Millimeter Water (4 °C) to Foot Water (4 °C)
mmH2O
ftH2O
Conversion History
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|---|---|---|
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Quick Reference Table (Millimeter Water (4 °C) to Foot Water (4 °C))
| Millimeter Water (4 °C) (mmH2O) | Foot Water (4 °C) (ftH2O) |
|---|---|
| 1 | 0.0032808399 |
| 10 | 0.032808399 |
| 25 | 0.082020997 |
| 100 | 0.32808399 |
| 250 | 0.82020997 |
| 1,000 | 3.2808399 |
| 10,332 | 33.897638 |
Results rounded to a maximum of 8 significant figures
About Millimeter Water (4 °C) (mmH2O)
The millimeter of water at 4 °C (mmH₂O) is the pressure exerted by a 1 mm column of pure water at 4 °C under standard gravity. Using the fixed reference density of 999.972 kg/m³, it equals 9.8063754138 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 specifies a temperature close to water's maximum density for reproducible reference conversions.
HVAC supply duct static pressures typically range from 25 to 250 mmH₂O. A forced exhalation against resistance generates roughly 10–50 mmH₂O.
About Foot Water (4 °C) (ftH2O)
The foot of water at 4 °C (ftH₂O) equals 2,988.98322612624 pascals under the fixed-density reference and standard gravity. This converter derives the factor from 9.8063754138 Pa/mm × exactly 304.8 mm/ft. It is used in US hydraulic engineering, pump head specifications, and well-drilling. Total dynamic head (TDH) in American water system design is expressed in feet of water. One ftH₂O equals 12 inH₂O with the same reference conditions. Firefighting system pressures and potable water distribution designs commonly reference feet of head.
A residential well pump typically delivers 40–60 ft of head. A standard building fire-sprinkler system requires 15–25 ftH₂O of minimum pressure.
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 is densest near 4 °C, but its density is not exactly 1 gram per cubic centimeter. Column pressure follows p = ρgh, so both density and gravity matter. This converter combines a fixed reference density of 999.972 kg/m³ with standard gravity (9.80665 m/s²), giving 98.063754138 Pa per centimeter without rounding the reference calculation. NIST publishes the shorter approximate value 98.0638 Pa/cm. The result also differs from the conventional 98.0665 Pa/cm factor that assumes a density of 1,000 kg/m³. Real manometer readings may require temperature and local-gravity corrections.
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)?
For water-column units with the same temperature and gravity reference, 1 inch of water = 25.4 mmH₂O, since 1 inch = 25.4 mm. Thus 0.5 inches of water at 4 °C equals 12.7 mmH₂O at 4 °C. US HVAC specs often use inches of water gauge ("in. w.g."); European and Asian specs use mmH₂O. Check the stated reference before comparing factors: a water-column unit at 4 °C is slightly different from one at another temperature or the conventional density-based unit.
Foot Water (4 °C) – Frequently Asked Questions
Why does a gravity-fed water tower need to be so tall to supply decent pressure in ftH₂O?
Each foot of water head provides about 1 ftH₂O of static pressure when water density and gravity are close to the unit's reference conditions; actual pressure at a tap also depends on pipe losses. A comfortable shower needs about 20–25 ftH₂O, and a fire hydrant demands 40–60 ftH₂O. So a water tower serving a flat town typically stands 40–60 feet above rooftop level to provide pressure. Taller buildings in the service area need even more height — or booster pumps — because each story above ground "uses up" about 10 ftH₂O of the tower's gravity-supplied head.
How do you convert feet of water to psi?
1 ftH₂O ≈ 0.4335 psi. For an estimate, divide psi by 0.4335 (or multiply by 2.31) to get feet of head. A city water main at 60 psi delivers about 138 ft of head — enough to reach the 12th floor of a building by gravity alone. This 2.31 factor is worth memorising if you work in US plumbing or fire-protection engineering; it pops up in every pipe-sizing calculation.
Why do US well drillers and plumbers prefer feet of water over psi?
Because the physical setup is literally vertical — a well pump sits at the bottom of a hole and pushes water up. Saying "the pump needs 150 feet of head" maps directly to the well depth plus the elevation to the pressure tank. Converting to psi (65 psi) loses that physical clarity. Fire-sprinkler designers think the same way: "how high does water need to climb?" is answered in feet, not pounds.
What is the relationship between ftH₂O and inH₂O?
1 ftH₂O = 12 inH₂O, just as 1 foot = 12 inches. Inches of water are used for low-pressure air systems (HVAC ducts at 0.1–4 inH₂O), while feet of water handle higher liquid pressures (municipal water at 40–140 ftH₂O). The two scales cover different engineering domains but share the same underlying physics — pressure from a column of water at 4 °C under standard gravity.
How much pressure does 33.9 feet of seawater exert?
About 1 atmosphere (14.7 psi). Divers learn the "33 feet" rule: every 33 feet of seawater adds 1 atm of pressure. (Fresh water is slightly less dense, so the equivalent is about 34 feet.) At 100 feet, a diver is under roughly 4 atm total — 3 gauge plus 1 atmospheric. This is why recreational dive limits are set at 130 ft (about 5 atm) — beyond that, nitrogen narcosis becomes a serious risk.