How this instrument works
Inches of water column measures pressure the direct way: how high a column of water rises under it, the same principle behind a mercury barometer but built for pressures far too small for mercury to register usefully. It dominates the low end of American pressure measurement — HVAC duct static pressure, industrial fan and blower ratings, gas-appliance regulator settings and cleanroom pressurisation are all read in inches of water column, because the numbers involved would otherwise be an awkward fraction of a psi.
The catch is that a column of water's height under a given pressure depends on the water's density, and density depends on temperature — so 'one inch of water' is not one single, universal figure the way an inch or a pascal is. This page uses the reference most commonly published for general engineering use: water at 4°C (39.2°F), its point of maximum density, giving exactly 249.082 pascals per inch, the value behind the 27.6806726025 factor shown here. Convert psi to inH2O anywhere without checking which reference applies and you're implicitly assuming this one.
It is not the only convention in circulation. The natural-gas metering and orifice industry, following API and AGA standards built around imperial field practice, traditionally references water at 60°F instead, giving a very slightly smaller figure — about 248.84 Pa per inch — because water is marginally less dense at 60°F than at its 4°C maximum. The difference between conventions is small, roughly a tenth of one percent, but it is not zero: a precise gas-metering calculation and a general HVAC static-pressure reading can legitimately use two different 'inches of water' and both be internally correct.
- Enter your reading into the PSI (psi) field; it opens at 1 psi, near the top of the range where inches-of-water gauges are typically used.
- Read Inches of water (inH2O) below — calculated using the 4°C (39.2°F) reference, the figure most commonly published in general engineering tables.
- Working with a gas-metering or orifice-plate calculation instead? Check whether your source standard specifies the 60°F reference, which gives results about 0.1% smaller than this page's.
- Going the other way, from inches of water to psi, divide your inH2O figure by 27.6806726025.
- For very low pressures, well under 1 psi, expect answers with several digits before the decimal; that's expected, since an inch of water is a far smaller pressure unit than a psi.
Worked example — a 1 psi blower rating, read on a manometer
An industrial dust-collection blower carries a manufacturer's maximum static-pressure rating of 1 psi on its spec sheet, but the technician commissioning the system reads duct pressure off a manometer graduated in inches of water column, the standard field instrument for this range. Enter 1 into PSI (psi) and Inches of water (inH2O) reads 27.6806726025 — the figure to compare against the manometer, using this page's 4°C reference.
That is a genuinely high static pressure for ductwork — roughly ten times what a comfort-ventilation system would ever see — which is exactly why the blower's rated maximum matters: exceeding it risks collapsing lighter-gauge ducting or overloading the fan motor. Had the technician instead been working from a gas-metering table referenced to 60°F, the same 1 psi would read about 27.7076 inH2O, a difference of roughly 0.027 inches — invisible on a field manometer but the kind of gap a metrology lab would document.
Questions
Why does the 'inches of water' figure change with temperature?
Because a column of water's height under a given pressure depends on how dense the water is, and density itself shifts with temperature — warmer water is slightly less dense and so needs to stand slightly taller to balance the same pressure. Water reaches its maximum density at 4°C (39.2°F), which is why that temperature became the reference point for the 'conventional' inch of water figure of 249.082 Pa used on this page; any other reference temperature gives a very slightly different number.
Which reference temperature should I actually use — 4°C or 60°F?
Check the standard or industry the figure came from. General engineering, HVAC and NIST reference tables typically default to the 4°C (39.2°F) convention used here, giving 249.082 Pa per inch. The natural-gas metering and orifice-plate industry, following API and AGA practice, traditionally references 60°F instead, giving about 248.84 Pa per inch. For most fieldwork the difference — about a tenth of one percent — is invisible on a manometer, but a metrology lab or gas custody-transfer calculation should match the convention its governing standard specifies.
Is 'inch of water' the same everywhere, or does it have other names?
Same physical idea, several names: inches of water column, inches WC, inH2O, and inches of water gauge (inWG) all mean the same unit, distinguished mainly by which trade is using it — HVAC favours 'inches WC,' gas utilities often say 'inches WG.' All of them inherit the same temperature-reference ambiguity discussed above, so the name alone doesn't reveal which reference a particular figure used; the source document usually does, in a footnote most readers skip.
Why not just use pascals for these low-pressure readings?
Pascals would remove the temperature-reference question entirely, and some fields — European ventilation engineering, acoustics — do exactly that. American trade practice, however, grew up around inches of water column for a physically direct reason: cheap, robust manometers and pressure gauges could be built and read using an actual water column long before precise electronic pressure sensors existed, and the habit and the instrumentation both outlived the reasons for the original choice.
How does inches of water compare to inches of mercury?
Mercury is about 13.6 times denser than water, so the same pressure produces a column of mercury about 13.6 times shorter than the equivalent water column — which is why barometric and vacuum readings use inches or millimetres of mercury while low-pressure duct and gas readings use inches of water instead. One psi comes to roughly 2.036 inHg, compared with 27.68 inH2O at the 4°C reference — a useful way to sanity-check which unit a given figure is actually in if it isn't labelled clearly.
How much precision does a field manometer actually need?
Rarely more than two decimal places, and often just one. A typical inclined manometer or digital gauge used for duct or gas-line testing resolves to about 0.01 inH2O at best, so 27.6806726025 inH2O is honestly 27.68 in the field — the extra digits exist so repeated conversions and calibration paperwork don't accumulate rounding error, not because any instrument on a job site can actually read them.