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Free flammable gas tool

Gas Mixture Flammability Calculator

Add the fuels in your gas and their percentages. The calculator estimates the lower and upper flammable limits of the blend with Le Chatelier's rule, adjusts for inert gas, and tells you whether a given air mix can burn.

  • Le Chatelier's rule
  • 33 fuels
  • Inert gas adjustment
  • Free, no sign-up
Gas Mixture Flammability Calculator Le Chatelier rule for blended fuels
0%Inert gas in the mix (the rest, such as N2 or CO2)
Optional. Shows whether that air mix can burn.
Planning tool only. Le Chatelier's rule gives estimates. It is less accurate for hydrogen, acetylene, ethylene oxide, high temperature, high pressure, and heavy inert dilution. Use tested data for design, and follow your site safety procedures for alarms and entry.
Le Chatelier rule applied to a natural gas blend Bars show the flammable range of methane, ethane, propane and butane, and the blended range of 4.53 to 14.46 percent. A formula panel explains the rule on the right. Blend the limits with Le Chatelier Example. 90% methane, 6% ethane, 3% propane, 1% butane. Log scale, percent in air. 1%2%5%10%20%Methane 90%515Ethane 6%312.5Propane 3%2.19.5n-Butane 1%1.68.4Blend4.5314.46 The rule LFL mix = 1 / Σ( x / LFL ) UFL mix = 1 / Σ( x / UFL ) x is each fuel's share of thefuel part of the gas. Result for this blend LFL 4.53%UFL 14.46% Watch out Inert gas raises the LFL.Hydrogen blends need testing. Results are estimates.
Infographic. Le Chatelier's rule blends four fuel ranges into one. The blend sits close to methane but with a lower LFL.

How the gas mixture flammability calculator works

This gas mixture flammability calculator estimates the flammable limits of a blend of fuels. It uses Le Chatelier's rule, a formula chemists have used for over a century. You enter each fuel and its share of the gas. The tool returns a blended lower flammable limit and a blended upper flammable limit.

It also handles inert gas such as nitrogen or carbon dioxide. Anything you leave out of the fuel list counts as inert. Then you can test a real concentration in air and see whether that air mix can burn.

Le Chatelier's rule in plain words

The rule says each fuel adds its own share to the mix. The formula is LFL = 1 / the sum of (x / LFL of each fuel). Here x is that fuel's fraction of the total fuel. The same form works for the upper limit.

A gas with a low LFL pulls the blend toward a low LFL, even in small amounts. That is why a little propane in methane lowers the lower limit of the mix. In our example blend of 90 percent methane, 6 percent ethane, 3 percent propane, and 1 percent butane, the LFL falls to 4.53 percent. Pure methane sits at 5 percent. The UFL falls from 15 to 14.46 percent.

How inert gas changes the picture

Inert gas such as nitrogen and carbon dioxide dilutes the fuel. In terms of the whole gas, the LFL rises. If half the gas is inert, the gas needs twice as much volume in air to reach the same fuel level. The tool divides the fuel-only LFL by the fuel fraction to account for this.

The upper limit is trickier. Inert gas shrinks the flammable range from the top, and it can erase the range at high inert levels. The tool shows a fuel-only UFL and an upper bound, because a full answer needs test data or charts for each inert.

The preset blends

The presets give you a quick start. They are illustrative, so replace them with your gas analysis.

  • Pipeline natural gas. 90 percent methane, 6 percent ethane, 3 percent propane, 1 percent butane.
  • LPG. 60 percent propane, 40 percent butane.
  • Biogas. 60 percent methane, with the other 40 percent treated as inert carbon dioxide.
  • Syngas. 35 percent hydrogen, 35 percent carbon monoxide, 10 percent methane, and the rest inert.

Where the rule works and where it does not

Le Chatelier's rule works best for mixtures of similar fuels, such as hydrocarbons. It gets weaker when hydrogen, acetylene, ethylene oxide, or carbon disulfide are in the blend. It also gets weaker at high temperature or pressure, and with halogenated compounds.

Treat the output as a planning estimate. For process design, rely on tested data, such as limits measured under ASTM E681. Safety professionals also use software and published charts for blends with inert gases.

Worked example for a gas blend

Take the pipeline blend above.

  1. Divide each fuel by its LFL. Methane gives 0.90 / 5.0 = 0.180. Ethane gives 0.06 / 3.0 = 0.020. Propane gives 0.03 / 2.1 = 0.0143. Butane gives 0.01 / 1.6 = 0.0063.
  2. Add them. The sum is 0.2205.
  3. Take the inverse. The blended LFL is 1 / 0.2205 = 4.53 percent.

If air in a boiler room holds 3 percent of this gas, the mix reads 3 / 4.53 = 66 percent of the LFL. That is too lean to burn but far above the 10 percent action level, and a leak that grows by half would reach the limit.

Using the result with gas detection

A detector calibrated for methane reads a blend wrongly if the blend has other fuels. Ask the maker for response factors, or calibrate on a gas that matches the mix. Use the blended LFL to convert a reading to percent by volume. Our LEL and UEL planning lookup shows how that conversion works for one gas at a time.

For a Texas plant, tie the final alarm levels to your process hazard analysis and to guidance from OSHA's process safety rule, 29 CFR 1910.119.

Frequently asked questions

What is Le Chatelier's rule?

It is a formula that estimates the flammable limits of a fuel blend from the limits of each fuel. The blended limit equals one divided by the sum of each fuel's fraction over its own limit.

How do I find the LEL of a gas mixture?

Convert each fuel to a fraction of the total fuel. Divide each fraction by that fuel's LEL. Add the results and take the inverse. The answer is the mix LEL.

Does nitrogen or carbon dioxide change the LEL?

Yes. Inert gas dilutes the fuel, so the LEL of the gas as supplied goes up. It also narrows the range from the top, and at high levels the mix may not burn at all.

Is the rule accurate for hydrogen mixtures?

It is weaker for hydrogen, acetylene, and ethylene oxide. Use tested data or process safety software for those blends.

Can I use this for a gas detector alarm setting?

You can use it to estimate the blend LEL. Pair it with the detector maker's response factors and your site hazard analysis before you set any alarm.

Which fuels does the calculator include?

It includes the 33 gases and vapors in our LEL and UEL lookup, from hydrogen and methane to gasoline and toluene.

Related Texas fire safety guides

Sources and standards

These references shaped the numbers and rules on this page. Always read the edition your local fire marshal has adopted.

Last reviewed October 13, 2026.

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