Unit converter
Choose the substance or type its molar mass and lower explosive limit, then enter the value you want to convert. The calculation runs in your browser; nothing you enter is sent anywhere.
| % (by volume) | ppm | g/m³ | %LEL |
|---|---|---|---|
Threshold values for this substance
| Level | % (by volume) | ppm | g/m³ |
|---|
Detailed calculation
Every conversion step is shown with its formula and the numbers substituted.
LEL of a gas mixture
Enter the share and the lower explosive limit of at least two components. Enter flammable components only; non-flammable components such as nitrogen do not enter the calculation.
Lower explosive limit of the mixture: % (by volume)
What LEL and %LEL mean
LEL is the lower explosive limit: the lowest concentration of a flammable gas or vapour in air that can explode. Standards often call it LFL, the lower flammable limit. It is given in per cent by volume; for methane it is about 4.4 %.
Gas detectors usually display %LEL, the concentration as a share of the lower explosive limit. A reading of 20 %LEL means that the gas has reached one fifth of the concentration at which it could explode.
Conversion formulas
- ppm = % by volume × 10,000
- %LEL = % by volume / LEL × 100
- g/m³ = % by volume × 10 × M / Vm
M is the molar mass of the substance (g/mol) and Vm the volume of one mole of gas (L/mol). Vm depends on temperature: from the ideal gas law Vm = R · T / p, which gives 24.06 L/mol at 20 °C and atmospheric pressure. The calculator works it out for the temperature you enter.
Conversion between % by volume, ppm and %LEL does not depend on temperature. Only the g/m³ value does.
Worked example
The detector in a boiler room alarms at 20 %LEL for methane. What is that in other units?
- % by volume: 20 × 4.4 / 100 = 0.88 %
- ppm: 0.88 × 10,000 = 8,800 ppm
- g/m³ (20 °C): 0.88 × 10 × 16.0 / 24.06 ≈ 5.85 g/m³
Press Fill in the example in the calculator to see it.
LEL of a mixture: Le Chatelier's rule
When several flammable gases are present together, the lower explosive limit of the mixture is not the simple average of the individual limits. It is calculated with Le Chatelier's rule:
LEL = 100 / ( y₁/LEL₁ + y₂/LEL₂ + … )
Here y is the share by volume of each flammable component among the flammables (they add up to 100).
Example. A gas whose flammable part is 70 % methane (LEL 4.4), 20 % ethane (2.4) and 10 % propane (1.7):
LEL = 100 / (70/4.4 + 20/2.4 + 10/1.7) = 3.32 %
This has a practical consequence. If the detector alarm is set for pure methane at 20 %LEL, that is 0.88 %, the same concentration is about 26.5 % of the mixture's own limit. The alarm comes later than assumed.
Where this calculation sits in the explosion protection document
When we prepare an explosion protection document, we use these conversions and the mixture calculation in the following sections:
- Substance data. The explosion limits of every flammable gas and vapour are recorded in % by volume and in g/m³, with their source.
- Zone calculation. How far the released gas is diluted by ventilation is assessed relative to the lower explosive limit.
- Detector and ventilation conditions. Alarm thresholds are given in %LEL. Where the detector is calibrated for a different gas from the one present, the document shows what the threshold really corresponds to.
- Mixtures. For mixtures such as natural gas, LPG or thinners, the limit is calculated from the composition.
Converting units is the easy part. The hard part is choosing the right substance data and establishing where, and how much, gas can be released.
When using the results
- LEL values differ between sources. The list uses the lower, more cautious end of the range found in the sources. If your safety data sheet gives a different value, you can type it in; when in doubt, use the lower one.
- The lower explosive limit falls as temperature rises. Table values apply at room temperature. In a hot process the real limit is lower than the tabulated one.
- Le Chatelier's rule does not work for every mixture. It gives good results for similar hydrocarbons. For mixtures that contain hydrogen or very different substances it is less reliable.
- A detector reads relative to the gas it was calibrated with. With another gas the reading can differ from the true value. Check the manufacturer's correction factors.
Sources
- ISO/IEC 80079-20-1 (material characteristics for gas and vapour classification; formerly IEC 60079-20-1)
- GESTIS substance database (IFA): explosion limits and molar masses
This tool is for information. For detector settings and zone decisions, the manufacturer's data, the current safety data sheet of the substance and the relevant standards apply.
Frequently asked questions
How do I convert ppm to %LEL?
Divide the ppm value by 10,000 to get % by volume, then divide by the lower explosive limit of the substance and multiply by 100. For methane, 8,800 ppm is 0.88 %, and 0.88 / 4.4 × 100 = 20 %LEL.
Are LEL and LFL the same?
Yes. LEL stands for lower explosive limit and LFL for lower flammable limit. Both describe the same value; standards tend to use LFL, while site practice and detectors mostly use LEL.
What should the detector alarm be set to?
There is no single right answer. Thresholds follow the detector manufacturer's recommendation, the relevant standards and the site's risk assessment, and they are recorded in the explosion protection document. This tool shows what a chosen threshold corresponds to in other units.
Does temperature change the result?
Not for the conversion between % by volume, ppm and %LEL. Only the g/m³ value depends on temperature, because gas expands as it warms. The calculator takes the temperature you enter into account.