Inches of Water to PSI Converter
One inch of water column (inH₂O, also written as in.w.c. or in.wg) at 4°C equals exactly 0.0361263 psi under standard gravity. This unit is the de facto standard for low-pressure...
Formula
Source: Engineering Toolbox | Last reviewed: June 7, 2026
Examples
1 in H₂O
= 0.0361 psi
1 inH₂O = 0.0361 psi
27.68 in H₂O
= 1 psi
27.68 inH₂O = 1 psi
100 in H₂O
= 3.613 psi
Quick Reference Table
| inH₂O | psi |
|---|---|
| 1 | 0.036 |
| 5 | 0.181 |
| 10 | 0.361 |
| 20 | 0.723 |
| 50 | 1.807 |
| 100 | 3.613 |
Where is this used?
A typical VAV air-handling unit might show 3.5 inH₂O of external static pressure at design airflow; converting to 0.126 psi puts this in context with the AHU casing structural rating (often ±8 inH₂O or ±0.29 psi maximum), confirming the casing is operating within its structural limit.
For industrial dust collection and fume extraction systems designed per SMACNA guidelines, duct static pressures can reach 10–20 inH₂O (0.36–0.72 psi) at the fan inlet, and the system designer must convert these values to psi when specifying the explosion relief panel burst rating (typically 1–2 psi) to ensure the panel does not prematurely rupture during normal operation yet relieves before the ductwork reaches its yield pressure.
In cleanroom and isolation room pressurization per ASHRAE 170 for healthcare facilities, the required differential pressure is typically 0.01 to 0.05 inH₂O between spaces — converting to 0.00036 to 0.0018 psi illustrates just how sensitive these measurements are and why stray air currents from door openings or elevator piston effect can disrupt the pressure cascade.
In natural gas distribution, the utility service regulator at a commercial building drops delivery pressure from medium pressure (5–60 psig) to 7 inH₂O (0.253 psi) at the meter outlet, and appliance burner manifolds typically require 3.5 inH₂O (0.126 psi).
A contractor who misreads 7 inH₂O as 7 psi and installs a regulator accordingly would deliver uncontrolled high pressure to the building, creating an immediate explosion hazard — the conversion is a literal life-safety step.
In laboratory fume hood certification per ANSI/ASHRAE 110, face velocity is correlated with hood static pressure drop, typically measured in inH₂O, and the conversion to psi allows the safety officer to compare the hood's pressure drop against the laboratory's exhaust fan capability curve.
In pharmaceutical manufacturing, lyophilizer (freeze dryer) chamber vacuum is often measured in microns of mercury or inches of water during the drying cycle crossover, and the inch-of-water range (0 to 1 inH₂O absolute during primary drying) must be converted to psia for integration with the PLC control system if the pressure transmitter outputs in psi.
Filter monitoring in air-handling systems uses differential pressure switches set in inH₂O (e.g., 1 inH₂O for pre-filters, 2 inH₂O for final filters per ASHRAE Guideline 26), which trigger alarms when filters load — the conversion to psi or kPa is necessary when the BAS or DDC controller expects a universal pressure unit for trending and alarm setpoint entry.
Real-World Usage Scenarios
AHU Casing Pressure Limit Verification
A custom air-handling unit serves a pharmaceutical manufacturing suite with a design external static pressure of 6.5 inH₂O. The AHU casing is shop-rated for ±10 inH₂O (0.36 psi) per SMACNA Class 10. During commissioning, the TAB contractor measures 7.2 inH₂O at the fan discharge under a temporary higher-resistance filter configuration. Converting 7.2 inH₂O to 0.26 psi confirms the casing is still within its structural limit, but the commissioning agent notes that the 10 inH₂O rating leaves only a 28% safety margin — prompting a recommendation to add casing reinforcement or reduce filter loading trigger points in the BAS sequence of operations.
Cleanroom Pressure Cascade Design
An ISO Class 5 aseptic filling room requires +0.05 inH₂O positive pressure relative to the adjacent ISO Class 7 buffer room, which itself requires +0.03 inH₂O relative to the unclassified corridor. The cascade: 0.0018 psi (filling), 0.00108 psi (buffer), 0 psi (corridor). The HVAC controls engineer programs the VAV box pressure control loop setpoints in the BAS — which only accepts psi inputs — by converting each cascade step. A 0.001 psi rounding error at each step could invert the pressure cascade direction, compromising sterility assurance. Precision conversion with at least four decimal places of psi resolution is mandatory.
