Gas Concentration Converter
Convert gas concentrations between parts per million (ppm), percent volume (%vol), percent of lower explosive limit (%LEL), and milligrams per cubic meter (mg/m³). Select a gas to include molecular weight and LEL data for accurate conversions at 25°C, 1 atm.
Quick select (ppm):
Carbon Monoxide (CO)
Common combustion byproduct in parking garages, boiler rooms, and loading docks.
ppm ↔ %vol
%vol = ppm ÷ 10,000 | ppm = %vol × 10,000
ppm ↔ %LEL
%LEL = (ppm ÷ (LELvol × 10,000)) × 100 | ppm = (%LEL ÷ 100) × LELvol × 10,000
ppm → mg/m³ (at 25°C, 1 atm)
mg/m³ = (ppm × MW) ÷ 24.45
mg/m³ → ppm (at 25°C, 1 atm)
ppm = (mg/m³ × 24.45) ÷ MW
Where MW = molecular weight (g/mol) and 24.45 L/mol is the molar volume of an ideal gas at 25°C and 1 atm.
Gas Concentration Reference
Properties, exposure limits, and example conversions at 25°C, 1 atm
| Unit | Typical Use |
|---|---|
| ppm | Toxic gas monitoring (CO, H₂S, NH₃, NO₂) and occupational exposure limits |
| %vol | High concentrations — O₂ monitoring (20.9% normal), CO₂, and expressing LEL values |
| %LEL | Combustible gas detection — normalizes gases to a common explosion risk scale |
| mg/m³ | European OELs, industrial hygiene, environmental monitoring (mass-based, depends on MW) |
Exposure Limit Definitions
| Term | Definition | Source |
|---|---|---|
| PEL-TWA | Maximum average concentration for an 8-hour workday. | OSHA 29 CFR 1910.1000 |
| STEL | 15-minute TWA, must not be exceeded during any 15-minute period. | ACGIH / NIOSH |
| Ceiling | Must never be exceeded, even instantaneously. | OSHA / NIOSH |
| IDLH | Concentration from which a worker could escape within 30 minutes without irreversible health effects. | NIOSH |
| LEL | Minimum concentration that can ignite. Detectors typically alarm at 10–25% LEL. | NFPA 497 |
Gas Properties
| Gas | Formula | MW (g/mol) | LEL (%vol) | PEL-TWA (ppm) | IDLH (ppm) |
|---|---|---|---|---|---|
| Carbon Monoxide | CO | 28.01 | 12.5 | 50 | 1,200 |
| Carbon Dioxide | CO₂ | 44.01 | N/A | 5,000 | 40,000 |
| Hydrogen Sulfide | H₂S | 34.08 | 4 | 10 | 100 |
| Ammonia | NH₃ | 17.03 | 15 | 50 | 300 |
| Methane | CH₄ | 16.04 | 5 | Simple asphyxiant | Simple asphyxiant |
| Propane | C₃H₈ | 44.1 | 2.1 | 1,000 | 2,100 |
| Hydrogen | H₂ | 2.02 | 4 | Simple asphyxiant | Simple asphyxiant |
| Nitrogen Dioxide | NO₂ | 46.01 | N/A | 5 | 20 |
| Ethylene | C₂H₄ | 28.05 | 2.7 | 200 | N/A |
| Oxygen | O₂ | 32 | N/A | Normal: 20.9% | Deficient <19.5%, Enriched >23.5% |
Example Conversions: Methane (CH₄) and Carbon Monoxide (CO)
| ppm | %vol | %LEL (CH₄) | mg/m³ (CO) |
|---|---|---|---|
| 1 | 0.000100 | <0.1 | 1.1 |
| 10 | 0.001000 | <0.1 | 11.5 |
| 100 | 0.0100 | 0.20 | 114.6 |
| 500 | 0.0500 | 1.0 | 572.8 |
| 1,000 | 0.1000 | 2.0 | 1145.6 |
| 5,000 | 0.5000 | 10.0 | 5728.0 |
| 10,000 | 1.00 | 20.0 | 11456.0 |
| 50,000 | 5.00 | 100.0 | 57280.2 |
| 100,000 | 10.00 | 200.0 | 114560.3 |
Frequently Asked Questions
Divide the ppm value by 10,000. For example, 5,000 ppm equals 0.5%. This works because 1% equals 10,000 parts per million by volume.
One percent equals exactly 10,000 ppm. This is a simple ratio: 1% = 1 part per 100 = 10,000 parts per 1,000,000.
Yes. 10,000 ppm is exactly equal to 1% by volume. This conversion applies to any gas measured by volume fraction at any temperature and pressure.
Percent of Lower Explosive Limit (%LEL) expresses a gas concentration as a fraction of the minimum concentration needed for ignition. For methane (LEL = 5.0% vol), 10% LEL equals 5,000 ppm. The conversion depends on the specific gas's LEL value.
Multiply ppm by the gas's molecular weight and divide by 24.45 (the molar volume at 25°C, 1 atm). For carbon monoxide (MW = 28.01): 50 ppm × 28.01 / 24.45 = 57.3 mg/m³. This formula varies with temperature and pressure.
It depends on the gas and the hazard. Toxic gas detectors (CO, H₂S, NH₃) typically alarm in ppm at fractions of the occupational exposure limit. Combustible gas detectors (methane, propane) alarm in %LEL, usually at 10% and 25% LEL.
Ammonia and CO are toxic at concentrations far below their LEL, so ppm is the relevant safety unit. Methane is a simple asphyxiant — its primary hazard is explosion, not toxicity — so %LEL is the appropriate measurement scale.
PEL-TWA (Permissible Exposure Limit, Time-Weighted Average) is the maximum 8-hour average concentration allowed by OSHA. STEL (Short-Term Exposure Limit) is a 15-minute average. IDLH (Immediately Dangerous to Life or Health) is the concentration from which a worker could escape within 30 minutes without irreversible health effects. These are defined by OSHA (29 CFR 1910.1000) and NIOSH, respectively.
Related CET Products
cGas Detector
Addressable gas detector for CO, NO₂, combustible gases, and refrigerants. Wall or duct mount with BACnet/Modbus output.
View Products →FCS Flexible Control System
Multi-channel gas detection controller for parking garages, mechanical rooms, and refrigeration facilities.
View Products →See also: ASHRAE 15 Refrigerant Detection Guide | A2L Refrigerant Transition Guide
Sources & Standards
- OSHA 29 CFR 1910.1000 Table Z-1 — Permissible Exposure Limits
- NIOSH Pocket Guide to Chemical Hazards — IDLH values, RELs
- NFPA 497 — Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors (LEL values)
- ACGIH — Threshold Limit Values and Biological Exposure Indices (TLVs)
Advisory Disclaimer
This tool is provided for educational and reference purposes only. It summarizes requirements from published standards as of their most recent edition date and does not replace the full text of the applicable standard, local code, or the judgment of a qualified design professional. Values shown here must be verified against the current edition of the relevant standard (ASHRAE 15, ASHRAE 34, IIAR 2, etc.) and the Authority Having Jurisdiction for the project location. Critical Environment Technologies assumes no liability for decisions made using this tool. Always consult a licensed mechanical engineer for system design and compliance determinations.