Millimeter Water (4 °C) to Pound per Square Inch
mmH20
psi
Conversion History
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Quick Reference Table (Millimeter Water (4 °C) to Pound per Square Inch)
| Millimeter Water (4 °C) (mmH20) | Pound per Square Inch (psi) |
|---|---|
| 1 | 0.00142229517053597597626090596439463056 |
| 10 | 0.01422295170535975976260760926656899004 |
| 25 | 0.03555737926339939940651684760035650176 |
| 100 | 0.14222951705359759762607029115618063816 |
| 250 | 0.35557379263399399406517427751307427984 |
| 1,000 | 1.42229517053597597626069856042967443492 |
| 10,332 | 14.69515370197770378672553966711640517936 |
About Millimeter Water (4 °C) (mmH20)
The millimeter of water at 4 °C (mmH₂O) is the pressure exerted by a 1 mm column of pure water at its maximum density, equal to approximately 9.807 pascals. It is used for very low pressure measurements where even pascals give large numbers: HVAC duct static pressures, spirometry and respiratory mechanics, building ventilation system balancing, and manometer readings in laboratory work. The 4 °C reference ensures maximum water density and measurement reproducibility.
HVAC supply duct static pressures typically range from 25 to 250 mmH₂O. A forced exhalation against resistance generates roughly 10–50 mmH₂O.
About Pound per Square Inch (psi)
The pound per square inch (psi) is the dominant pressure unit in US customary practice, equal to approximately 6,894.76 pascals. It is universally used in the United States for tire pressures, compressed-air systems, hydraulic machinery, pipe pressure ratings, and industrial gauges. Standard atmospheric pressure is 14.696 psi. Pressure is expressed as gauge (psig, relative to atmosphere) or absolute (psia). Natural gas pipeline pressures, boiler ratings, and municipal water supply pressures in the US are all quoted in psi.
Car tire inflation is typically 30–35 psi. A standard garden hose delivers water at 40–60 psi.
Etymology: A straightforward compound of "pound" (from Latin pondus, weight) and "square inch", arising naturally from US customary engineering practice. There is no single defining event; the unit crystallised alongside the development of American steam engineering in the early 19th century.
Millimeter Water (4 °C) – Frequently Asked Questions
Why is HVAC duct pressure measured in millimeters of water instead of pascals?
HVAC technicians originally measured duct pressure with a simple U-tube manometer filled with water — you literally read the height difference in millimeters. One mmH₂O ≈ 9.81 Pa, so a typical 25–250 mmH₂O duct pressure range corresponds to 245–2,450 Pa. The water column scale is still used because the instruments are cheap, intuitive, and field-rugged, even though digital gauges now display the same numbers electronically.
What does the "at 4 °C" part of the unit mean?
Water reaches maximum density at 3.98 °C (roughly 4 °C), where one cubic centimeter weighs exactly 1 gram. Specifying 4 °C ensures the pressure per millimeter of column height is reproducible and standardized. At 20 °C, water is about 0.2% less dense, introducing a tiny error. For most HVAC and lab work the difference is negligible, but calibration labs insist on the 4 °C reference for traceability.
How do you read a water manometer in mmH₂O?
Connect one side of a U-tube to the duct and leave the other open to atmosphere. The water level drops on the pressurized side and rises on the open side. The total height difference in millimeters is the gauge pressure in mmH₂O. Inclined (slant) manometers amplify small readings by tilting the tube — a 10:1 slope makes each millimeter of travel represent 0.1 mmH₂O, improving resolution for filter pressure-drop testing.
What mmH₂O range indicates a clogged HVAC filter?
A clean residential furnace filter creates 12–50 mmH₂O of pressure drop. When the drop exceeds 125–250 mmH₂O (varies by manufacturer), the filter is restricting airflow enough to hurt efficiency and strain the blower motor. Commercial systems set alarms at specific mmH₂O thresholds — when the differential pressure sensor hits the limit, a "replace filter" indicator lights up on the building management system.
How does mmH₂O relate to inches of water column (inH₂O)?
1 inch of water = 25.4 mmH₂O (since 1 inch = 25.4 mm). US HVAC specs use inches of water gauge (often written "in. w.g."); European and Asian specs use mmH₂O. If a US furnace manual says "maximum 0.5 in. w.g. static pressure," that is 12.7 mmH₂O. The conversion is just the familiar inch-to-millimeter factor applied to a column of water.
Pound per Square Inch – Frequently Asked Questions
Why do race car teams obsess over half-psi differences in tire pressure?
Because at racing speeds, tiny pressure changes transform tire behavior. Half a psi less inflates the contact patch, generating more grip but also more heat — potentially pushing the rubber past its optimal temperature window within a few laps. Half a psi more stiffens the sidewall, improving response but shrinking the contact patch and reducing peak grip. Formula 1 teams adjust pressures in quarter-psi increments based on track temperature, fuel load, and stint length. In NASCAR, a 1 psi difference between left and right tires is a deliberate setup tool that changes how the car rotates through banked turns.
What psi should car and bicycle tires be inflated to?
Passenger car tires: 30–35 psi (check the door jamb sticker, not the tire sidewall — the sidewall shows the maximum, not the recommended). Truck tires: 80–100 psi. Road bike tires: 80–130 psi. Mountain bike tires: 25–35 psi. Under-inflation by just 5 psi increases fuel consumption by about 2% and accelerates edge wear. Over-inflation reduces grip and makes the ride harsh.
What is the difference between psi, psig, and psia?
PSI is the generic unit. PSIG ("gauge") means pressure above atmospheric — what your tire gauge reads. PSIA ("absolute") includes atmospheric pressure on top: PSIA = PSIG + 14.7. A tire at 32 psig is actually at 46.7 psia. The distinction matters in engineering calculations involving gas laws (PV=nRT uses absolute pressure) and in vacuum work where gauge readings go negative.
What psi range do common household and industrial systems use?
City water supply: 40–80 psi. Home espresso machine: ~130 psi (9 bar). Pressure washer: 1,000–4,000 psi. Scuba tank: 3,000 psi. Hydraulic car jack: 3,000–10,000 psi. Diesel fuel injector rail: up to 30,000 psi. Waterjet cutter: 60,000+ psi. The range from a garden hose to an industrial waterjet spans roughly three orders of magnitude.
How do you quickly convert between psi and bar in your head?
Divide psi by 14.5 to get bar, or multiply bar by 14.5 to get psi. For even quicker estimates: 15 psi ≈ 1 bar, 30 psi ≈ 2 bar, 45 psi ≈ 3 bar. This "15 psi per bar" shortcut is within 2% of exact and is used by mechanics, divers, and engineers worldwide when a calculator is not handy.