Natural Gas Appliance Commissioning
A commercial kitchen receives natural gas at 7 inH₂O from the utility regulator. The appliance manifold requires 3.5 inH₂O per the manufacturer, and an in-line appliance regulator reduces pressure at each unit. A technician using a digital manometer set to psi inadvertently reads 0.126 psi at the appliance inlet (correctly 3.5 inH₂O) and reports that pressure is within spec. In reality, the regulator had failed open and was delivering 14 inH₂O (0.506 psi) — the technician's unit-mode error masked a hazardous over-pressure condition. Post-incident, the kitchen's commissioning checklist was updated to require dual-unit verification (inH₂O and psi) for all gas manifold checks.
Common Mistakes to Avoid
Misreading inH₂O gauge value as psi
A technician measuring duct static pressure reads 1.5 on the manometer and records '1.5 psi' on the TAB report. The actual pressure is 1.5 inH₂O = 0.054 psi — an error factor of 28×. If the AHU fan was then selected based on 1.5 psi instead of 1.5 inH₂O, the selected motor would be grossly oversized, wasting thousands in capital cost and operating energy over the equipment lifetime. Always verify the unit label on the instrument and the report form before accepting a value.
Confusing inH₂O with inches of mercury (inHg)
Mercury is 13.6 times denser than water, so 1 inHg = 13.6 inH₂O. A technician who converts a 2 inHg vacuum reading using the inH₂O factor (×0.03613) gets 0.072 psi instead of the correct 0.982 psi — a 14× error. This mistake commonly occurs when interpreting medical gas vacuum system readings (often in inHg) or legacy boiler draft gauges that use mercury-filled manometers.
Ignoring water density correction for precision manometry
At 100°F, water density drops to 61.99 lb/ft³ from 62.43 lb/ft³ at 4°C. For a 10 inH₂O measurement at 100°F, the true pressure is 0.3586 psi instead of 0.3613 psi — a 0.75% difference. While negligible for HVAC TAB work, in laboratory flow measurement using water manometers for primary calibration of critical flow venturis, such errors propagate through the calibration hierarchy and must be corrected per NIST Technical Note 1297 guidelines on uncertainty analysis.
Using the conversion for fluids other than water without SG correction
A manometer filled with indicating fluid other than water (e.g., red gauge oil with SG = 0.826, or Meriam 827 with SG = 2.95) displays a column height but the conversion factor changes. An inH₂O-equivalent reading on such a manometer must be multiplied by the fluid SG before converting to psi. Numerous field errors occur when technicians use 0.03613 directly on the reading without the SG step.
Industry Standards Referenced
Frequently Asked Questions
What temperature does this conversion assume?
This conversion assumes water at 4°C (maximum density). At room temperature (20°C), the factor is about 0.03609 psi/inH₂O — a negligible difference for most applications.
What is the difference between inH₂O and inHg?
Inches of water measures low pressure using water. Inches of mercury uses mercury (13.6× denser). 1 inHg = 13.6 inH₂O ≈ 0.491 psi.
How is inH₂O measured?
Traditionally with a U-tube manometer filled with water. Modern electronic pressure transmitters are calibrated in inH₂O for low-pressure ranges (0–10 inH₂O typical).
Why does HVAC use inches of water instead of psi?
HVAC system pressures are very low — typically 0.5 to 5 inH₂O for supply ductwork and 0.05 to 0.5 inH₂O for return air paths. Expressing these as 0.018 to 0.18 psi would require cumbersome decimal notation on plans, instruments, and TAB reports. Inches of water provides intuitive whole-number values and directly represents the water-column height visible on a manometer, making field verification straightforward for technicians.
What is the SMACNA duct pressure classification system?
SMACNA classifies ductwork by operating pressure in inches of water: Class 0.5 (<0.5 inH₂O), Class 1 (0.5–1 inH₂O), Class 2 (1–2 inH₂O), Class 3 (2–3 inH₂O), Class 4 (3–4 inH₂O), Class 6 (4–6 inH₂O), and Class 10 (6–10 inH₂O). Each class specifies minimum sheet metal gauge, reinforcement spacing, and seal class requirements. Converting these to psi: Class 10 = 0.36 psi maximum — a surprisingly low number that underscores why psi is rarely used in duct design despite being the underlying physical unit.
Reviewed for accuracy
Reviewed against ASHRAE 90.1 and SMACNA standards · Last reviewed: June 7, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.