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Confined Space %LEL Calculator

Convert a measured flammable-gas concentration in parts per million (ppm) to a percentage of its Lower Explosive Limit (LEL) and check it against common alarm thresholds. It is a planning and training aid — real confined-space entry always relies on a calibrated detector and a permit.

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How to use this calculator

Enter Values

Minimum 0

Minimum 0

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the measured gas concentration in ppm.
  2. Optionally enter the gas's Lower Explosive Limit as a percentage by volume (default 5.0% for methane).
  3. Read the %LEL, the LEL expressed in ppm, and the alarm status band.

How it works

The LEL entered as a percentage by volume is converted to ppm using the fact that 1% by volume equals 10,000 ppm, so LEL-in-ppm = LEL% × 10,000. The percentage of LEL is then the measured concentration divided by the LEL-in-ppm, times 100. The result is graded into bands: below 5% LEL is OK, 5–10% is caution, and 10% or above means evacuate — 10% LEL is the level at which many portable gas detectors alarm.

Worked example

Worked example. A methane reading of 2500 ppm with a 5.0% LEL: the LEL in ppm is 5.0 × 10,000 = 50,000 ppm, so %LEL = 2500 ÷ 50,000 × 100 = 5% (caution). A higher reading of 6000 ppm gives 6000 ÷ 50,000 × 100 = 12%, which is above the 10% threshold and means evacuate and do not enter.

Common mistakes

  • Entering the LEL in ppm instead of percent by volume — the field expects a percentage (e.g. 5.0 for methane), which the tool converts to 50,000 ppm.
  • Assuming a low %LEL means the space is safe — oxygen level and toxic gases must also be tested; a flammable-gas reading alone does not clear an atmosphere for entry.
  • Trusting a reading from an uncalibrated or un-bumped detector, or one taken outside a confined-space entry permit by an untrained person.

Frequently asked questions

Why is 10% LEL treated as the evacuate level?

Most gas detectors are set to alarm at 10% LEL to give a large safety margin below the concentration that could actually ignite. Reaching 10% LEL means conditions are trending toward danger and the space should be evacuated and re-ventilated before any further testing.

Does a different gas change the result?

Yes — each gas has its own LEL. Methane is about 5% by volume, but gases such as hydrogen, acetylene or petrol vapour have very different LELs, so enter the correct LEL for the gas being measured. Detectors are also calibrated to a specific gas and may read differently for others.

Related tools

Explore more in Safety, Workplace Risk & Compliance Helpers.

Cite this tool

Zerdly. (2026). Confined Space %LEL Calculator. Retrieved from https://www.zerdly.com/tools/confined-space-gas-lel-calculator/

Tip: Enter any known values to calculate the remaining results.

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