What Is Inch of Water (inH₂O)?
Inch of water is a unit used to measure small amounts of pressure. It tells you how much force is made by a column of water that is exactly one inch high.
Definition
One inch of water is the pressure made at the bottom of a water column that is:
- 1 inch tall
- Filled with pure water
- At normal gravity on Earth
In science numbers, 1 inch of water is about:
- 249 pascals Pa
- 0.249 kilopascal kPa
- 0.0361 pounds per square inch psi
This unit is useful for measuring gentle pressures that are too small for everyday units like psi to show clearly.
History / Origin
Before modern pressure units like the pascal were used, people often measured pressure with simple water or mercury columns. They saw that the higher the water column, the stronger the pressure at the bottom.
In places that used the inch for length, it was natural to talk about the height of water in inches. So inch of water became a handy way to show low pressures in pipes, heating and cooling systems, and lab tools.
Even after the metric system and the pascal became standard, many fields kept using inch of water because workers and old tools were already based on it.
Symbol & Abbreviation
The most common way to write inch of water is:
- inH₂O or in H₂O
Other forms you might see include:
- in WC inch water column
- inch H₂O
- ” w.c. or ” water older style
All of these mean the same type of pressure based on a one inch high water column.
Current Use Around the World
Inch of water is not an SI unit, but it is still widely used in many countries, especially where inches are common, such as the United States.
Today, inch of water is often used in:
- HVAC systems heating, ventilation, and air conditioning to measure air pressure in ducts and filters
- Gas lines low pressure natural gas and propane systems
- Building systems checking room pressure in hospitals, clean rooms, and labs
- Industrial processes small pressure differences in tanks and pipes
- Draft measurement pressure in chimneys, boilers, and burners
Engineers and technicians often work with both inch of water and pascals, using simple conversion factors to switch between them.
Example Conversions
Here are some simple example conversions using rounded numbers for easy understanding.
Inch of water to pascals Pa
1 inH₂O ≈ 249 Pa
- 2 inH₂O ≈ 498 Pa
- 5 inH₂O ≈ 1 245 Pa about 1.25 kPa
- 10 inH₂O ≈ 2 490 Pa about 2.49 kPa
Inch of water to psi
1 inH₂O ≈ 0.0361 psi
- 10 inH₂O ≈ 0.361 psi
- 27.7 inH₂O ≈ 1 psi close to
- 50 inH₂O ≈ 1.80 psi
Inch of water to millimeters of water
1 inch is 25.4 millimeters, so:
- 1 inH₂O ≈ 25.4 mmH₂O
- 4 inH₂O ≈ 101.6 mmH₂O
- 10 inH₂O ≈ 254 mmH₂O
Inch of water to millimeters of mercury mmHg
1 inH₂O ≈ 0.0735 mmHg
- 10 inH₂O ≈ 0.735 mmHg
- 100 inH₂O ≈ 7.35 mmHg
Related Units
Other pressure units that are related or often used with inch of water include:
- Pascal Pa the SI unit for pressure, used in science and engineering
- Kilopascal kPa 1 000 pascals, common in weather and engineering
- Pounds per square inch psi common in tires, tools, and many US systems
- Bar 1 bar ≈ 100 kPa, often used in industry
- Millimeter of water mmH₂O same idea as inch of water but using millimeters and metric system
- Meter of water mH₂O pressure from a 1 meter high water column
- Millimeter of mercury mmHg pressure based on mercury, common in medicine and weather
In low pressure work, inch of water and pascal are the most common pair to convert between.
FAQs
What is inch of water used for?
Inch of water is used to measure very small pressures, such as air pressure in ducts, gas pressure in home lines, and tiny pressure differences in buildings and labs.
Why use inch of water instead of psi?
Psi is good for higher pressures, like in car tires. For gentle pressures, psi numbers become small and hard to read. Inch of water gives larger, easier numbers for the same small pressures.
Is inch of water an SI unit?
No. Inch of water is not an SI unit. The official SI unit for pressure is the pascal Pa. Inch of water is a practical unit that is still used in some fields.
How do I convert inch of water to pascals?
Multiply inch of water by about 249 to get pascals. For example, 3 inH₂O × 249 ≈ 747 Pa. For a quick estimate, you can also think 4 inH₂O ≈ 1 kPa.
Why is water used in this unit?
Water is easy to find, safe, and has a known weight. Long ago, many pressure tools used water columns. So height of water became a simple way to describe pressure.
Is inch of water the same everywhere?
Yes, as long as you use standard conditions, such as normal gravity on Earth and pure water. In real life, small differences in temperature or location change the value only a little.