Your weather app says the barometer sits at 29.92. Your tire gauge says 32. Both numbers describe pressure, yet they look nothing alike. One speaks the language of mercury columns; the other speaks the language of force on a surface. If you work across aviation, weather, HVAC, or mechanical systems, you will eventually need to translate between them. This guide shows you exactly how inch of mercury (inHg) and pounds per square inch (psi) relate, when each unit appears, and how to convert between them without second-guessing yourself.
What Is an Inch of Mercury?
An inch of mercury measures pressure by asking a simple question: how tall a column of mercury can this pressure hold up? When atmospheric pressure pushes down on a pool of liquid mercury inside a glass tube, it lifts the mercury to a certain height. If that height is one inch, the pressure equals one inch of mercury.
The symbol is inHg. The idea dates back to the earliest barometers, which used real mercury tubes to weigh the invisible weight of the air. Modern instruments have gone digital, but the unit stuck around.
Where You'll See inHg in Real Life
- Aviation: Pilots set their altimeters using barometric pressure reported in inches of mercury. The standard sea-level value, 29.92 inHg, is memorized by every pilot on Earth.
- Meteorology: U.S. weather broadcasts still report barometric pressure in inHg, especially during storm coverage.
- HVAC and refrigeration: Technicians pull vacuums on systems and read the results in inches of mercury on their manifold gauges.
- Automotive diagnostics: Engine vacuum at idle is often measured in inHg, giving mechanics a fast health check on an engine.
What Is a Pound Per Square Inch?
Pounds per square inch takes a more direct approach. Instead of lifting a liquid, it measures raw force: imagine one pound of force pressing evenly on a one-inch-by-one-inch square. That is one psi.
The symbol is psi, and it belongs to the imperial and U.S. customary measurement systems. Because it describes force per area in plain mechanical terms, engineers and technicians reach for it constantly.
Where You'll See psi in Real Life
- Tires: The sticker on your driver's door frame lists recommended tire pressure in psi.
- Hydraulics: Excavators, presses, and brake systems all specify operating pressures in psi.
- Compressed air: Shop compressors, nail guns, and spray rigs run on psi ratings.
- Scuba and gas cylinders: Tank pressure gauges read in psi so divers know exactly how much breathing gas remains.
How inHg and psi Relate to Each Other
Here is the key insight: these units are not rivals. They measure the identical physical quantity—pressure—just through different reference frames. One counts the height of a mercury column. The other counts force spread over an area. Any pressure expressed in one unit can be expressed in the other.
Under standard conditions, a column of mercury one inch tall presses down with just under half a pound of force on every square inch beneath it. That gives us the working relationship:
1 inHg ≈ 0.4912 psi
Flip it around and you get the reverse view: one psi supports a mercury column of roughly 2.04 inches. Notice how convenient the first number is. Since 0.4912 sits just under one-half, you can estimate conversions in your head by cutting the inHg value roughly in half.
A Mental Math Shortcut
- Take your inHg value.
- Halve it.
- Nudge the result up by about 2%.
Example: 20 inHg. Half is 10. Add 2% and you land near 9.82 psi—which matches the precise figure. For casual estimates, plain halving gets you close enough. For engineering work, always use the full factor.
Putting the Conversion to Work
Real-world scenarios make the relationship click. Consider these everyday cases:
- Standard atmosphere: Sea-level pressure of 29.92 inHg converts to approximately 14.70 psi. That second number may look familiar—it is the classic figure for atmospheric pressure at sea level.
- Engine health check: A healthy gasoline engine typically pulls 17 to 21 inHg of vacuum at idle. Converted, that is roughly 8.3 to 10.3 psi below atmospheric pressure.
- HVAC evacuation: A technician pulling a deep vacuum might see readings climb well past 25 inHg on the gauge, equivalent to more than 12 psi of suction relative to the atmosphere.
- Storm coverage: A powerful hurricane can drive barometric readings down toward 27 inHg, or about 13.26 psi—a drop of nearly 1.5 psi from normal conditions. Small-sounding, but spread over an entire coastline, that difference moves enormous volumes of air and water.
Quick Reference Conversion Table
The table below covers the most commonly requested inHg values, converted to psi. Figures are rounded to four decimal places.
| Inches of Mercury (inHg) | Pounds Per Square Inch (psi) |
|---|---|
| 1 | 0.4912 |
| 2 | 0.9823 |
| 5 | 2.4558 |
| 10 | 4.9115 |
| 20 | 9.8231 |
| 50 | 24.5577 |
| 100 | 49.1154 |
| 500 | 245.5770 |
Four Mistakes to Avoid
1. Ignoring Temperature Effects
The inHg scale assumes mercury at a standard reference temperature. Mercury expands when warm and contracts when cold, which slightly shifts what a given pressure reads on a physical mercury column. Precision instruments compensate for this automatically; old-school glass barometers do not.
2. Mixing Up Gauge and Absolute Pressure
Many psi readings are gauge pressures, meaning they measure above ambient atmospheric pressure. Vacuum readings in inHg usually run the other direction, measuring below atmospheric pressure. Before converting, confirm which side of atmospheric pressure each value represents.
3. Confusing inHg with inH₂O
HVAC and airflow work frequently uses inches of water (inH₂O), a much smaller unit than inches of mercury. Water is far less dense than mercury, so the two units differ by a large margin. Always read the gauge label twice.
4. Over-Rounding Critical Values
Rounding 0.4912 to 0.5 works fine for ballpark estimates. In aviation, hydraulics, or any safety-related calculation, carry the full precision through to the end of your work.
The Bottom Line
Inch of mercury and pounds per square inch are two dialects of the same language. Mercury tells the story through the height of a liquid column; psi tells it through direct force on an area. The bridge between them is simple: multiply inHg by about 0.4912 to get psi, or remember that one inHg is just shy of half a psi. Whether you are setting an altimeter, diagnosing an engine, or checking a storm report, that single relationship turns confusing numbers into clear answers. Ready to convert a specific value? Use the converter above and get an instant, precise result.
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