Enter a pressure in atmospheres (atm) to convert it instantly to pascals (Pa), kilopascals, megapascals, bar, hectopascals and PSI, using the exact SI definition of 1 atm = 101,325 Pa. You can also enter Pa or kPa to convert back to atm.
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SI Definition
1 atm = 101,325 Pa exactly, per the International System of Units (SI). No rounding applied.
Enter Pressure in Atmospheres
atm
Or Enter Pascals to Convert Back
Pa
kPa
Conversion Results
Pascals (Pa)
101,325
Pa = N/m²
Kilopascals (kPa)
101.325
kPa = 1,000 Pa
Megapascals (MPa)
0.101325
MPa = 1,000,000 Pa
Bar
1.01325
1 bar = 100,000 Pa
Hectopascals (hPa)
1,013.25
hPa = mbar (weather unit)
Millibar (mbar)
1,013.25
1 mbar = 1 hPa = 100 Pa
PSI
14.69595
Pounds per square inch
ATM (from Pa input)
1.0000
Atmospheres
Common ATM to Pa Conversions
Atmospheres (atm)
Pascals (Pa)
Kilopascals (kPa)
Bar
PSI
Worked Example: Default Inputs
Input: 1 atm
Formula: Pa = atm x 101,325
Pa = 1 x 101,325 = 101,325 Pa
kPa = 101,325 / 1,000 = 101.325 kPa
MPa = 101,325 / 1,000,000 = 0.101325 MPa
bar = 101,325 / 100,000 = 1.01325 bar
hPa = 101,325 / 100 = 1,013.25 hPa
PSI = 1 x 14.69595 = 14.69595 PSI
These match the calculator output above for 1 atm.
What is a Standard Atmosphere (atm)?
The standard atmosphere (symbol: atm) is a unit of pressure defined as exactly 101,325 pascals (Pa). It was originally intended to approximate the mean atmospheric pressure at sea level on Earth. It has been the internationally accepted definition since 1954. While it is no longer an SI unit, it remains widely used in chemistry, physics, and engineering, particularly when describing pressures close to ambient.
What is a Pascal (Pa)?
The pascal is the SI unit of pressure, named after the French mathematician and physicist Blaise Pascal. It is defined as one newton of force per square metre of area (1 Pa = 1 N/m²). Because atmospheric pressure is approximately 101,325 Pa, the pascal is quite small for everyday use. Kilopascals (kPa = 1,000 Pa) and megapascals (MPa = 1,000,000 Pa) are more practical in engineering contexts. Hectopascals (hPa = 100 Pa) are the standard unit in meteorology.
The Exact Conversion Formula
The conversion between atmospheres and pascals is exact, not an approximation:
Pa = atm x 101,325
kPa = atm x 101.325
MPa = atm x 0.101325
bar = atm x 1.01325
hPa = atm x 1,013.25
PSI = atm x 14.695948775... (approximately 14.69595 to 7 sig. figs.)
To convert pascals back to atmospheres: atm = Pa / 101,325.
Pressure Unit Reference Table
Unit
Symbol
Relation to Pa
1 atm equals
Pascal
Pa
1 Pa
101,325 Pa
Kilopascal
kPa
1,000 Pa
101.325 kPa
Megapascal
MPa
1,000,000 Pa
0.101325 MPa
Hectopascal
hPa
100 Pa
1,013.25 hPa
Millibar
mbar
100 Pa
1,013.25 mbar
Bar
bar
100,000 Pa
1.01325 bar
Pounds per square inch
psi
6,894.757... Pa
14.69595 psi
Millimetre of mercury
mmHg / Torr
133.322... Pa
760.000 mmHg
When to Use Pascals vs Atmospheres
Pascals are required when working with SI unit systems, such as the ideal gas law in SI form (PV = nRT, where R = 8.314 J/(mol K) and P must be in Pa). Atmospheres are convenient when describing conditions close to ambient pressure, such as stating that a reaction occurs at 3 atm rather than 303,975 Pa. In meteorology, hectopascals (hPa) are the international standard; standard sea-level pressure is 1,013.25 hPa. In engineering, kilopascals (kPa) and megapascals (MPa) are common for structural loads and hydraulic systems.
Sources and method: The conversion factor 1 atm = 101,325 Pa is an exact definition from the International System of Units (SI), as established by the 10th General Conference on Weights and Measures (CGPM) in 1954 and confirmed in the SI Brochure (Bureau International des Poids et Mesures, 9th edition). The PSI conversion uses 1 lbf = 4.44822162 N and 1 in = 0.0254 m, giving 1 psi = 6,894.757... Pa.
This calculator is for reference and educational use. The conversion 1 atm = 101,325 Pa is exact by SI definition. Results for PSI and other non-SI units are accurate to at least 6 significant figures.
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