How ATM Conversions Work
The standard atmosphere (atm) is an internationally defined unit of pressure equal to exactly 101,325 pascals (Pa). It was originally defined as the pressure exerted by a column of mercury 760 millimetres tall at 0 degrees Celsius at sea level under standard gravity (9.80665 m/s2). All conversion factors for the atm derive from this exact definition.
To convert from atm to any other unit, multiply the atm value by the conversion factor. To convert from another unit back to atm, divide by that same factor.
Conversion Table: All Units from 1 atm
| Unit | Symbol | Value for 1 atm | Exact or approximate? |
|---|
| Pascal | Pa | 101,325 | Exact (definition) |
| Kilopascal | kPa | 101.325 | Exact |
| Bar | bar | 1.01325 | Exact |
| Millibar | mbar | 1,013.25 | Exact |
| Pounds per square inch | psi | 14.6959 | Approximate (14.695948...) |
| Millimetres of mercury | mmHg (torr) | 760 | Exact (by definition) |
| Inches of mercury | inHg | 29.9213 | Approximate (29.92126...) |
Key Pressure Units Explained
Pascal (Pa) and kilopascal (kPa): The SI unit of pressure, named after Blaise Pascal. One pascal equals one newton per square metre. Most scientific work uses pascals or kilopascals. Weather maps in New Zealand use hectopascals (hPa), where 1 hPa = 1 mbar.
Bar and millibar (mbar): The bar is a metric unit equal to exactly 100,000 Pa (or 100 kPa). One standard atmosphere is 1.01325 bar, so the bar and atm are close but not equal. Millibar (mbar) is commonly used in meteorology; standard sea-level pressure is 1,013.25 mbar.
PSI (pounds per square inch): An imperial unit used widely in the US and UK for tyre pressures, boiler pressures, and industrial applications. Standard atmosphere is 14.6959 psi. Car tyre pressures are typically 30 to 35 psi on the gauge, which is about 2.0 to 2.4 atm of gauge pressure (roughly 3.0 to 3.4 atm absolute).
mmHg (millimetres of mercury) and torr: Historically used in medicine (blood pressure) and vacuum science. One mmHg and one torr are nearly identical (they differ by about 0.000015%). Standard atmosphere is exactly 760 mmHg. Blood pressure readings like 120/80 are in mmHg.
inHg (inches of mercury): Used in aviation and US weather reporting for barometric pressure. Standard pressure is 29.9213 inHg. Altimeters are set to local QNH in inHg in US aviation, and in hPa in New Zealand.
Absolute vs Gauge Pressure
Pressure can be expressed as absolute or gauge. Absolute pressure is measured relative to a perfect vacuum (zero pressure). Gauge pressure is measured relative to ambient atmospheric pressure. A car tyre inflated to 32 psi gauge is at about 46.7 psi absolute, which is 32 psi gauge plus 14.7 psi for the 1 atm of atmospheric pressure. This calculator works with absolute pressure. When reading tyre pressure gauges, add 1 atm (14.696 psi or 101.325 kPa) to get absolute pressure.
Standard vs Non-Standard Atmosphere
The standard atmosphere (atm) described on this page should not be confused with the technical atmosphere (at), which equals 98,066.5 Pa (one kilogram-force per square centimetre). The standard atmosphere at 101,325 Pa is the one used in chemistry, physics, and the ISO standard definition. In everyday speech, "atmosphere" almost always refers to the standard atmosphere (atm).
Related Pressure Converters
Sources and method: Conversion factors from the National Institute of Standards and Technology (NIST) SI unit definitions and the International Union of Pure and Applied Chemistry (IUPAC) definition of the standard atmosphere (1 atm = 101,325 Pa, exact). The psi and inHg values are derived from this exact definition and are rounded to six significant figures.
This tool converts between common pressure units using internationally accepted exact or near-exact conversion factors. Results for psi and inHg are rounded to six significant figures due to their non-SI nature. For scientific or engineering work, verify values against NIST or ISO standards.
