ppm to mg/m³ calculator, step by step
ppm and mg/m³
- Formula: mg/m³ = (ppm × molecular weight) ÷ molar volume
- At 25 °C and 760 mmHg, one mole of gas takes up 24.45 liters.
- Put in the numbers: (100 × 92.14) ÷ 24.45
- Multiply the top: 9,214 ÷ 24.45
- Answer: 376.9 mg/m³
Percent and ppm
- 1% by volume = 10,000 ppm (1 part per hundred = 10,000 parts per million).
- Multiply by 10,000: 0.5 × 10,000
- Answer: 5,000 ppm
%LEL reading to ppm
- The gas is flammable from 1.1% by volume (its LEL). A reading of 10% LEL is 10% of that.
- Percent by volume: (10 ÷ 100) × 1.1% = 0.11%
- Convert to ppm: 0.11 × 10,000 = 1,100 ppm
- Answer: 1,100 ppm
Formulas and sources
- mg/m³ = (ppm × MW) ÷ 24.45
- ppm = (mg/m³ × 24.45) ÷ MW
- Other conditions: 24.45 × (°C + 273.15) ÷ 298.15 × 760 ÷ mmHg
- 1% by volume = 10,000 ppm
- ppm = (%LEL ÷ 100) × LEL% × 10,000
The formula comes from
NIOSH, Occupational Exposure Banding e-Tool: Calculator (mg/m³ to ppm)(opens wwwn.cdc.gov in a new tab)“The conversion equation is based on 25 ºC and 1 atmosphere: X ppm = (Y mg/m3)(24.45)/(molecular weight) or Y mg/m3 = (X ppm)(molecular weight)/24.45”
Source
CCOHS (Canadian Centre for Occupational Health and Safety), Converting Occupational Exposure Limits from mg/m³ to ppm(opens ccohs.ca in a new tab)“The number 24.45 in the equations above is the volume (litres) of a mole (gram molecular weight) of a gas or vapour when the pressure is at 1 atmosphere (760 torr or 760 mm Hg) and at 25°C. To convert mg/m3 to ppm at other temperatures and pressures, one must calculate the volume of one gram molecular weight of the airborne contaminant (e.g., 92.13 grams of toluene) at that temperature and pressure by using the ideal gas law formula: PV = nRT”
Source
NIOSH, Criteria for a Recommended Standard: Occupational Exposure to Carbon Dioxide (1976)(opens stacks.cdc.gov in a new tab)“The recommended standards include an exposure limit of 10,000 ppm (1 percent) as a time-weighted average concentration for up to a 10- hour work shift in a 40-hour work week, with a ceiling concentration of 30,000 ppm (3 percent) for 10 minutes.”
Source
29 CFR 1910.146(b), definition of hazardous atmosphere(opens ecfr.gov in a new tab)“Hazardous atmosphere means an atmosphere that may expose employees to the risk of death, incapacitation, impairment of ability to self-rescue (that is, escape unaided from a permit space), injury, or acute illness from one or more of the following causes: (1) Flammable gas, vapor, or mist in excess of 10 percent of its lower flammable limit (LFL);”
Formulas and calculators on the BCSP exams
- BCSP gives you no formula sheet, and you can't bring one.
- When a question needs a formula, the formula is written into the question, unless the question is testing whether you know that formula.
- You get an on-screen calculator that works like the TI-30XS, plus a whiteboard. You can't bring your own calculator.
- On the ASP, Mathematical Calculations is its own domain, worth 10% of the exam (ASP11 blueprint).
BCSP on formulas
BCSP, What to Expect When Sitting for a BCSP Exam (item 7)(opens bcsp.org in a new tab)“For any question requiring a mathematical formula, the formula will be included within the exam question (unless the question is specifically testing your knowledge of a formula). You will not be provided or permitted to bring a formula sheet.”
BCSP on calculators
BCSP, What to Expect When Sitting for a BCSP Exam (item 6)(opens bcsp.org in a new tab)“You will be provided an on-screen calculator, emulating the TI-30XS scientific calculator. You will also have access to a white board and dry erase marker. The testing center will not provide a physical calculator or permit candidates to bring their own.”
FAQ
How do you convert ppm to mg/m³?
Multiply ppm by the chemical's molecular weight, then divide by 24.45: mg/m³ = (ppm × MW) ÷ 24.45. The 24.45 is the liters one mole of gas takes up at 25 °C and 1 atmosphere. Example: 100 ppm of toluene (MW 92.14) is 100 × 92.14 ÷ 24.45 = 376.9 mg/m³.
How do you convert mg/m³ to ppm?
Turn the formula around: ppm = (mg/m³ × 24.45) ÷ MW. Example: 40 mg/m³ of carbon monoxide (MW 28.01) is 40 × 24.45 ÷ 28.01 = 34.9 ppm.
What if the temperature or pressure isn't 25 °C and 760 mmHg?
Change the 24.45. Gas takes up more room when it's warmer or the pressure is lower, so scale it: 24.45 × (°C + 273.15) ÷ 298.15 × 760 ÷ mmHg. Then use that number in place of 24.45. The calculator does this when you change the temperature or pressure.
How many ppm is 1 percent?
1% by volume is 10,000 ppm. Multiply percent by 10,000 to get ppm, or divide ppm by 10,000 to get percent. NIOSH writes its carbon dioxide limit as 10,000 ppm (1 percent).
How do I turn a %LEL reading into ppm?
Multiply the reading (as a fraction) by the gas's LEL in percent, then by 10,000. Toluene's LEL is 1.1%, so a 10% LEL reading is 0.10 × 1.1 × 10,000 = 1,100 ppm. OSHA's confined space rule treats more than 10% of the lower flammable limit as a hazardous atmosphere.
Can I convert dusts and fumes to ppm?
No. ppm only makes sense for gases and vapors. Dusts, fumes and mists stay in mg/m³.
Molecular weights in the list are worked out from each chemical's formula. This tool is for study and estimates. It doesn't replace air sampling or a meter reading.