Water Column To Psi

Converting Water Column to PSI

The conversion itself is straightforward, but the place where people mess it up is usually downstream of the math. Before I get into the formula, I want to talk about why this conversion comes up in the first place and what goes wrong when you use it blindly. Water column, often written as w.c. or wc, is a unit of pressure used almost exclusively for low-pressure gas systems. Furnace manifolds, gas valves, pilot lights, natural gas supply pressure — these are all measured in water column. PSI is the standard pressure unit for everything else in the building trades. When you're troubleshooting a system that bridges those two worlds, or when a spec sheet lists one and the other, you need to convert.

Water Column To Psi Formula

1 inch of water column equals approximately 0.03609 psi at standard conditions. The exact conversion factor is derived from the density of water at 4 degrees Celsius, which is 62.43 pounds per cubic foot. Multiply your water column value by 0.03609 and you get psi. Divide psi by that same number and you get water column. For practical field work most people just remember that 27.7 inches of water column is roughly 1 psi. That's close enough for gas pressure work where tolerances are already loose. If you're doing something that requires more precision, keep more decimal places. The rounding error compounds when you're working with multiple conversions in a row. I ran into a specific problem last year that illustrates why the rounding matters more than you'd expect. I was diagnosing a malfunctioning commercial condensing boiler that kept locking out on high flame error. The manufacturer's specs called for 10.5 inches w.c. at the manifold. The technician before me had measured 0.38 psi and assumed that checked out because 0.38 times 27.7 gives you about 10.5. The problem was that 0.38 psi is actually 10.54 inches w.c., and this particular boiler's high-fire adjustment window was only 0.4 inches w.c. wide. The burner was running hot, causing the heat exchanger to exceed temperature limits and trip the safety. Switching to a digital manometer that reads in inches w.c. directly eliminated the conversion error entirely. Never convert back and forth if your gauge can read the native unit.

Here's a practical conversion table for the range you'll actually see in the field: 0.5 inch w.c. = 0.018 psi
1 inch w.c. = 0.036 psi
3 inch w.c. = 0.108 psi
5 inch w.c. = 0.180 psi
7 inch w.c. = 0.253 psi
10 inch w.c. = 0.361 psi
11 inch w.c. = 0.397 psi
14 inch w.c. = 0.505 psi
28 inch w.c. = 1.010 psi Natural gas residential supply pressure typically runs between 3.5 and 7 inches w.c. LPG runs higher, around 10 to 13 inches w.c. at the appliance inlet. If your manifold pressure falls outside those ranges you've got a regulator problem, a supply problem, or an orifice problem — not a conversion problem.

Things That Break the Conversion

The 0.03609 factor assumes water at 4°C and standard atmospheric pressure. Temperature affects water density, and altitude affects the atmospheric reference point. In practice these variables matter way more than the math. When I'm working on a rooftop unit at altitude or in an unconditioned space during winter, the actual water column reading on my manometer shifts because the test fluid temperature has changed. A column of water at 80°F is less dense than one at 40°F, and that difference shows up on the gauge. The bigger issue is that water column measurements are inherently sensitive to static pressure in the system. If you're measuring supply pressure on a running furnace and your manometer reference leg isn't properly vented, you'll read incorrectly. This happens constantly. People forget that a U-tube manometer measures differential pressure, not absolute pressure. Connect both sides to the same point and it reads zero. Connect one side to pressure and leave the other open to atmosphere and it reads the static pressure relative to atmospheric. Get the setup wrong and your conversion to psi is meaningless regardless of how accurate the math is. For digital gauges, check the units setting. Many inexpensive models default to inches w.c. but switch to psi automatically at certain thresholds, and the transition isn't always marked clearly. I've lost time on jobs where the gauge had flipped to psi without me noticing, and I was multiplying a psi reading by the w.c. conversion factor thinking I was working in the native unit.

If you need a quick reference while working, the conversion calculator I use is available here: UnitConverters water column to psi. It handles the math instantly and saves you from carrying a conversion factor in your head while you're also dealing with a live gas system. One more thing worth noting: water column is technically a measure of pressure, not flow. People sometimes confuse it with flow rate when they're reading gauges on gas equipment. The pressure tells you how hard the gas is pushing. The flow tells you how much gas is actually moving. They're related through the orifice size and gas density, but converting water column to psi doesn't tell you anything about flow. If you're troubleshooting a flame that's too low or too high, pressure might be right on spec and you still have a problem. Check the orifice, check the gas composition, check the air ratio. Don't assume the pressure reading is the whole story.