inH2O – Inches of Water Column at 4 deg C Pressure Unit

Inches of water gauge or column is an english and american unit for measuring liquid level. 1 inch of water column at 4 degrees Celsius equals 249.089 pascals.

The hydrostatic pressure generated by a certain liquid level is typically represented by the equivalent height of a water column. Since the pressure exerted by one inch of water is dependent on its density and the local gravity it is not a fundamental unit of measure for pressure but a derived one which is called a manometric unit.

Inches of water column or water gauge are used throughout the world for measuring shallow liquid level and low pressures such as differential air pressures in ventilation systems.

Since the density of a liquid is affected by changes in temperature, inches of water column should be accompanied by the temperature of the liquid that the units were derived. A pure water density of 1000 kg/m3 at 4 deg C and standard gravity of 9.80665 m/s2 is used in the calculation of this pressure unit. The significance of 4 degrees Celsius (39.2 degrees Fahrenheit) is that it is very close to the temperature that water reaches its maximum density.

It is conventional practise to use 1000 kg/m3 as the density of pure water at 4 deg C which is very close to the precise density and for most measurements this does not introduce any significant error. In fact since the temperature can vary significantly, measuring pressure in inches of water is never going to be a precise representation of the true liquid height. Local gravity also varies at different geological locations, which also adds some minor uncertainties to the use of inches of water gauge as an indication of exact water level in different parts of the world.

Use the conversion factors below to convert from inH2O to other pressure units or vice versa. To convert a reading in inH2O to another unit multiply it by the relevant pressure conversion factor. To convert a reading in any pressure unit to inH2O divide it by the relevant pressure conversion factor.

Alternatively convert from inH2O into another unit using the inches of water converter table or the pressure unit converter.

See how inH2O are derived from SI units or check other forms for describing inH2O.

Conversion Factors

• 0.00249089 bar
• 0.0361273 psi
• 2.49089 mbar
• 249.089 N/m²
• 249.089 Pa
• 2.49089 hPa
• 0.249089 kPa
• 0.000249089 MPa
• 0.00254000 kg/cm²
• 25.4 mmH2O 4°C (39.2°F)
• 2.54 cmH2O 4°C (39.2°F)
• 0.0254 mH2O 4°C (39.2°F)
• 1 inH2O 4°C (39.2°F)
• 0.0833333 ftH2O 4°C (39.2°F)
• 1.86832 mmHg 0°C (32°F)
• 0.186832 cmHg 0°C (32°F)
• 0.0735559 inHg 0°C (32°F)
• 1.86832 Torr
• 1868.32 mTorr
• 0.00245832 atm
• 0.00254000 at
• 2490.89 dyn/cm²
• 0.578037 oz/in²
• 1868.32 µHg 0°C (32°F)
• 0.0000161283 tsi (uk, long)
• 0.0000180636 tsi (usa, short)
• 0.00260117 tsf (usa, short)
• 5.20233 psf
• 2.54 g/cm²

Please note that the conversion factors above are accurate to 6 significant figures.

Derivation

The calculation below shows how the pressure unit Inches of Water Column (inH2O) is derived from SI Units.

Formula

• Pressure = Force / Area
• Force = Mass x Acceleration
• Mass = Density x Volume
• Volume = Area x Height
• Acceleration = Distance / (Time x Time)

SI Units

• Mass: kilogram (kg)
• Length: metre (m)
• Time: second (s)
• Force: newton (N)
• Pressure: pascal (Pa)

Input Values

• Density = Water Density at 4degC = 1000 kg/m³
• Area = 1 m²
• Height = 1 in = 0.0254 m
• Acceleration = Standard Gravity = 9.80665 m/s²

Calculation

• 1 inH2O Mass = 1000 kg/m³ x 1 m² x 0.0254 m = 25.4 kg
• 1 inH2O Force = 25.4 kg x 9.80665 m/s² = 249.08891 N
• 1 inH2O Pressure = 249.08891 N / 1 m² = 249.08891 Pa

Alternate Descriptions

These are the different versions used for identifying inH2O that you may find elsewhere.

• Inches of H2O
• Inch of H2O
• Inches of Water Column
• Inch of Water Column
• Inches of Water Gauge
• Inch of Water Gauge
• inH2O
• inWC
• inWG
• inWS
• inAq
• in H2O
• in WC
• in WG
• in WS
• in Aq
• “H2O
• “WC
• “WG
• “WS
• “Aq

Conversion Tables

Select a look up table for converting a pressure reading in inches of water column to other measurement units.

• mmH2O, mbar, psi and mmHg
• bar  »  1 to 1000 inH₂O → 0.00249089 to 0.00249089 bar
• psi  »  1 to 1000 inH₂O → 0.00361273 to 3.61273 psi
• Pa  »  1 to 1000 inH₂O → 249.089 to 249,089 Pa
• kPa  »  1 to 1000 inH₂O → 0.249089 to 249.089 kPa

Help

4°C vs 20°C

When determining the accuracy of a pressure gauge or pressure transducer is it better to measure it against inches of Water Column at 4 deg C or 20 deg C?

4 degC tends to be the most popular worldwide, especially since it also corresponds very closely to a water density of 1000 kg/m3 at this temperature, which makes it easier when calculating unit conversions to and from inches of water. What’s best to use, depends on how you are using the measurement unit, and what your industry standard is for the unit.

In fact it’s no longer a recommended unit to use by standards authorities in either temperature, because it can be easily misinterpreted, and cause confusion if being used to measure the level of liquid directly and the actual temperature does not match the same temperature as the unit used. There is only one value defined for the units of psi, mbar and pascal and they are therefore, not dependent on temperature, and provide unambiguous measurement units.

Glossary of Low Pressure technical terms

• g Effect
• mmH2O – Millimetres of Water Column at 4 deg C Pressure Unit
• oz/in² – Ounce per Square Inch Pressure Unit
• Pa – Pascal Pressure Unit

Help from Low Pressure resources

• Measuring the difference in air pressure between rooms

Guidance with Low Pressure requirements

• HVAC
• Low Pressure Range