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Pa
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kPa
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MPa
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atm
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bar
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psi
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mmHg
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inHg
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Pa
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kPa
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MPa
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atm
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bar
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psi
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mmHg
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inHg
The Pressure Conversion Calculator converts pressure values among eight common units: pascals, kilopascals, megapascals, atmospheres, bar, pounds per square inch, millimetres of mercury, and inches of mercury. Pressure quantifies the force distributed over a surface area:
$$P = \frac{F}{A}$$
The SI unit is the pascal (Pa), equal to one newton per square metre. Because one pascal is a very small pressure — roughly the pressure exerted by a dollar bill resting flat on a table — multiples like kilopascals (kPa) and megapascals (MPa) are far more practical for real-world use.
Different fields favour different units. Meteorologists report atmospheric pressure in millibars (mbar) or inches of mercury (inHg). Engineers in the US use psi (pounds per square inch) for tire pressures, hydraulic systems, and HVAC. European automotive and industrial standards use bar. Medical professionals measure blood pressure in mmHg (millimetres of mercury), a unit tracing back to Torricelli's barometer. The standard atmosphere (atm) serves as a reference for gas law calculations.
Key exact and defined relationships include:
$$1\text{ atm} = 101325\text{ Pa} = 760\text{ mmHg (exactly)} = 1.01325\text{ bar}$$
$$1\text{ bar} = 100000\text{ Pa}, \quad 1\text{ psi} \approx 6894.757\text{ Pa}$$
$$1\text{ inHg} \approx 3386.389\text{ Pa}, \quad 1\text{ mmHg} \approx 133.3224\text{ Pa}$$
This calculator normalizes the input to pascals, then converts to all target units simultaneously. It supports engineering, scientific, meteorological, and medical applications where pressure data must be compared across unit systems.
The algorithm follows a two-step hub-and-spoke model with the pascal as the hub:
Step 1 — Convert to Pascals:
$$Pa_{base} = value \times f_{in}$$
where $$f_{in}$$ is the conversion factor from the source unit to pascals (e.g., 101325 for atm, 6894.757 for psi).
Step 2 — Convert to each target:
$$\text{target} = \frac{Pa_{base}}{f_{out}}$$
For example: $$\text{atm} = \frac{Pa_{base}}{101325}, \quad \text{psi} = \frac{Pa_{base}}{6894.757}$$
All conversions are algebraic divisions — no iterative methods or approximations beyond the defined conversion constants.
Standard atmospheric pressure at sea level is 101,325 Pa = 101.325 kPa = 1 atm = 1.01325 bar = 14.696 psi = 760 mmHg = 29.921 inHg. Tire pressure is typically 30–35 psi (207–241 kPa). Human blood pressure readings around 120/80 mmHg correspond to about 16/10.7 kPa. Hydraulic systems operate at thousands of psi (tens of MPa). Deep-ocean pressure at 10,000 m depth reaches about 1000 atm or 100 MPa.
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One standard atmosphere equals 101,325 Pa, 1.01325 bar, 14.70 psi, and 760 mmHg — fundamental reference values used across physics and engineering.
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A typical car tire pressure of 35 psi corresponds to about 241 kPa or 2.41 bar — values commonly seen on European tire sidewall markings.
One bar = 100,000 Pa exactly, while one atmosphere = 101,325 Pa exactly. They differ by about 1.3%. The bar is a metric unit defined for convenience (10⁵ Pa), while the atmosphere is defined as the average sea-level air pressure. In practice they are close enough that some contexts use them interchangeably, but precision work requires distinguishing them.
The tradition dates to the mercury sphygmomanometer invented in 1881. The height of mercury in a tube directly indicates pressure: 1 mmHg ≈ 133.32 Pa. Although digital instruments now dominate, the mmHg unit persists in clinical practice worldwide. A reading of 120/80 mmHg means the systolic pressure supports a 120 mm mercury column.
Divide psi by 14.5038: $$\text{bar} = \frac{\text{psi}}{14.5038}$$. Alternatively, multiply psi by 0.0689476. For example, 35 psi ≈ 2.41 bar. This conversion is common for tire pressures, where US labels show psi and European labels show bar.
The torr is defined as exactly 1/760 of a standard atmosphere: 1 torr = 101325/760 Pa ≈ 133.3224 Pa. This is very close to 1 mmHg (also ≈ 133.3224 Pa), and the two are often used interchangeably. The tiny difference (< 0.000015%) arises from the precise definition of mercury density, but for all practical purposes, 1 torr = 1 mmHg.
Meteorologists use hectopascals (hPa) or equivalently millibars (mbar) in most of the world (standard sea-level ≈ 1013.25 hPa). In the US, inches of mercury (inHg) is traditional, with standard pressure at 29.92 inHg. Falling pressure indicates approaching storms; rising pressure suggests fair weather.
Atmospheric pressure decreases roughly exponentially with altitude. The barometric formula gives: $$P = P_0 \exp\left(-\frac{Mgh}{RT}\right)$$, where M is molar mass of air, g is gravity, h is altitude, R is the gas constant, and T is temperature. At 5,500 m (18,000 ft), pressure is about half the sea-level value. This is why aircraft cabins are pressurized and why cooking times change at high altitude.
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