Gas Concentration Converter (ppm ⇄ mg/m³)

ppm to mg/m³ for gases and vapours, with molar masses computed from the IUPAC atomic weights for eighteen compounds and the 1.64% gap between the American 24.45 L/mol and the European 24.055 shown in every answer — plus where the published 24.45 actually came from.

Last updated 24 Sep 2026

ppm to mg/m³, by compound and reference condition

ppm + compound + reference → mg/m³

Which way?ppm for a gas is a VOLUME ratio, strictly micromoles per mole, which is why a molar mass has to come into it.

Your valueppm in the first mode, mg/m³ in the second.

Which compound?Molar masses computed from the IUPAC atomic weights, not copied from a chart. Every one reproduces the NIOSH Pocket Guide’s own printed conversion.

Or type a molar mass, g/molLeave at 0 to use the dropdown. Any gas or vapour: divide the molecular formula’s atomic weights.

Reference temperature and pressureThis is the regional split, and it moves every answer by 1.7%. NIOSH, OSHA and ACGIH use 25 °C; the EU and HSE use 20 °C.

Your own temperature, °C

Your own pressure, kPa

Two cubic metres of air, not a circuit, and the 1.64% made visible. Both boxes hold the same volume and the same concentration in parts per million — the same NUMBER of your molecules among a million air molecules. The left box is at 25 °C, the reference condition NIOSH, OSHA and ACGIH use, and the right one at 20 °C, which is the European one. Cooler air is denser, so the right box holds more moles of air in the same cubic metre, so the same parts-per-million figure is more milligrams. No fill line is drawn in either box, deliberately: at 1.64% the difference would be under two parts in a hundred of the box height, and exaggerating it to make it visible would misstate its size. The numbers beneath say it instead. That is the whole of the regional disagreement: not a measurement error, a choice of reference temperature, and it goes the same way for every compound.

79.871mg/m³Example

25 ppm of benzene at the NIOSH and OSHA reference condition

One division, two lookups

mg/m³ = ppm × M / V_m · ppm = mg/m³ × V_m / M · V_m = RT / P

ppm: parts per million BY VOLUME, that is µmol/mol. For a gas this is what ppm means
M: molar mass of the compound, g/mol. Computed here from the IUPAC atomic weights
V_m: molar volume of air at the reference condition, L/mol. 24.45 for NIOSH, OSHA and ACGIH; 24.055 for HSE and the EU
R: 8.314 462 618 J mol⁻¹ K⁻¹ exactly, since the 2019 SI redefinition

Worked example

25 ppm of benzene at the NIOSH and OSHA reference condition
Benzene is C₆H₆, so M = 6 × 12.011 + 6 × 1.0080 = 78.114 g/mol from the IUPAC atomic weights
At 25 °C and 760 mmHg, NIOSH, OSHA and ACGIH all state V_m = 24.45 L/mol
mg/m³ = 25 × 78.114 ÷ 24.45 = 79.87 mg/m³, so 1 ppm of benzene is 3.19 mg/m³ — which is exactly what the NIOSH Pocket Guide prints
At the British and European reference condition, 20 °C with V_m = 24.055, the same 25 ppm is 81.18 mg/m³ — 1.64% higher
That 1.64% is in every converted limit value, in the same direction, for every compound. It is the reason this page asks which regime you are in before it answers

