Pressure Converter: Pascal, Bar, PSI, atm, mmHg, and More
Pressure is measured in more units than almost any other physical quantity — different industries developed their own standards long before international coordination. Pa and kPa are the SI standard used in science and most global engineering. PSI dominates in the United States for tires, plumbing, and industrial equipment. Bar is common in European industry. mmHg (Torr) is used in medicine and chemistry. atm is the chemistry standard. inHg is used in US weather reporting and aviation altimetry.
Standard atmospheric pressure (1 atm) is the reference point for most pressure scales. Gauge pressure = absolute pressure − atmospheric pressure.
Pressure Units and Their Origins
Pascal (Pa) is the SI unit, named after Blaise Pascal. It's tiny — one Pa is about the pressure of a dollar bill lying flat. Engineering uses kPa or MPa. Atmosphere (atm) was originally defined as mean sea-level atmospheric pressure. PSI (pounds per square inch) uses the imperial force and area units common in the US. mmHg (millimeters of mercury) comes from mercury manometers used in early pressure measurement — the pressure that holds a column of mercury N millimeters tall. Bar was defined as 100,000 Pa for a round number close to 1 atm.
Absolute vs Gauge vs Differential Pressure
Absolute pressure is measured from true vacuum (0 Pa). Gauge pressure is measured above ambient atmospheric pressure — most practical pressure gauges (tire gauges, blood pressure cuffs) measure gauge pressure. Differential pressure is the difference between two pressures at different points (e.g., across a filter). When converting between these, always clarify which reference is being used — a tire at '35 psi gauge' is very different from '35 psi absolute' (which would be a near-vacuum).
Blood Pressure Explained
Blood pressure is expressed as systolic/diastolic in mmHg (e.g., 120/80 mmHg). Systolic is the peak pressure when the heart contracts; diastolic is the pressure when the heart rests between beats. These are gauge pressures above atmospheric. Normal range is below 120/80 mmHg. Hypertension is generally classified at 130/80 mmHg or above. The mmHg unit is maintained by international convention despite mercury no longer being used in most medical devices.
Tire Pressure and Altitude Effects
Car tire pressure should be checked when tires are cold (driven less than 1 mile or after sitting 3+ hours). Tire pressure increases by about 1 psi per 10°F (0.07 bar per 5.5°C) of temperature increase, so hot tires read higher than cold. At high altitude, reduced atmospheric pressure means gauge readings are the same but absolute pressure is lower — tire pressure specs account for this. Most passenger car tires run at 30–35 psi gauge (207–241 kPa).
Quick Reference
- Science & engineering (SI): Pa, kPa, MPa
- US industry, tires: psi
- European industry, HVAC: bar
- Medicine, chemistry: mmHg (Torr)
- Chemistry, aviation reference: atm
- US weather, altimetry: inHg
- Standard atmosphere: 1 atm = 101,325 Pa = 14.696 psi = 1.01325 bar
Frequently Asked Questions
What is Pascal (Pa)?
The Pascal is the SI unit of pressure, defined as one Newton per square meter (N/m²). It's a very small unit — standard atmospheric pressure is 101,325 Pa, so kPa (kilopascal) is more commonly used in engineering. Weather maps in metric countries show pressure in hPa (hectopascals), equivalent to millibars.
What is the difference between psi and bar?
Both measure pressure, but with different scales. 1 bar = 100,000 Pa = 14.504 psi. Bar is a metric unit used widely in European industry and HVAC. PSI (pounds per square inch) is the US customary unit, standard for tire pressure and plumbing in the US. 1 psi ≈ 0.069 bar.
What is gauge pressure vs absolute pressure?
Absolute pressure is measured relative to a perfect vacuum (0 Pa absolute). Gauge pressure is measured relative to ambient atmospheric pressure. Tire pressure gauges read gauge pressure — a tire at '35 psi' is 35 psi above atmospheric, or about 49.7 psi absolute. Gauge pressure = Absolute pressure − Atmospheric pressure (≈ 14.696 psi).
Why is blood pressure measured in mmHg?
Blood pressure is expressed in millimeters of mercury (mmHg) because early sphygmomanometers used a mercury column to measure pressure. A reading of 120/80 mmHg means the systolic pressure supports a column of mercury 120mm tall. Despite mercury's phase-out from medical equipment, the unit persists by convention worldwide.
What is standard atmospheric pressure?
Standard atmospheric pressure (1 atm) = 101,325 Pa = 14.696 psi = 1.01325 bar = 760 mmHg = 29.92 inHg. This is defined as the pressure at sea level at 0°C and is used as the reference point in chemistry, physics, and aviation (standard altitude pressure tables).
How does altitude affect pressure?
Atmospheric pressure decreases with altitude as there's less air above. At 5,500m (18,045 ft) — roughly the altitude of Everest base camp — pressure is about half of sea level (≈ 506 hPa vs 1013 hPa). At 10,000m (cruising altitude), pressure is about 26 kPa, which is why aircraft cabins are pressurized.
What pressure should my car tires be?
Most passenger car tires are inflated to 30–35 psi (207–241 kPa) gauge pressure. The correct value is printed on a sticker inside the driver's door jamb or in the owner's manual — not on the tire sidewall (that's the maximum allowed, not the recommended pressure). Always check when tires are cold.