Article — psi to Inches of Water Conversion Calculator
psi to inches of water: HVAC, gas pressure, and low-pressure measurement
One psi equals 27.7076 inches of water column at 60°F (15.6°C). To convert psi to inches of water, multiply by 27.7076. To convert inH2O to psi, divide by 27.7076. The factor comes from water density and gravity acting on a 1-inch column, giving 248.84 pascals per inH2O.
Inches of water column is the working unit of HVAC and natural gas, and a strange unit to anyone outside those trades. It sounds antique because it is — the original measurement tool was a glass U-tube filled with water, and pressure read out as a height in inches. The unit stuck because for low pressures (0 to 10 inH2O) it gives a sensible scale that the higher-resolution psi cannot match.
What is psi to inches of water conversion?
Inches of water (inH2O, inWC, or iwg) is a pressure unit defined by the height of a water column at 60°F (15.6°C) under standard gravity. One inch of water column equals exactly the pressure that a one-inch tall, room-temperature water column exerts at its base. In SI units, this is 248.84 pascals per inch.
Psi (pounds per square inch) is the imperial pressure unit, equal to 6894.757 pascals per psi. Dividing 6894.757 by 248.84 gives 27.7076 inches of water per psi. The conversion is exact; the only variable is water temperature, which shifts density by a tenth of a percent across the normal range.
The psi to inches of water formula
Convert psi to inH2O by multiplying by 27.7076. A 1 psi pressure equals 27.71 inH2O, a half-psi differential equals 13.85 inH2O, and a 0.25 psi natural gas line reads 6.93 inH2O on a manometer. The reverse formula divides: 10 inH2O is 0.361 psi, and the 7 inH2O of typical residential gas supply equals 0.253 psi.
inH2O = psi * 27.7076 psi = inH2O / 27.70761 psi = 27.71 inH2O 1 inH2O = 248.84 Pa7 inH2O = 0.253 psi 1 inH2O = 25.4 mmH2OFor mental math, treat 1 psi as 28 inH2O. The error is 1.05%, fine for casual reading on an HVAC site visit. Use 27.7076 (or the full 27.71) when writing numbers down for design calculations.
psi to inches of water in HVAC
HVAC ductwork operates at total static pressures between 0.5 and 1.5 inH2O across the entire system from blower to register. Within that budget, a clean air filter consumes 0.1 inH2O, a straight duct run takes 0.2 to 0.5 inH2O, and each elbow or fitting adds 0.1 to 0.3 inH2O. The total has to balance, or the blower works hard for poor airflow.
ASHRAE standards specify inH2O for all duct-design calculations, fan-performance curves, and filter ratings. A residential furnace fan might be rated at 1.0 inH2O at 1000 CFM. A commercial blower handles 4 to 6 inH2O at much higher CFM. The choice of inH2O over psi is practical: 1 psi exceeds the entire operating range of most HVAC systems.
A dirty air filter pulls 0.5 inH2O instead of 0.1 inH2O, a five-fold increase in pressure drop. The blower compensates by working harder, but airflow still falls by 15 to 25 percent and energy use rises. Change filters when pressure drop exceeds 0.3 inH2O above the clean value, regardless of the calendar interval.
psi to inches of water for natural gas
Residential natural gas service runs at 7 inches of water column, which equals 0.253 psi. The pressure is low for a reason: leaks at 0.25 psi disperse harmlessly outdoors, while leaks at line pressure (50+ psi) could pool and ignite. Appliance regulators step the pressure down further: ranges and water heaters want 3.5 to 4.0 inH2O, furnaces 3.5 to 7.0 inH2O.
Commercial gas supply runs higher, typically 14 to 60 inH2O (0.5 to 2.2 psi), with appliance regulators at each piece of equipment dropping back to working pressure. Industrial gas reaches several psi at the meter, requiring different piping and certified pressure-relief systems. The 0.25 psi versus 50 psi distinction is the difference between residential gas piping (black iron) and commercial gas piping (welded steel with safety valves).
psi to inches of water in cleanrooms
Pharmaceutical and electronics cleanrooms maintain positive pressure relative to surrounding spaces, typically 0.02 to 0.05 inH2O (about 5 to 12 Pa). The differential prevents unfiltered air from leaking in when doors open. Isolation rooms for infectious disease patients use negative pressure of the same magnitude in the opposite direction.
Pressure monitoring sensors in cleanrooms must read in the 0.01 to 0.1 inH2O range with sub-1% accuracy. Magnehelic gauges scaled 0 to 0.25 inH2O are the industry standard. Digital differential-pressure transducers with the same range have replaced gauges in modern installations because they integrate with building management systems.
Sign matters in cleanroom and HVAC work. A reading of -2 inH2O means the room is 2 inches below ambient, an inflow condition. Positive readings mean outflow. Confusing the two has shut down pharmaceutical batches because of presumed contamination, even when the room was clean. Always note the sign on monitor logs.
Manometers and the inH2O unit
A U-tube manometer is the simplest pressure-measuring instrument: a glass or plastic tube bent into a U shape, partly filled with water. Connect one end to the pressure source and the water column shifts by an amount equal to the pressure in inches. The instrument is the definition of the unit, which is why water-column pressure remained standard long after digital sensors arrived.
Inclined manometers extend the readable range. A tube tilted at 30 degrees from horizontal gives twice the visible deflection per inH2O, doubling resolution at the cost of overall range. Most field-service manometers cover 0 to 10 inH2O on a vertical scale and 0 to 1 inH2O on an inclined scale, both calibrated against the same physical water column.
Common psi-to-inH2O mistakes
Three errors show up regularly in HVAC and gas service work. The first is confusing inH2O with inHg (inches of mercury). The two units share the inch dimension and the column-based definition, but mercury is 13.6 times denser than water. A reading of 5 inHg equals 68 inH2O — an order-of-magnitude difference that leads to dangerously wrong calculations.
The second is dropping the gauge versus absolute distinction. HVAC pressures in inH2O are almost always gauge pressures, measured relative to room air. Adding the 407 inH2O of atmospheric pressure to get absolute pressure is rarely done because the gauge value alone tells the technician everything needed. Confusion appears when an engineer needs to plug a gauge value into a thermodynamic calculation that expects absolute pressure.
The third is using a regular psi gauge to read HVAC pressure. A 0 to 100 psi gauge marked in 1 psi increments cannot resolve 0.5 inH2O (0.018 psi) at all — the needle would not visibly move. Always use a manometer or a low-range pressure transducer scaled in inH2O for HVAC and gas-line work.
Memorize 1 psi = 27.71 inH2O = 6.895 kPa = 51.71 mmHg. These four values cover every conversion in HVAC, gas service, and chemistry pressure work. Any other low-pressure unit can be derived from these constants.
Comparison: HVAC vs natural gas vs cleanroom
Three industries use inches of water column at three different operating points.
Quick reference values
The psi to inches of water scale spans four orders of magnitude in practice, from cleanroom differentials to industrial gas-line pressures.
- 1 psi = 27.71 inH2O (reference)
- Cleanroom positive = 0.02 inH2O = 0.0007 psi
- Small duct drop = 1 inH2O = 0.036 psi
- HVAC reference = 0.5 psi = 13.85 inH2O
- Natural gas residential = 7 inH2O = 0.253 psi
- HVAC max ductwork = 3 inH2O = 0.108 psi
- Commercial gas line = 14 inH2O = 0.505 psi
- Industrial pneumatic = 10 psi = 277 inH2O
- Atmospheric pressure = 14.696 psi = 406.8 inH2O
- Inches of mercury = inHg, NOT inH2O (13.6x heavier)