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.

• Minus 0.25 inWG differential pressure transmitter for negative air pressure difference
• 1MPa, 10bar, 4000inH2O, 150psi, 1000kPa range pressure gauge
• +/-10 inH2O Air Pressure Sensor for Ventilation & Extraction
• Air scrubber 10 inH2O partial vacuum pressure transmitter

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.

• Low range vacuum pressure logger for 0 to -60 inH2O g
• 40 inH2O g OEM pressure transmitter with 4-20mA curent output
• 25 inH2Og range 4-20mA output natural gas pressure sensor for use in generator supply
• DP transmitter with for measuring 300inH2O gas flow across orifice plate

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.

• 20 inH2Og range 4-20mA output natural gas pressure transducer for commercial generator use
• -0.1 to +0.1 inH2O g range 4-20mA signal air pressure sensor and display for pharmaceutical manufacturing use
• 100 inches H2O pressure gauge
• 40 inH2O g peak/valley storing pressure gauge for monitoring gas feed fluctuations

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

• 1.5 inWG differential pressure transducer to control positive pressure inside a building
• +/- 30 inches of water compound range pressure transmitter
• Genset hydrostatic sensor for measuring level of small diesel tank up to 12 inches
• 20 inH2Og range 4-20mA output natural gas pressure transducer for commercial generator use

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

• 0.1 inH2O differential range 0-10Vdc output between rooms air pressure sensor
• Low range vacuum pressure logger for 0 to -60 inH2O g
• Air blower 100 inWG range differential pressure transducer
• 250″wc Orifice plate flow measurement dp transmitter

Measurement Unit Related Terms

More pages related to measurement unit technical terms.

• at – Technical Atmosphere Pressure Unit
• atm – Standard Atmosphere Pressure Unit
• bar Pressure Unit
• cmH2O – Centimetres of Water Column at 4 deg C Pressure Unit
• cmHg – Centimetres of Mercury at 0 deg C Pressure Unit
• daPa – Dekapascal Pressure Unit
• dyn/cm² – Dyne per Square Centimetre Pressure Unit
• ftH2O – Feet of Water Column at 4 deg C Pressure Unit
• g – Acceleration due to Gravity
• g/cm² – Grams per Square Centimetre Pressure Unit
• hPa – Hectopascal Pressure Unit
• inHg – Inches of Mercury at 0 degrees C Pressure Unit
• kg/cm² – Kilogram per Square Centimetre Pressure Unit
• kg/mm² – Kilogram per Square Millimetre Pressure Unit
• kPa – Kilopascal Pressure Unit
• ksi – Kilopounds per Square Inch Pressure Unit
• mbar – Millibar Pressure Unit
• mH2O – Metres of Water Gauge at 4 deg C Pressure Unit
• mmH2O – Millimetres of Water Column at 4 deg C Pressure Unit
• mmHg – Millimetres of Mercury at 0 deg C Pressure Unit
• MPa – Megapascal Pressure Unit
• mPa – Millipascal Pressure Unit
• mTorr – Millitorr Pressure Unit
• N/cm² – Newton per Square Centimetre Pressure Unit
• N/m2 – Newton per Square Metre Pressure Unit
• N/mm² – Newton per Square Millimetre Pressure Unit
• oz/in² – Ounce per Square Inch Pressure Unit
• Pa – Pascal Pressure Unit
• Pressure
• Pressure Units
• psf – Pounds per Square Foot Pressure Unit
• psi – Pounds per Square Inch Pressure Unit
• Specific Gravity
• Torr Pressure Unit
• tsf – Tons per Square Foot (USA, Short) Pressure Unit
• tsi – Tons per Square Inch (UK, Long) Pressure Unit
• tsi – Tons per Square Inch (USA, Short) Pressure Unit
• Water Density
• µHg – Micron of Mercury at 0 deg C Pressure Unit