Pressure Conversion Chart
The three pressure worlds HVAC works in, converted: inches of water column (duct static, gas trains) to Pa, kPa, psi, and oz/in²; psi (hydronics, refrigerant gauges) to metric and feet of head; and the reference points every gauge conversation lands on. 1 psi = 27.68 in.wc.
Inches of water column → Pa / kPa / psi / oz-in²
| in. w.c. | Pa | kPa | psi | oz/in² |
|---|---|---|---|---|
| 0.05 | 12.45 | 0.01245 | 0.00181 | 0.0289 |
| 0.1 | 24.91 | 0.02491 | 0.00361 | 0.0578 |
| 0.2 | 49.82 | 0.04982 | 0.00723 | 0.116 |
| 0.3 | 74.73 | 0.07473 | 0.0108 | 0.173 |
| 0.5 | 124.5 | 0.1245 | 0.0181 | 0.289 |
| 0.8 | 199.3 | 0.1993 | 0.0289 | 0.462 |
| 1 | 249.1 | 0.2491 | 0.0361 | 0.578 |
| 2 | 498.2 | 0.4982 | 0.0723 | 1.16 |
| 3 | 747.3 | 0.7473 | 0.108 | 1.73 |
| 5 | 1245 | 1.245 | 0.181 | 2.89 |
| 10 | 2491 | 2.491 | 0.361 | 5.78 |
psi → kPa / bar / in.wc / ft of water
| psi | kPa | bar | in. w.c. | ft of water |
|---|---|---|---|---|
| 1 | 6.895 | 0.06895 | 27.68 | 2.307 |
| 2 | 13.79 | 0.1379 | 55.36 | 4.613 |
| 5 | 34.47 | 0.3447 | 138.4 | 11.53 |
| 10 | 68.95 | 0.6895 | 276.8 | 23.07 |
| 15 | 103.4 | 1.034 | 415.2 | 34.6 |
| 20 | 137.9 | 1.379 | 553.6 | 46.13 |
| 30 | 206.8 | 2.068 | 830.4 | 69.2 |
| 40 | 275.8 | 2.758 | 1,107 | 92.27 |
| 50 | 344.7 | 3.447 | 1,384 | 115.3 |
| 60 | 413.7 | 4.137 | 1,661 | 138.4 |
| 80 | 551.6 | 5.516 | 2,214 | 184.5 |
| 100 | 689.5 | 6.895 | 2,768 | 230.7 |
| 125 | 861.8 | 8.618 | 3,460 | 288.3 |
| 150 | 1034 | 10.34 | 4,152 | 346 |
Reference points
| Reference point | psi | kPa | in. w.c. | in. Hg |
|---|---|---|---|---|
| 1 atmosphere (sea level) | 14.696 | 101.325 | 406.78 | 29.921 |
| 1 bar | 14.504 | 100 | 401.46 | 29.53 |
| 1 psi | 1 | 6.89476 | 27.68 | 2.036 |
| 7 in.wc — natural-gas manifold | 0.25289 | 1.74362 | 7 | 0.51489 |
| 11 in.wc — propane manifold | 0.3974 | 2.73998 | 11 | 0.80912 |
Why HVAC uses three pressure units
The scales differ by orders of magnitude, and each unit fits its range. Duct static and gas manifolds live below half a psi, where a psi gauge would barely move — hence inches of water column. Hydronic loops and refrigerant circuits run tens to hundreds of psi. And metric documentation states everything in Pa or kPa. The tables above are sliced so each unit's real working range reads naturally in its own column.
Work duct statics with the static pressure calculator and piping with the pressure loss calculator. The component-by-component drops are in the static pressure drop chart, and gas-line sizing at these manifold pressures is in the gas pipe sizing chart.
Common questions
How many inches of water column in 1 psi?
One psi equals 27.68 inches of water column (conventional water column; sources quoting 27.6807 use true 4 °C water density — the difference is under 0.003%). Going the other way, 1 in.wc is about 0.036 psi, which is why ductwork and gas trains use the finer water-column unit in the first place.
What is a Pascal compared to inches of water column?
One inch of water column is 249.1 pascals, so metric duct static specs in Pa divide by roughly 250: a 125 Pa external static is about 0.5 in.wc. European and rooftop-unit documentation often lists Pa; North American field gauges read in.wc.
What are standard gas manifold pressures in inches of water column?
Natural gas appliances typically run a 3.5 in.wc manifold with 7 in.wc supply; propane runs a 10–11 in.wc manifold. The reference-point table pins 7 and 11 in.wc because they are the two values checked at every gas appliance startup — always confirm against the rating plate.