Foot Water (4 °C) to Centimeter Water (4 °C)

Because every foot of elevation equals exactly 1 ftH₂O of pressure at the tap below. 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 guarantee adequate pressure during peak demand. 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.

1 ftH₂O = 0.4335 psi. So 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.

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.

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.

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.

Respiratory medicine adopted cmH₂O because the original ventilators literally used water columns to regulate pressure — a jar of water with a submerged tube set the pressure at whatever depth the tube was immersed. A CPAP setting of 10 cmH₂O meant the air bubbled out at 10 cm depth. The unit stuck even after electronics replaced water seals, because clinicians, patients, and device manuals all speak the same scale.

Most adults are prescribed between 6 and 14 cmH₂O, with 10 cmH₂O being a common starting point. Severe obstructive sleep apnoea may require 15–20 cmH₂O. Auto-titrating (APAP) machines vary pressure within a set range — typically 4–20 cmH₂O — adjusting breath by breath. Higher pressures are more effective at splinting the airway open but can cause discomfort and air swallowing.

ICU ventilators also use cmH₂O. Positive end-expiratory pressure (PEEP) is usually set at 5–15 cmH₂O to keep alveoli open. Peak inspiratory pressure above 30–35 cmH₂O raises the risk of lung injury. Plateau pressures are monitored to stay below 30 cmH₂O. The entire field of mechanical ventilation runs on this single unit because it directly corresponds to the pressures inside the lung.

Measured via lumbar puncture with the patient lying on their side, normal CSF pressure is 10–18 cmH₂O in adults. Above 25 cmH₂O suggests raised intracranial pressure — potentially from a tumor, meningitis, or hydrocephalus. Below 6 cmH₂O indicates low pressure, often from a CSF leak. Neurologists use cmH₂O rather than mmHg because spinal fluid is essentially water, making the unit a direct physical analogue.

1 cmH₂O ≈ 0.981 mbar ≈ 0.0981 kPa. For bedside estimates, 1 cmH₂O ≈ 1 mbar is close enough (error under 2%). A CPAP setting of 12 cmH₂O is about 11.8 mbar or 1.18 kPa. Since respiratory equipment universally reads cmH₂O, conversion is mainly needed when interfacing with industrial instruments or when charting pressures alongside blood gas data reported in mmHg.