Compressibility Factor (Z) Calculator

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Select a gas to load its critical properties automatically.
Peng-Robinson and SRK are the industry standards for hydrocarbons. Van der Waals is the historical reference. Ideal gas assumes Z = 1.
Operating temperature. Must be above absolute zero.
K
Absolute pressure of the gas.
bar
Compressibility Factor (Z)Attractive dominance (Z < 1)
0.9018

Ratio of actual molar volume to ideal-gas molar volume at same T and P. Z = 1 for a perfect ideal gas.

Reduced Temperature (Tr)1.574
Reduced Pressure (Pr)1.087
Molar volume (real gas)0.45L/mol
Molar volume (ideal gas)0.499L/mol
Deviation from ideal (%)-9.82%
Z (gas phase)0.9018
Z (liquid phase)0.9018
0.9018
Strong attraction (Z << 1)<0.8Moderate attraction0.8-0.95Near-ideal gas0.95-1.05Moderate repulsion1.05-1.25Strong repulsion (Z >> 1)1.25+

Z = 0.9018 for Methane (CH4) (Peng-Robinson)

  • Z < 1: intermolecular attractions dominate. The gas occupies about 9.8% less volume than an ideal gas would.
  • Reduced temperature Tr = 1.57: well above the critical point, so the gas is in a supercritical or vapor region.

Next stepUse Z in the real-gas law PV = ZnRT in place of PV = nRT when calculating flow rates, tank volumes, or compressor work at non-ambient conditions.

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