How do you calculate natural gas density?

Natural gas density is calculated using the Real Gas Equation of State: ρ = (P × MW) / (Z × R × T), where P is absolute pressure, MW is molecular weight, Z is the compressibility factor, R is the gas constant, and T is absolute temperature.

What is the Z-factor in gas density calculations?

The Z-factor (compressibility factor) is a correction term that accounts for the deviation of a real gas from ideal gas behavior. At high pressures, natural gas becomes denser than an ideal gas would predict, making Z less than 1.0.

Why is gas density important in pipelines?

Gas density is critical for determining mass flow rates, calculating pipeline inventory (line pack), sizing compressors, and performing accurate custody transfer measurements.

Natural Gas Density Calculator | Real Gas Z-Factor (Z)

Operating Conditions

Pressure

psig

Temperature

°F

Gas Properties

Gas Specific Gravity (Air=1.0)

Typical: 0.55-0.70 (sweet gas), 0.70-0.90 (rich gas)

Z-Factor (Compressibility)

Typical range: 0.70-1.00. Obtain from AGA-8 or Standing-Katz chart.

Calculation Method

Equation of State

Real gas: Use for P > 100 psig or custody transfer
Ideal gas: Only for preliminary estimates at low pressure

Calculated Gas Properties

Density Units:
lb/ft³ (US Customary), kg/m³ (Metric/SI), Specific Volume (ft³/lb).

Gas Physics:
Z-Factor (Compressibility), Molecular Weight (MW), Absolute Pressure (psia), Absolute Temperature (°R).

Calculation Standards:
AGA Report No. 8, GPSA Section 23, ISO 12213-2, GPA 2145 physical constants.

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Understand ideal gas law, real gas behavior, Z-factor correlations, and equation of state methods

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Formula

ρ = (P × MW) / (Z × R × T)

ρ = Gas density (lb/ft³)

P = Absolute pressure (psia)

MW = SG × 28.9647 (lb/lbmol)

Z = Compressibility factor (0.7-1.0)

R = 10.7316 psia·ft³/lbmol·°R

T = °F + 459.67 (°R)

Standards & References

AGA Report No. 8
Compressibility Factors - Detail Characterization Method
GPSA
Section 23: Physical Properties
ISO 12213-2
Natural Gas Compression Factor Calculation
GPA 2145
Physical Constants for Paraffin Hydrocarbons

Engineering Notes

Z-factor source: Use AGA-8, Standing-Katz chart, or process simulator for accurate Z values
Pressure basis: Calculator adds 14.73 psia to convert gauge to absolute pressure
Z < 1 effect: Real gas is denser than ideal gas prediction (ρ_real = ρ_ideal / Z)
Accuracy: ±0.5% with accurate Z-factor; ±5-10% using ideal gas at high pressure
Standard conditions: 14.73 psia, 60°F per GPA 2145

Typical Gas Gravities

Pure Methane: 0.554
Dry Natural Gas: 0.58-0.65
Wet/Rich Gas: 0.70-0.85
Associated Gas: 0.75-1.00

Gas Density Calculator

Operating Conditions

Gas Properties

Calculation Method

Calculated Gas Properties

📚 Learn the Theory

Results

Formula

Standards & References

Engineering Notes

Typical Gas Gravities

Related Calculators

Ideal Gas Law

Gas Velocity

Gas Conversions

Compressor Sizing