Inch Mercury to Foot Water (4 °C)

The US National Weather Service inherited the convention from early American meteorology, which used mercury barometers calibrated in inches. A typical sea-level reading of 29.92 inHg is easy to remember and fits weather maps without decimal clutter. Most other countries switched to millibars or hectopascals, but the US stuck with inHg for the same reason it kept Fahrenheit — familiarity and institutional inertia.

US air traffic controllers broadcast the local barometric pressure in inches of mercury — for example, "altimeter two niner niner two" means 29.92 inHg. Pilots dial this into their altimeter so the instrument reads correct altitude above sea level. If the setting is wrong by just 0.1 inHg, the altimeter reads roughly 100 feet off — enough to matter during instrument approaches in fog.

At sea level, 29.92 inHg is standard. Readings above 30.20 inHg are high-pressure (clear skies, calm winds). Below 29.50 inHg is considered low pressure and often signals approaching storms. The lowest sea-level pressure ever recorded was Typhoon Tip in 1979 at 25.69 inHg (870 mbar). A household barometer swinging from 30.50 down to 29.30 is a reliable sign that weather is deteriorating.

Refrigeration techs evacuate AC system lines to remove moisture before charging with refrigerant. They measure the vacuum in inHg below atmospheric pressure — a reading of 29 inHg (out of 29.92 max) means near-total vacuum. Industry best practice requires pulling to at least 29.92 inHg (or equivalently, below 500 microns on a micron gauge) to ensure all moisture has boiled off at room temperature.

1 inHg ≈ 33.86 mbar ≈ 0.491 psi. So standard atmosphere (29.92 inHg) is about 1013 mbar or 14.7 psi. For quick mental math: multiply inHg by 34 to get millibars, or divide by 2 to get a rough psi estimate. These conversions come up constantly when comparing US weather data with international sources or converting aviation altimeter settings for foreign aircraft.

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.