa.
- Kilopascal (kPa) 1 kPa = 1000 Pa. 1 kPa ≈ 102 mmH₂O.
- Bar 1 bar = 100 000 Pa. 1 bar ≈ 10 197 mmH₂O.
- Pound per square inch (psi) common in the United States. 1 psi ≈ 703 mmH₂O.
- Meter of water (mH₂O) based on a 1 meter high water column. 1 mH₂O = 1000 mmH₂O.
- Millimeter of mercury (mmHg) based on a mercury column. 1 mmHg is much larger, about 13.6 times 1 mmH₂O.
FAQs
What does millimeter of water measure exactly?
It measures how strong a pressure is by comparing it to the pressure made by a 1 millimeter high column of water. The taller the water column, the higher the pressure.
Is mmH₂O an SI unit?
No. The official SI unit of pressure is the pascal (Pa). Millimeter of water is a non SI unit, but it is still allowed and used in some fields.
How do I convert mmH₂O to pascal?
Multiply the value in mmH₂O by about 9.81. For example, 20 mmH₂O × 9.81 ≈ 196 Pa.
How do I convert mmH₂O to psi?
Divide the value in mmH₂O by about 703. For example, 1406 mmH₂O ÷ 703 ≈ 2 psi.
What is the difference between mmH₂O and mmHg?
Both use the height of a liquid column, but mmH₂O uses water and mmHg uses mercury, which is much heavier. For the same height, mmHg shows a much higher pressure than mmH₂O.
Where is mmH₂O most commonly used today?
It is often used for low pressure measurements in air systems, filters, fans, and some gas or liquid systems, especially where older measuring devices are still in use.
Does water temperature change the value of mmH₂O?
Yes, slightly. Water density changes with temperature. In practice, a standard temperature and gravity are assumed so that 1 mmH₂O can be used with fixed conversion factors.