ppm ↔ mg/m³ Converter

Convert airborne contaminants between ppm and mg/m³ with adjustable temperature and pressure. Industrial hygiene, occupational chemistry, no signup.

ppm
g/mol

Common molecular weights

Click a row to load its molecular weight into the field.

How It Works

The converter uses the ideal gas law to calculate the molar volume Vm at the specified temperature and pressure, then applies the standard conversion formula:

Vm (L/mol) = R × T (K) / P (Pa) × 1000
where R = 8.314 J/(mol·K)
mg/m³ = (ppm × MW) / Vm
ppm = (mg/m³ × Vm) / MW
where MW = molecular weight (g/mol)

At the default conditions (25 °C, 101.325 kPa), the molar volume is Vm = 24.45 L/mol. This is the value used in the ACGIH TLV/BEI Documentation and the NIOSH Manual of Analytical Methods (5th edition) for reporting atmospheric concentrations at near-ambient conditions. Some older references use 22.4 L/mol (STP: 0 °C, 1 atm) — the advanced panel lets you replicate any reference condition.

All internal calculations use SI units (Kelvin, Pascals). Temperature and pressure are converted to SI at the edges, ensuring numerical stability regardless of the display unit chosen.

Frequently Asked Questions

What is Vm and why does it depend on temperature and pressure?
Vm is the molar volume — the volume occupied by one mole of an ideal gas at given conditions. It is derived from the ideal gas law: Vm = RT/P, where R = 8.314 J/(mol·K). At higher temperatures, gas molecules move faster and occupy more space (larger Vm), so a given mass of contaminant spreads across more air — ppm goes up while mg/m³ stays the same. At higher pressures, the same number of molecules is packed into less volume (smaller Vm) — mg/m³ increases while ppm stays the same. This is why field conditions matter for accurate exposure assessment.
Which conditions does the default Vm = 24.45 L/mol use?
Vm = 24.45 L/mol corresponds to 25 °C (298.15 K) and 1 atm (101.325 kPa). This is the reference condition used by the ACGIH TLV/BEI documentation and the NIOSH Manual of Analytical Methods, 5th edition. It represents typical indoor air conditions. Older references (pre-1990s) sometimes used Vm = 22.4 L/mol, which corresponds to 0 °C and 1 atm (Standard Temperature and Pressure, STP). If you are comparing results against an older source, select 0 °C in the advanced panel to reproduce STP conditions.
Why do exposure limits use both ppm and mg/m³?
ppm (parts per million by volume) describes how many molecules of contaminant are present per million air molecules — it is dimensionless with respect to air density and independent of temperature and pressure. mg/m³ describes the actual mass of contaminant in a cubic metre of air — it changes with temperature and pressure. For toxicological purposes (dose to the lung), mg/m³ is more directly meaningful. For field measurements with direct-reading instruments, ppm is more common. Regulatory bodies like ACGIH publish TLVs in both units for gases and vapors. The NIOSH Manual of Analytical Methods uses mg/m³ as the primary reporting unit at 25 °C / 1 atm.
Does this account for non-ideal gas behavior?
No. The conversion formula assumes ideal gas behavior (Vm = RT/P). For occupational hygiene work at typical ambient conditions (0–50 °C, pressures near 1 atm), the deviation from ideal behavior is less than 1% for most industrial gases and vapors — well within the uncertainty of field sampling and analytical methods. Non-ideal corrections (e.g., using the van der Waals equation or compressibility factor Z) are only necessary at very high pressures (above ~10 atm) or near the critical point of the substance, conditions that are rarely encountered in routine exposure assessment.

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