Free Engineering Tool · NFPA 69
Gas Mixture LFL Calculator
Find the lower and upper flammable limits of a fuel gas mixture from its composition, adjust the lower limit for temperature, and compare a measured concentration with the NFPA 69 limits of 25 and 60 percent of the LFL.
- Based on NFPA 69
- Free, runs in your browser; your inputs stay on your device
- Screening tool, not reviewed or sealed by a licensed engineer
- Last updated October 2026
Screening tool, not engineering advice. This calculator estimates results with simplified published methods. It has not been reviewed or sealed by a licensed engineer for your project, and DGA does not warrant that its results are accurate, complete or suitable for any use. Check every result independently and have the licensed professional responsible for your project review it before you rely on it. Use is subject to the Terms of Use.
Advanced method. The correlations behind this tool come from research and test data with narrow ranges of validity. Outside those ranges results can be wrong by a wide margin, and they are no substitute for a fire model or an analysis by a qualified fire protection engineer.
Fuel Gas Composition
| Gas | % of fuel | LFL % | UFL % |
|---|
List only the flammable gases; the percentages are by volume of the fuel and are scaled to total 100. Limits load from US Bureau of Mines data at about 25 °C and can be edited.
Conditions
Result
| Gas | Share | LFL at T | In air at mixture LFL |
|---|
Flammable Ranges in Air
About This Tool
Le Chatelier’s rule treats each flammable gas in a mixture as using up its share of the flammability budget. If gas i makes up a fraction fi of the fuel and has a lower flammable limit LFLi in air, the mixture’s limit is LFLm = 1 / Σ(fi / LFLi). The rule was proposed by Le Chatelier in 1891 and confirmed for mixed gases by Coward, Carpenter and Payman in 1919; it works well for the lower limits of hydrocarbons, hydrogen and carbon monoxide in air. The same form applied to upper limits gives the mixture UFL, which is shown with a caution because rich mixtures follow the rule less closely.
Lower limits fall as the mixture gets hotter. Burgess and Wheeler observed in 1911 that lean limit mixtures of the paraffins release about the same heat per unit volume, so they burn at about the same flame temperature, roughly 1,600 K for most hydrocarbons. Holding that limit flame temperature Tf constant gives LFL(T) = LFL(T0) × (Tf − T) / (Tf − T0) in kelvin, applied to each gas before mixing. Hydrogen, carbon monoxide and acetylene burn at the limit with cooler flames, so the tool uses 980 K, 1,300 K and 1,280 K for them. The preset limits are US Bureau of Mines values at about 25 °C and atmospheric pressure, and every limit can be edited.
NFPA 69 allows deflagration prevention by keeping the combustible concentration at or below 25 percent of the LFL, or up to 60 percent where automatic instrumentation with safety interlocks is provided. Enter a measured or predicted concentration of the fuel in air and the tool reports it as a percent of the mixture LFL and shows which side of those limits it falls on.
Worked example
A synthesis gas is 40 percent carbon monoxide, 30 percent hydrogen, 20 percent methane and 10 percent ethane. At 25 °C, LFLm = 1 / (0.40/12.5 + 0.30/4.0 + 0.20/5.0 + 0.10/3.0) = 5.55 percent and the UFL is about 32 percent. The NFPA 69 limits are 1.39 percent (25 percent of the LFL) and 3.33 percent (60 percent), so a sensor reading of 1.0 percent is 18 percent of the LFL. In a duct at 150 °C (302 °F) the LFL drops to 4.77 percent, the 25 percent limit to 1.19 percent, and the same reading becomes 21 percent of the LFL.
Assumptions and limits
Enter only the flammable gases. Nitrogen, carbon dioxide and other inerts change the limits in ways this rule does not capture; use flammability diagrams or test data for diluted mixtures. Limits are for upward flame propagation in air at about atmospheric pressure. The temperature adjustment assumes no reaction before ignition, so it applies only well below the autoignition temperature of each gas, and it has not been evaluated for sulfur or nitrogen fuels. Combustible concentration reduction under NFPA 69 also requires the measurement, safety factor and interlock design that the standard sets out.
When to bring in an engineer
Ventilation rates, gas detection setpoints and interlocks that depend on the LFL are part of an explosion prevention design that the AHJ will review. DGA provides code consulting on NFPA 69 and hazardous process protection and third-party review of explosion prevention designs.
References
- NFPA 69, Standard on Explosion Prevention Systems, National Fire Protection Association, 2024 edition: deflagration prevention by combustible concentration reduction (25 percent of the LFL, or 60 percent with automatic instrumentation and safety interlocks).
- M.G. Zabetakis, Flammability Characteristics of Combustible Gases and Vapors, Bulletin 627, US Bureau of Mines, 1965: limits of flammability in air and the effect of temperature.
- H. Le Chatelier, “Estimation du grisou par les limites d’inflammabilité,” Annales des Mines, Series 8, Vol. 19, pp. 388 to 395, 1891.
- H.F. Coward, C.W. Carpenter and W. Payman, “The Dilution Limits of Inflammability of Gaseous Mixtures, Parts III and IV,” Journal of the Chemical Society, Transactions, Vol. 115, pp. 27 to 36, 1919.
- C.V. Mashuga and D.A. Crowl, “Derivation of Le Chatelier’s Mixing Rule for Flammable Limits,” Process Safety Progress, Vol. 19, No. 2, pp. 112 to 117, 2000.
- M.J. Burgess and R.V. Wheeler, “The Lower Limit of Inflammation of Mixtures of the Paraffin Hydrocarbons with Air,” Journal of the Chemical Society, Transactions, Vol. 99, pp. 2013 to 2030, 1911.
- M.G. Zabetakis, S. Lambiris and G.S. Scott, “Flame Temperatures of Limit Mixtures,” Seventh Symposium (International) on Combustion, Butterworths, London, pp. 484 to 487, 1959.
Frequently Asked Questions
How do you calculate the LFL of a gas mixture?
Use Le Chatelier’s rule: divide each gas’s fraction of the fuel by its own lower flammable limit, add the results, and take the reciprocal. For 50 percent methane (LFL 5.0), 25 percent carbon monoxide (12.5) and 25 percent hydrogen (4.0), the mixture LFL is 1 / (0.50/5.0 + 0.25/12.5 + 0.25/4.0) = 5.48 percent.
What does NFPA 69 say about percent of LFL?
For deflagration prevention by combustible concentration reduction, NFPA 69 keeps the combustible concentration at or below 25 percent of the LFL. It allows up to 60 percent of the LFL where automatic instrumentation with safety interlocks is provided.
How does temperature change the lower flammable limit?
The LFL decreases as temperature rises because the incoming mixture needs less heat from combustion to reach the limit flame temperature. With a limit flame temperature of about 1,600 K, a hydrocarbon’s LFL drops by roughly 7 to 8 percent of its room temperature value for every 100 C of heating.
Can I include nitrogen or carbon dioxide in the mixture?
Not with this rule. Le Chatelier’s rule combines flammable gases only. Inert gases narrow the flammable range, and their effect depends on the gas, so diluted fuels need flammability diagrams or test data.
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Engineering aid only, provided as is and without warranty. Results depend on the inputs and on the simplifying assumptions of the method summarized on each tool page, and DGA has not verified them for your application. They do not replace code-required design, testing or review by the licensed engineer responsible for the project. Use of the tools is subject to the Terms of Use.
Related Reading
Articles from DGA Insights on the code and the method behind this calculator.