# Pressure converter

> Convert psi to bar, bar to psi, kPa, atm, hPa, mmHg and inHg for tyre, weather and blood pressure readings, with the exact factor chain.

Interactive version: https://www.calcopenly.com/conversion/pressure-converter
Subject: Unit converters

Each pressure unit is a fixed number of pascals (newtons per square metre). The bar is exactly 100,000 Pa, the standard atmosphere 101,325 Pa and the torr 1/760 atm. The psi, one pound-force per square inch, is about 6,894.757 Pa, fixed exactly by the 1959 pound, standard gravity (9.80665 m/s²) and the 25.4 mm inch.

The default converts a 32 psi tyre pressure to 2.2063 bar. Weather maps use hectopascals (1 hPa = 1 mbar; standard sea-level pressure is 1,013.25 hPa), and blood pressure is recorded in millimetres of mercury.

Gauge and absolute readings convert with the same factor but are not interchangeable: a tyre gauge shows pressure above the surrounding air, so 32 psi on the gauge is about 46.7 psi absolute at sea level. The inch of mercury uses the conventional value at 0 °C and is marked approximate.

## Inputs

- **Value**: A number in the “from” unit, or with its own unit
- **From** (options: pascal (Pa), hectopascal (hPa), kilopascal (kPa), megapascal (MPa), bar, standard atmosphere (atm), pound per square inch (psi), millimetre of mercury (mmHg), torr (Torr), inch of mercury (inHg), kilogram-force per cm² (kgf/cm²))
- **To** (options: pascal (Pa), hectopascal (hPa), kilopascal (kPa), megapascal (MPa), bar, standard atmosphere (atm), pound per square inch (psi), millimetre of mercury (mmHg), torr (Torr), inch of mercury (inHg), kilogram-force per cm² (kgf/cm²))

## Results

- Converted value — main result
- Conversion
- Conversion factor
- Exactness

## Formula

$$
\begin{gathered} x_{\text{to}} = x_{\text{from}} \times \dfrac{f_{\text{from}}}{f_{\text{to}}} \\ f = \text{one unit expressed in } \text{Pa} \end{gathered}
$$

## Worked examples

### 32 psi tyre pressure in bar

- Value: 32 psi
- From: pound per square inch (psi)
- To: bar
- **Converted value: 2.2063223338 bar**
- Checked against: 1 psi = 4.4482216152605 N ÷ (0.0254 m)² (NIST SP 811 App. B); 32 psi = 8896443230521/4032250000000 bar (Python 3.8 fractions from the legal definitions)

### 1 atm in psi

- Value: 1 atm
- From: standard atmosphere (atm)
- To: pound per square inch (psi)
- **Converted value: 14.6959487755 psi**
- Checked against: 101325 Pa ÷ 6894.757293168… Pa (Python 3.8 fractions from the legal definitions)

### 760 torr is 1 atm

- Value: 760 Torr
- From: torr (Torr)
- To: standard atmosphere (atm)
- **Converted value: 1 atm**
- **Exactness: Exact**
- Checked against: 1 torr = 101325/760 Pa by definition (NIST SP 811 App. B)

### 1 inHg (conventional value)

- Value: 1 inHg
- From: inch of mercury (inHg)
- To: pascal (Pa)
- **Converted value: 3,386.389 Pa**
- **Exactness: Approximate**
- Checked against: NIST SP 811 App. B.9: inch of mercury (0 °C) = 3.386389 E+03 Pa

## Questions

### How do I convert psi to bar?

Divide by 14.5038, or multiply by 0.0689476. A bar is 100,000 Pa and a psi 6,894.757 Pa, so 1 bar = 14.5038 psi. A 32 psi tyre is 2.2063 bar, and a 2.5 bar tyre is 36.26 psi.

### Is 1 bar the same as 1 atmosphere?

No, though they are within 1.3%. The bar is exactly 100,000 Pa, while the standard atmosphere is exactly 101,325 Pa, which is 1.01325 bar, 14.696 psi or 760 mmHg. The atmosphere was set to match average sea-level air pressure, which is why weather charts centre on 1,013.25 hPa.

### What is the difference between psig and psia?

psig is gauge pressure, measured relative to the surrounding air; psia is absolute pressure, measured from a vacuum. At sea level psia = psig + 14.696. Tyre and compressor gauges read psig, so a flat tyre shows 0 psig while still holding 14.7 psia of air. The converter treats both alike, so keep input and output the same kind.

### Are mmHg and torr the same?

Almost. A torr is exactly 1/760 of a standard atmosphere, 133.322368 Pa, while the conventional millimetre of mercury is 133.322387415 Pa; they differ by about 0.14 parts per million. For blood pressure the difference never matters: 120/80 mmHg is 16.0/10.7 kPa either way.

### How do I convert inches of mercury to hPa?

Multiply by 33.8639: 29.92 inHg is 1,013.2 hPa, the standard altimeter setting in US aviation. This converter uses the conventional inch of mercury at 0 °C, 3,386.389 Pa (NIST SP 811), and marks results that use it as approximate because mercury's density changes with temperature.

### How accurate is the pressure converter?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 4 worked examples whose answers come from independent sources; for example, “32 psi tyre pressure in bar” is checked against 1 psi = 4.4482216152605 N ÷ (0.0254 m)² (NIST SP 811 App. B); 32 psi = 8896443230521/4032250000000 bar (Python 3.8 fractions from the legal definitions).

### Where does the method come from?

NIST SP 811 (2008), Guide for the Use of the SI, Appendix B — conversion factors; NIST Handbook 44, Appendix C — General tables of units of measurement.

## Sources

- [NIST SP 811 (2008), Guide for the Use of the SI, Appendix B — conversion factors](https://www.nist.gov/pml/special-publication-811)
- [NIST Handbook 44, Appendix C — General tables of units of measurement](https://www.nist.gov/pml/owm/nist-handbook-44-current-edition)