How the LEL and UEL lookup works
This lookup returns the lower explosive limit and upper explosive limit for 33 common gases and vapors. Pick a substance and the tool shows its flammable range, 10 and 25 percent of the LEL, and its vapor density. It also converts a detector reading from percent LEL to percent by volume and parts per million.
A sizing line shows how much vapor would bring a room to a chosen level. The table below the calculator lists every substance, and a search box filters it as you type.
What LEL and UEL mean
The lower explosive limit, or LEL, is the thinnest mix of fuel and air that can burn. The upper explosive limit, or UEL, is the richest mix that still burns. Between the two lies the flammable range. Below the LEL, the mix is too lean. Above the UEL, it is too rich. Some sources use the terms LFL and UFL for the same limits.
Methane has an LEL of 5 percent and a UEL of 15 percent. That is a range of 10 points. Hydrogen runs from 4 to 75 percent, a range of 71 points. The wider the range, the easier it is for a leak to find a burnable mix.
Percent LEL on a gas detector
Gas detectors show readings in percent of LEL, not percent by volume. A reading of 100 percent LEL means the air is at the lower limit. For methane, that is 5 percent by volume, or 50,000 ppm. A reading of 10 percent LEL for methane is just 0.5 percent by volume.
The conversion is simple. Percent by volume equals percent LEL divided by 100, times the LEL. To get ppm, multiply percent by volume by 10,000. The lookup does both steps for you.
Action levels you will see in the field
OSHA lists a flammable atmosphere above 10 percent of the LEL as a hazardous atmosphere in permit-required confined spaces. The rule is in 29 CFR 1910.146. Hot work programs often use the same 10 percent line. Many sites set a low alarm near 10 or 20 percent LEL and a high alarm near 20 to 40 percent. These are common practices, not universal rules, so follow your site's written procedure.
Sensor type matters. Catalytic bead sensors need oxygen to work and respond differently to each gas. If you calibrate to methane and then measure something else, apply a correction factor from the maker. Infrared sensors work in low oxygen, but they miss hydrogen.
Vapor density tells you where to look
Vapor density compares a gas to air, which is set at 1. Hydrogen, at 0.07, rises fast. Methane, at 0.55, also rises. Propane, at about 1.55, sinks. Gasoline vapor, at 3 to 4, creeps along the floor and into drains and pits.
Put sensors where the gas will collect. That means near the ceiling for hydrogen and methane, and near the floor for propane and heavy solvent vapors.
Temperature and pressure change the limits
The values in the table apply near room temperature and normal pressure. Heat widens the range. As a rough guide, the LEL drops about 7 percent for every 100 degrees Celsius of temperature rise. Higher pressure usually widens the range too, and oxygen-rich air widens it a lot.
Process lines, dryers, and hot tanks can sit well outside table conditions. Use test data for those cases.
Worked example for a propane leak
A shop measures 50 by 20 by 10 feet, or 10,000 cubic feet. Propane has an LEL of 2.1 percent. To reach 10 percent of the LEL, the room needs 0.21 percent propane by volume.
That is 10,000 x 0.0021, or 21 cubic feet of propane vapor. A 20 pound cylinder holds roughly 170 cubic feet of vapor when released. So about one eighth of that cylinder can bring a well-mixed room to the 10 percent action level. Real leaks mix poorly, and propane pools low, so floor-level readings climb sooner.
Texas context
Texas has dense oil, gas, and petrochemical operations, and many small shops with propane, solvents, and paint booths. Fire marshals look for flammable gas controls under the International Fire Code and NFPA 30 and 58. Hazardous locations for electrical equipment follow NFPA 70, Article 500. Ask your local fire marshal which editions apply.
Frequently asked questions
What does LEL stand for?
LEL stands for lower explosive limit. It is the lowest concentration of a gas or vapor in air that can burn. Some sources call it the lower flammable limit, or LFL.
What is the difference between LEL and UEL?
The LEL is the leanest burnable mix. The UEL is the richest burnable mix. Between the two limits lies the flammable range. Outside it, the mix cannot burn.
What does 10 percent LEL mean?
It means the air holds one tenth of the gas needed to reach the lower limit. For methane, that is 0.5 percent by volume. OSHA treats 10 percent LEL as a hazardous atmosphere in permit-required confined spaces.
How do I convert percent LEL to ppm?
Multiply the percent LEL by the LEL, divide by 100, and then multiply by 10,000. For propane at 10 percent LEL, that is 10 / 100 x 2.1 x 10,000 = 2,100 ppm.
Is a rich mixture safe because it is above the UEL?
No. A rich mix can dilute with air as it leaks and move back into the flammable range. Treat any rich space as dangerous.
Do the limits change with temperature?
Yes. Heat lowers the LEL and raises the UEL, which widens the range. The table values apply near room temperature and normal pressure.
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.
- NIOSH Pocket Guide to Chemical Hazards
- OSHA 29 CFR 1910.146, Permit-required confined spaces
- NFPA 497, Classification of Flammable Liquids, Gases, or Vapors
Last reviewed October 13, 2026.