psi to inH₂O Converter
1 psi = 27.6806726 inH₂O at 4°C (39.2°F, water's maximum density) under standard gravity. The conversion factor is derived from: water density at 4°C = 1,000 kg/m³ = 62.42796...
Formula
Source: NIST SP 811, ASHRAE Handbook, Fundamentals, ASHRAE Standard 170 | Last reviewed: June 27, 2026
Examples
1 psi
= 27.68 in H₂O
1 psi = 27.68 inH₂O
0.0361 psi
= 1 in H₂O
0.0361 psi = 1 inH₂O (alternate reference)
0.5 psi
= 13.84 in H₂O
0.5 psi = 13.84 inH₂O, typical natural gas line pressure drop
5 psi
= 138.4 in H₂O
5 psi = 138 inH₂O, cooling coil waterside pressure drop
Quick Reference Table
| psi | inH₂O | ft H₂O | Context |
|---|---|---|---|
| 0.001 | 0.0277 | 0.0023 | Cleanroom differential pressure (0.03-0.05 inH₂O typical) |
| 0.01 | 0.277 | 0.023 | Low duct static pressure |
| 0.0361 | 1 | 0.0833 | 1 inH₂O exactly |
| 0.1 | 2.77 | 0.231 | Moderate duct pressure |
| 0.5 | 13.84 | 1.15 | Gas piping allowable drop |
| 1 | 27.68 | 2.31 | 1 psi reference |
| 5 | 138.4 | 11.5 | Water coil pressure drop |
| 14.7 | 406.9 | 33.9 | Atmospheric pressure (34 ft H₂O) |
Where is this used?
The conversion factor is derived from: water density at 4°C = 1,000 kg/m³ = 62.42796 lb/ft³, so the height of a water column exerting 1 psi = (1 lb/in² × 144 in²/ft²) / 62.42796 lb/ft³ = 2.30666 ft = 27.680 inches.
The inches of water column (inWC, inH₂O, or in w.g., inches water gauge) is the universal unit for low-pressure measurements in HVAC, cleanroom, and industrial ventilation applications.
A typical residential HVAC duct system operates at 0.5-1.5 inH₂O of static pressure, tiny fractions of a psi (0.018-0.054 psi).
A cleanroom requiring 0.05 inH₂O positive differential pressure relative to the adjacent space (per ISO 14644) is maintaining just 0.0018 psi, a pressure difference too small to measure with a standard Bourdon tube gauge but easily measured with an inclined manometer or micro-manometer calibrated in inH₂O.
A hospital operating room maintaining 0.03 inH₂O positive pressure relative to the corridor (per ASHRAE 170), the equivalent of 0.0011 psi, prevents airborne contaminants from entering the sterile field.
This illustrates why inH₂O exists: it's a practical unit for pressures far below 1 psi.
The conversion from psi to inH₂O is commonly needed when a US mechanical engineer knows a system's pressure drop in psi (from pipe friction calculations, pump head, or compressor specifications) and needs to communicate it in inH₂O for HVAC equipment selection and duct design.
A cooling coil with a waterside pressure drop of 5 psi converts to 5 × 27.68 = 138.4 inH₂O (11.5 ft H₂O), this pressure drop is added to the pump head requirement.
A natural gas piping system with a allowable pressure drop of 0.5 psi (per IFGC for low-pressure gas systems) converts to 13.8 inH₂O, and gas pressure regulators and appliance gas valves are often calibrated in inH₂O (a typical residential natural gas appliance manifold pressure is 3.5-5 inH₂O).
The conversion is also critical in air pollution control: a baghouse or scrubber with a pressure drop of 6 inH₂O across the filter media is losing 6 / 27.68 = 0.217 psi, this pressure loss must be added to the fan static pressure requirement, expressed in inH₂O (the fan industry standard) or in psi (for the mechanical equipment schedule).
The factor 27.68 (or roughly 27.7) is worth memorizing alongside 2.31 (feet of water per psi), the two are related by a factor of 12 in/ft.
Real-World Usage Scenarios
HVAC system fan static pressure calculation
An HVAC engineer is designing the duct system for a 10,000 CFM commercial air handler. The total external static pressure (ESP) is calculated from component pressure drops: filter (pre-filters 0.3 inH₂O + HEPA filters 1.0 inH₂O = 1.3 inH₂O), cooling coil (0.8 inH₂O), heating coil (0.5 inH₂O), supply ductwork (2.5 inH₂O including fittings and run length), return ductwork (1.2 inH₂O), terminal boxes (1.5 inH₂O at design airflow), dampers (0.5 inH₂O for modulating outdoor air damper). Total ESP: 1.3 + 0.8 + 0.5 + 2.5 + 1.2 + 1.5 + 0.5 = 8.3 inH₂O. Converting to psi for the fan selection (US manufacturers): 8.3 / 27.68 = 0.300 psi. The fan selection software inputs the ESP in psi, but the duct design report presents the breakdown in inH₂O for clarity. The conversion between the two units is performed at every fan selection iteration. A 1% error in the conversion (using 27.4 instead of 27.68) corresponds to about 0.003 psi error in the ESP, small individually, but it could affect the fan speed selection and the motor brake horsepower calculation, which scales with pressure. For a 10 HP motor at $0.10/kWh over 20 years of operation, a 1% efficiency loss is about $1,500, meaningful for the lifetime operating cost.
Industry Standards Referenced
Frequently Asked Questions
Why does HVAC use inches of water instead of psi?
Because HVAC pressures are tiny. A typical residential duct system operates at 0.5-1.5 inH₂O, that's 0.018-0.054 psi. A standard 0-100 psi pressure gauge wouldn't even register these values. Inches of water provide a scale where the numbers are readable integers in the 0-10 range for most HVAC work. A cleanroom differential pressure of 0.05 inH₂O is a small but critical pressure, in psi, it's 0.0018, a value too small for most pressure gauges to resolve accurately.
Is the conversion factor different at different temperatures?
Slightly. The standard factor (27.68) assumes water at 4°C (39.2°F, maximum density: 1,000 kg/m³). At 20°C (68°F), water density drops to 998.2 kg/m³, and the factor becomes 27.73 inH₂O per psi, a 0.2% difference. At 100°C (212°F), water density drops to 958.4 kg/m³, and the factor becomes 28.88 inH₂O per psi, a 4.3% difference. For most HVAC and industrial ventilation work, the 27.68 factor is standard and the temperature correction is negligible. For precision work (laboratory manometry at non-standard temperatures), apply the water density correction.
What does this converter do?
This converter performs the unit conversion at standard conditions using the exact conversion factor. The result is displayed with appropriate precision for engineering use.
Reviewed for accuracy
Verified against ASHRAE and NIST pressure conversion standards · Last reviewed: June 27, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.