Pressure Converter — PSI, Bar, Atm, kPa, mmHg, Pa, Torr
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How Pressure Units Work
Pressure is the force applied perpendicular to a surface per unit area. The SI unit is the pascal (Pa), defined as one newton per square meter. Since one pascal is an extremely small amount of pressure, practical applications use kilopascals (kPa), bars, or other derived units. According to the International Bureau of Weights and Measures (BIPM), the pascal is the only SI-derived pressure unit, while other units like bar, atmosphere, and PSI remain in widespread use due to industry convention.
Standard atmospheric pressure at sea level is defined as exactly 101,325 Pa by the International Organization for Standardization (ISO 2533). This reference value equals 1 atmosphere (atm), 1.01325 bar, 14.696 PSI, 760 mmHg, or 29.92 inHg. Atmospheric pressure decreases by approximately 12 Pa per meter of altitude gain, which is why pressure readings are so important in aviation and meteorology. At the summit of Mount Everest (8,849 meters), atmospheric pressure drops to roughly 33,700 Pa, about one-third of sea-level pressure.
This converter handles all common pressure units with precise conversion factors, enabling engineers, scientists, automotive technicians, HVAC professionals, and medical practitioners to translate between the unit systems used in their respective fields. Enter any value and select your source and target units for instant, accurate conversion. All results are shown alongside a complete table of conversions to every supported unit. For related conversions, try our temperature converter or energy converter.
How Pressure Conversion Is Calculated
All pressure conversions use pascals as an intermediate base unit. The process works in two steps:
Step 1: Convert the input value to pascals by multiplying by the unit's conversion factor.
Step 2: Convert from pascals to the target unit by dividing by the target unit's conversion factor.
The conversion factors are: 1 kPa = 1,000 Pa; 1 bar = 100,000 Pa; 1 atm = 101,325 Pa; 1 PSI = 6,894.757 Pa; 1 mmHg = 133.322 Pa; 1 inHg = 3,386.389 Pa.
Worked example: Convert 35 PSI to bar. Step 1: 35 x 6,894.757 = 241,316.5 Pa. Step 2: 241,316.5 / 100,000 = 2.413 bar. Quick shortcut: PSI x 0.0689476 = bar, so 35 x 0.0689476 = 2.413 bar.
Key Terms You Should Know
- Pascal (Pa): The SI unit of pressure equal to one newton per square meter. Named after French mathematician Blaise Pascal. One atmosphere equals 101,325 Pa.
- PSI (Pounds per Square Inch): The most common pressure unit in the United States for tire pressure, hydraulic systems, and compressed gas. One PSI = 6,894.757 Pa.
- Bar: A metric unit equal to exactly 100,000 Pa. Widely used in Europe and in weather reporting (as millibars). Standard atmosphere is 1.01325 bar.
- Atmosphere (atm): A reference unit defined as the average atmospheric pressure at sea level. Exactly 101,325 Pa. Used in chemistry, physics, and diving.
- mmHg (Millimeters of Mercury): A unit based on the height of a mercury column. Used primarily for blood pressure readings (e.g., 120/80 mmHg) and in vacuum science. Essentially identical to the torr.
- Gauge vs. Absolute Pressure: Gauge pressure is measured relative to atmospheric pressure (0 PSI gauge = 14.7 PSI absolute). Absolute pressure includes atmospheric pressure. Most everyday gauges read gauge pressure.
Pressure Unit Quick Reference Table
This table shows one standard atmosphere expressed in all common pressure units, along with where each unit is typically used.
| Unit | 1 atm Equals | Primary Use |
|---|---|---|
| Pascal (Pa) | 101,325 | Scientific research, SI standard |
| Kilopascal (kPa) | 101.325 | Engineering, tire pressure (metric) |
| Bar | 1.01325 | Meteorology, European industry |
| PSI | 14.696 | US automotive, hydraulics, HVAC |
| mmHg / Torr | 760 | Medicine (blood pressure), vacuum |
| inHg | 29.92 | US weather reports, aviation |
| Millibar (mbar) | 1,013.25 | International weather, climate |
Practical Examples
Example 1: Tire Pressure. A car tire sticker says 32 PSI. You have a European tire gauge reading in bar. Convert: 32 x 0.0689476 = 2.21 bar. Alternatively, in kPa: 32 x 6.89476 = 220.6 kPa. Most modern tire pressure monitoring systems (TPMS) can display in PSI, bar, or kPa depending on the vehicle's market of origin.
Example 2: Blood Pressure Reading. A doctor measures blood pressure at 120/80 mmHg (systolic/diastolic). In kPa: 120 x 0.133322 = 16.0 kPa systolic and 80 x 0.133322 = 10.7 kPa. While mmHg remains the universal clinical standard, some European countries have adopted kPa for blood pressure reporting. Normal blood pressure is below 120/80 mmHg according to the American Heart Association.
