Real Gas Density Calculator

Calculator

Pressure (P)

Use absolute pressure for best accuracy.

Temperature (T)

C and F are converted to Kelvin internally.

Molar mass (M)

Example: CO₂ is 44.01 g/mol.

Model for Z

Cubic EOS is useful near high pressure or non-ideal regions.

Phase selection (EOS root)

For gases, vapor root is typical.

Density output unit

1 kg/m³ equals 1 g/L.

Known compressibility factor (Z)

Use a value from charts, tables, or simulation.

Tip

When Z is known, density is direct with ρ = P·M / (Z·R·T).

Critical temperature (Tc)

CO₂ Tc ≈ 304.13 K.

Critical pressure (Pc)

CO₂ Pc ≈ 73.77 bar.

Acentric factor (ω)

CO₂ ω ≈ 0.225 (set 0 if unknown).

EOS note

Peng–Robinson solves a cubic in Z. Near saturation, multiple Z roots can occur; choose vapor or liquid to select a root consistently.

Example Data Table

These examples are illustrative. Use your own properties for best results.

Gas	P	T	M	Model	Inputs for model	Expected note
CO₂	10 bar	25 °C	44.01 g/mol	Peng–Robinson	Tc 304.13 K, Pc 73.77 bar, ω 0.225	Non-ideal Z < 1 at moderate pressure.
N₂	1 atm	20 °C	28.013 g/mol	Ideal	Z = 1	Often close to ideal near ambient conditions.
Propane	20 bar	40 °C	44.097 g/mol	Known Z	Z from chart/table, e.g., 0.80	Use data sources appropriate for your region.

Formula Used

The calculator uses the real-gas form of the ideal gas relationship:

ρ = (P · M) / (Z · R · T)

ρ is density, P is absolute pressure, M is molar mass, T is absolute temperature, R is the universal gas constant, and Z is the compressibility factor.

When the Peng–Robinson option is selected, Z is computed by solving the EOS cubic and choosing a root based on the phase selection.

How to Use This Calculator

Enter pressure, temperature, and molar mass with units.
Select a Z method: EOS, known Z, or ideal.
If using EOS, provide Tc, Pc, and ω for the gas.
Choose vapor or liquid root when near saturation.
Press Submit to view results above the form.
Download the CSV or PDF report for documentation.

FAQs

1) What is real-gas density?

It is gas density corrected for non-ideal behavior using the compressibility factor Z, which accounts for intermolecular forces and finite molecular size at given P and T.

2) When should I avoid the ideal assumption?

Avoid Z = 1 at elevated pressures, low temperatures near condensation, or for strongly non-ideal gases. Under these conditions, EOS or a known Z improves accuracy.

3) What does the compressibility factor Z mean?

Z compares real behavior to ideal behavior. Z < 1 often indicates attractive forces dominate, while Z > 1 suggests repulsive effects dominate at the selected conditions.

4) Why does the EOS have vapor and liquid roots?

Near saturation, cubic equations can yield multiple Z solutions that correspond to different phases. Selecting vapor chooses the largest root; selecting liquid chooses the smallest.

5) What inputs are required for Peng–Robinson?

You need critical temperature Tc, critical pressure Pc, and an acentric factor ω. These are tabulated for common gases in property databases and engineering handbooks.

6) What units does the calculator use internally?

Pressure is converted to pascals, temperature to kelvin, and molar mass to kg/mol. Density is first computed in kg/m³ and then converted to your chosen output unit.

7) Can I use this for gas mixtures?

This version targets a single pure component. For mixtures, you typically need mixing rules and component interaction parameters to compute EOS parameters consistently.

8) Why might my results differ from another tool?

Different tools may use different EOS models, root-selection strategies, or property sources. Ensure Tc, Pc, ω, and pressure basis (absolute vs gauge) match before comparing results.

Built for quick checks, teaching, and lightweight reporting.