What is the Wobbe Index?

The Wobbe Index (WI) is a measure of gas interchangeability defined as the higher heating value (HHV) divided by the square root of specific gravity: WI = HHV / sqrt(SG). Two gases with the same Wobbe Index will deliver the same thermal input to a burner at the same gas pressure, making it the primary criterion for fuel gas substitution.

What is the typical Wobbe Index range for natural gas?

Pipeline-quality natural gas typically has a Wobbe Index between 1,310 and 1,390 Btu/scf. The acceptable interchangeability band is usually plus or minus 5% of the target gas Wobbe Index. LNG regasification gas ranges from 1,350 to 1,420 Btu/scf depending on source.

What are the AGA interchangeability indices?

The American Gas Association defines three interchangeability indices: IL (Lifting Index) for flame stability, IF (Flashback Index) for flashback tendency, and IY (Yellow Tip Index) for incomplete combustion. All three must fall within acceptable ranges (typically IL = 0.8-1.2, IF < 1.2, IY < 0.8) for safe gas substitution.

What is the Modified Wobbe Index (MWI)?

The Modified Wobbe Index accounts for gas temperature by including a temperature ratio: MWI = HHV / sqrt(SG x T_gas/T_ref). It is commonly used for gas turbine fuel specifications where fuel gas temperature varies. Typical gas turbine MWI spec is 40-55 MJ/m3 or equivalent in Btu/scf.

Wobbe Index Calculator | Gas Interchangeability (ISO 6976, AGA)

Calculation Mode

Input Method

Reference Conditions

Target Gas for Comparison

Known Gas Properties

Gross Heating Value (HHV)

Btu/scf

Specific Gravity (Air = 1.0)

Net Heating Value (LHV) - optional

Btu/scf

If omitted, LHV estimated as HHV x 0.905

Gas Temperature - for Modified Wobbe

°F

Gas Composition (mol%)

Enter mole percentages. Total must equal 100%.

Methane (C1)

Ethane (C2)

Propane (C3)

i-Butane (iC4)

n-Butane (nC4)

i-Pentane (iC5)

n-Pentane (nC5)

C6+ (Hexanes+)

Nitrogen (N2)

Carbon Dioxide (CO2)

Hydrogen Sulfide (H2S)

Hydrogen (H2)

Helium (He)

Oxygen (O2)

Total: 100.00%

Gas Temperature (for Modified Wobbe)

Gas Supply Temperature

°F

Reference temperature (60°F) used if equal. Different values affect Modified Wobbe Index.

Understanding the Wobbe Index

What is Wobbe Index?
The Wobbe Index (WI = HHV / sqrt(SG)) measures gas interchangeability. Two gases with the same WI deliver equal heat through the same burner orifice at the same pressure.

Typical Ranges (Btu/scf):
Pipeline gas: 1,310 - 1,390
LNG regas: 1,350 - 1,420
Propane-air: 1,330 - 1,380

Where Wobbe Index is Used:
Gas turbine fuel specs, burner design, LNG cargo assessment, gas blending optimization, pipeline gas quality tariffs, distribution network interchangeability, custody transfer quality specifications.

Learn the Theory

Understand gas interchangeability, Wobbe Index significance, AGA indices, and gas blending applications

Read Engineering Guide →

Results

Gross Wobbe Index

Btu/scf

Net Wobbe Index -

Modified Wobbe Index -

Gross Heating Value (HHV) -

Net Heating Value (LHV) -

Specific Gravity -

Molecular Weight -

AGA Interchangeability Indices
IL (Lifting): -
IF (Flashback): -
IY (Yellow Tip): -

Formula

WI = HHV / √SG

WI = Wobbe Index (Btu/scf)

HHV = Gross heating value (Btu/scf)

SG = Specific gravity (air = 1.0)

MWI = HHV / √(SG × T_gas/T_ref)

WI_net = LHV / √SG

Standards & References

ISO 6976
Natural Gas - Calculation of Calorific Values
EN 437
Test Gases, Test Pressures, Appliance Categories
GPA 2172
Calculation of Gross Heating Value
GPA 2145
Physical Constants for Hydrocarbons
AGA Report No. 5
Natural Gas Interchangeability
AGA Bulletin No. 36
Interchangeability of Other Fuel Gases

Engineering Notes

Interchangeability: Gases within ±5% Wobbe Index are generally interchangeable
Gas turbines: Modified Wobbe Index is critical for GT fuel specs (typically MWI 40-55 MJ/m³)
LNG quality: Wobbe Index determines whether LNG cargo can be delivered to specific markets
Hydrogen blending: H2 addition lowers both HHV and SG, affecting Wobbe differently than simple dilution
Inerts: N2 and CO2 reduce HHV and increase MW, lowering Wobbe Index
C3+ content: Higher heavies increase HHV and SG; net effect depends on composition

Quick Reference — Typical Wobbe Values

Pure methane: WI ≈ 1,363 Btu/scf
Lean pipeline gas (SG 0.58): WI ≈ 1,340 Btu/scf
Rich pipeline gas (SG 0.65): WI ≈ 1,380 Btu/scf
LNG (Qatar): WI ≈ 1,400 Btu/scf
10% H2 blend: WI ≈ 1,310 Btu/scf

Wobbe Index Calculator

Calculation Mode

Known Gas Properties

Gas Composition (mol%)

Gas Temperature (for Modified Wobbe)

Understanding the Wobbe Index

Learn the Theory

Results

Formula

Standards & References

Engineering Notes

Quick Reference — Typical Wobbe Values

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MCF to MMBTU