Example 3: Weather Report. A U.S. weather report says barometric pressure is 30.12 inHg (high pressure). In millibars: 30.12 x 33.8639 = 1,020 mbar. In hPa (hectopascals, numerically identical to millibars): 1,020 hPa. International weather stations report in hPa, while U.S. stations report in inHg. Standard sea-level pressure (29.92 inHg or 1,013.25 hPa) is the reference for altimeter settings in aviation. Use our length converter for altitude-related calculations.
Tips and Strategies for Pressure Conversions
- Know your gauge vs. absolute: Most practical gauges read gauge pressure (psig), where 0 equals atmospheric pressure. Scientific calculations often require absolute pressure (psia). Convert by adding 14.696 PSI (or 101.325 kPa) to gauge readings.
- Use quick mental shortcuts: 1 bar is roughly 14.5 PSI. 1 atm is roughly 15 PSI. 1 PSI is roughly 7 kPa. These approximations are useful for quick estimates when exact values are not critical.
- Match units to your instrument: Always verify that your pressure gauge or sensor reads in the expected units before recording values. Mixing PSI and bar readings is a common and potentially dangerous error in industrial settings.
- Remember temperature affects pressure: Gas pressure increases with temperature (Gay-Lussac's law). Tire pressure rises approximately 1 PSI per 10 degrees Fahrenheit increase in temperature. Check tire pressure when tires are cold for accurate readings.
- Understand precision requirements: Engineering and industrial applications may require 4-6 significant figures for conversion accuracy. Casual applications like tire pressure only need 1 decimal place. This converter provides full precision for professional use.
Frequently Asked Questions
What is standard atmospheric pressure?
Standard atmospheric pressure at sea level is exactly 101,325 Pa (101.325 kPa), as defined by the International Organization for Standardization. This equals 1 atm, 1.01325 bar, 14.696 PSI, 760 mmHg, 29.92 inHg, or 1,013.25 millibars. This reference value is used for calibrating instruments, defining standard conditions in chemistry and physics, and setting sea-level pressure for aviation altimeters. Actual atmospheric pressure varies constantly with weather conditions and altitude.
How do I convert PSI to bar?
To convert PSI to bar, multiply the PSI value by 0.0689476. For example, 30 PSI equals approximately 2.07 bar, and 35 PSI equals 2.41 bar. One bar equals exactly 100,000 pascals. For a quick mental estimate, divide PSI by 14.5 to get an approximate bar value. This conversion is commonly needed when switching between American and European pressure gauges for automotive, industrial, and HVAC applications.
What is the difference between mmHg and torr?
For practical purposes, mmHg and torr are the same unit, both equal to approximately 133.322 pascals. The torr was named after Evangelista Torricelli, who invented the mercury barometer in 1643. Technically, 1 torr is defined as exactly 1/760 of a standard atmosphere (133.3224 Pa), while 1 mmHg is defined by the density of mercury at 0 degrees Celsius under standard gravity (133.3224 Pa). The difference is less than 0.000015% and is negligible for all practical applications.
What pressure unit is used for tire pressure?
In the United States, tire pressure is measured in PSI (pounds per square inch), with typical passenger car tires inflated to 30-35 PSI. In Europe and most of the world, bar or kPa are standard, with the same tires rated at 2.1-2.4 bar or 210-240 kPa. Australian and Asian markets typically use kPa. The recommended pressure is listed on a sticker inside the driver's door jamb and in the vehicle owner's manual. Always check pressure when tires are cold, as driving heats tires and raises pressure by 3-5 PSI.
Why do weather stations use different pressure units?
U.S. weather stations report barometric pressure in inches of mercury (inHg) due to historical convention from mercury barometer usage. International weather organizations and most countries use hectopascals (hPa) or millibars (mbar), which are numerically identical. Aviation worldwide uses inHg for altimeter settings in the United States and hPa internationally (with the exception of a few countries). Standard sea-level pressure is 29.92 inHg or 1,013.25 hPa. The World Meteorological Organization recommends hPa as the standard unit.
What is the difference between gauge and absolute pressure?
Gauge pressure measures pressure relative to atmospheric pressure, so 0 psig equals atmospheric pressure (about 14.7 psia). Absolute pressure measures from a perfect vacuum, so 0 psia is a complete absence of pressure. Most everyday instruments like tire gauges and blood pressure cuffs read gauge pressure. Scientific calculations, thermodynamic equations, and gas law problems require absolute pressure. To convert: absolute pressure = gauge pressure + atmospheric pressure. For example, 32 psig tire pressure = 32 + 14.7 = 46.7 psia.