Millimeter Mercury to Pound per Square Inch
mmHg
psi
Conversion History
| Conversion | Reuse | Delete |
|---|---|---|
1 mmHg (Millimeter Mercury) → 0.0193367212698464995682223460339917998 psi (Pound per Square Inch) Just now |
Quick Reference Table (Millimeter Mercury to Pound per Square Inch)
| Millimeter Mercury (mmHg) | Pound per Square Inch (psi) |
|---|---|
| 1 | 0.0193367212698464995682223460339917998 |
| 10 | 0.19336721269846499568221910920778605132 |
| 80 | 1.54693770158771996545774997290753377944 |
| 120 | 2.32040655238157994818662495936130066916 |
| 760 | 14.69590816508333967184862619299894822024 |
| 1,000 | 19.33672126984649956822187611172154955856 |
| 3,000 | 58.01016380953949870466562833516464867568 |
About Millimeter Mercury (mmHg)
The millimeter of mercury (mmHg) is the pressure exerted by a 1 mm column of mercury at 0 °C under standard gravity, equal to approximately 133.322 pascals. It is the universal unit for clinical blood pressure measurement and intraocular pressure in ophthalmology. Normal blood pressure is approximately 120/80 mmHg (systolic/diastolic). The unit is also used in vacuum technology, barometry, and respiratory physiology for reporting partial pressures of oxygen and carbon dioxide in blood. It remains entrenched in clinical medicine globally despite SI adoption.
Normal human blood pressure is about 120/80 mmHg. Standard atmospheric pressure is 760 mmHg.
Etymology: Derives from Evangelista Torricelli's 1643 mercury barometer experiment, in which he first measured atmospheric pressure as the height of mercury column it could support — approximately 760 mm. The unit is named after the instrument's working fluid rather than its inventor.
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 Mercury – Frequently Asked Questions
Why is blood pressure still measured in mmHg instead of kilopascals?
Clinical medicine is deeply conservative about units because misreadings kill people. Doctors, nurses, and patients worldwide have memorized "120/80 is normal" in mmHg. Converting to kPa (16.0/10.7) would require retraining millions of clinicians and rewriting every guideline. The WHO considered the switch and decided the risk of transcription errors during transition outweighed the elegance of SI compliance. So mmHg stays — likely for decades more.
What do the two numbers in a blood pressure reading actually mean?
The top number (systolic) is the peak pressure when the heart contracts and pushes blood into the arteries — typically 90–120 mmHg. The bottom number (diastolic) is the lowest pressure between beats when the heart relaxes — typically 60–80 mmHg. A reading of 140/90 mmHg or above is classified as hypertension. The gap between the two (pulse pressure) also matters: a wide gap above 60 mmHg may signal stiff arteries.
How did Torricelli's mercury barometer lead to the mmHg unit?
In 1643, Evangelista Torricelli filled a glass tube with mercury, inverted it into a dish of mercury, and watched the column drop to about 760 mm. The empty space above was the first laboratory vacuum. The height of the mercury column became the measurement of atmospheric pressure — 760 mmHg at sea level. Nearly 400 years later, we still use his column height as a pressure unit in medicine and vacuum science.
What is the difference between mmHg and torr?
For all practical purposes, they are identical — 1 torr = 1/760 atm ≈ 133.322 Pa, and 1 mmHg ≈ 133.322 Pa. The difference is about 0.00015% and arises from the torr being defined from the atmosphere while mmHg is defined from mercury density. Medicine uses mmHg; vacuum physics uses torr. They are interchangeable in any real-world measurement.
What mmHg values are important in medicine beyond blood pressure?
Intraocular pressure (glaucoma screening): normal is 10–21 mmHg, above 21 is suspicious. Partial pressure of oxygen in arterial blood (PaO₂): normal is 80–100 mmHg. Central venous pressure: 2–6 mmHg. Intracranial pressure: normal below 15 mmHg, dangerous above 20 mmHg. Carbon dioxide in blood (PaCO₂): 35–45 mmHg. The unit pervades clinical monitoring far beyond the blood pressure cuff.
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.