Molar masses, and the conversion factor under each convention

Every molar mass in the third column is computed from the IUPAC abridged standard atomic weights — H 1.0080, C 12.011, N 14.007, O 15.999, F 18.998, S 32.06, Cl 35.45 — and not taken from a conversion chart. The fifth column is the check: it is the “Conversion: 1 ppm = x mg/m³” that the NIOSH Pocket Guide prints on that substance’s own page, and all eighteen agree with the computed fourth column to the precision NIOSH prints. That is what establishes both that the atomic weights are being used correctly and that NIOSH’s molar volume really is 24.45. Compare the fourth and sixth columns for the regional gap: a European limit in mg/m³ is 1.64% higher than the American one for the same ppm figure, every time, for every compound.
Compound	Formula	Molar mass, g/mol	mg/m³ per ppm at 24.45	NIOSH prints	mg/m³ per ppm at 24.055	ppm per mg/m³ at 24.45
Carbon monoxide	CO	28.010	1.1456	1.15	1.1644	0.87290
Carbon dioxide	CO2	44.009	1.8000	1.80	1.8295	0.55557
Nitrogen dioxide	NO2	46.005	1.8816	1.88	1.9125	0.53146
Sulfur dioxide	SO2	64.058	2.6200	2.62	2.6630	0.38169
Hydrogen sulfide	H2S	34.076	1.3937	1.39	1.4166	0.71751
Ammonia	NH3	17.031	0.6966	0.70	0.7080	1.43562
Ozone	O3	47.997	1.9631	1.96	1.9953	0.50941
Formaldehyde	HCHO	30.026	1.2281	1.23	1.2482	0.81429
Benzene	C6H6	78.114	3.1948	3.19	3.2473	0.31300
Toluene	C7H8	92.141	3.7685	3.77	3.8304	0.26535
Xylene (any isomer)	C8H10	106.168	4.3422	4.34	4.4135	0.23030
Methanol	CH3OH	32.042	1.3105	1.31	1.3320	0.76306
Acetone	C3H6O	58.080	2.3755	2.38	2.4144	0.42097
n-Hexane	C6H14	86.178	3.5247	3.52	3.5825	0.28372
Styrene	C8H8	104.152	4.2598	4.26	4.3297	0.23475
Hydrogen chloride	HCl	36.458	1.4911	1.49	1.5156	0.67063
Chlorine	Cl2	70.900	2.8998	2.90	2.9474	0.34485
Hydrogen fluoride	HF	20.006	0.8182	0.82	0.8317	1.22213

The molar volume, and where the published numbers came from

Two findings in this table, and the second one this page would most like to be remembered for. First, the 1.64% gap between the American and European conventions is real, it is in every converted limit, and it is not measurement scatter: the two regimes chose different reference temperatures and neither is wrong. Second, the published 24.45 is not RT/P. Compute RT/P at 25 °C and 101.325 kPa with the exactly fixed gas constant and it is 24.4654, which rounds to 24.47, not 24.45. Row three shows where 24.45 actually comes from: 22.4 L/mol — the schoolroom figure — scaled from 0 °C to 25 °C, which gives 24.4502. Row four shows that scaling the REAL 22.413 97 the same way does not. So the American convention carries a rounding made before the SI fixed the gas constant, and it makes every North American conversion 0.063% low. That is far smaller than the regional gap and it is still worth knowing about; this page offers both figures and defaults to the published one, because a limit value is compared against a measurement made the regulator’s way.
Convention	Temperature	Pressure	V_m, L/mol	Against that regime’s published figure	Note
NIOSH, OSHA and ACGIH — as published	25 °C	760 mmHg (101.325 kPa)	24.45000	—	The figure printed in the NIOSH Pocket Guide and the ACGIH TLV booklet
The same condition, RT/P with modern constants	25 °C	101.325 kPa	24.46540	0.0630%	R = 8.314 462 618 exactly since 2019
22.4 L/mol scaled from 0 °C to 25 °C	25 °C	101.325 kPa	24.45016	0.0006%	This is where the published 24.45 comes from
22.413 97 L/mol scaled the same way	25 °C	101.325 kPa	24.46540	0.0630%	The real molar volume at 0 °C, scaled — and it does NOT give 24.45
HSE EH40 — as published	20 °C	101 325 Pa	24.05526	—	EH40 paragraph 68 states the constant and the pressure explicitly
The same condition, RT/P	20 °C	101.325 kPa	24.05512	-0.00060%	EH40’s constant is right to six parts per million
The EU directives’ wording taken literally	20 °C	101.3 kPa	24.06105	0.0241%	Directive 2000/39/EC footnote 6 says 101,3 KPa — 0.025% from EH40
Normal conditions	0 °C	101.325 kPa	22.41397	-8.327%	For gas metering and emissions, not for exposure limits

Gases only, and why that is not pedantry

This page is for gases and vapours only, and the first thing to say is why that matters. For a gas, ppm is a ratio of volumes — strictly micromoles per mole, one molecule of your compound among a million molecules of air. Converting that to a mass per cubic metre needs to know what each of those molecules weighs, which is the compound’s molar mass. For an aqueous solution ppm is a ratio of masses instead, and a litre of dilute solution weighs about a kilogram, so 1 ppm is 1 mg/L for everything and no molar mass enters at all. Two different conventions wearing the same three letters. The lab utilities section owns the aqueous case; everything below is gases.

The conversion, and the two things it needs that a search engine does not have. mg/m³ = ppm × M / V_m. The molar mass M is a property of your compound. The molar volume V_m is a property of air at a stated reference temperature and pressure — and there is no single right answer, because the regimes that set exposure limits do not agree about it. NIOSH, OSHA and ACGIH work at 25 °C and 760 mmHg and state V_m = 24.45 L/mol. Every EU occupational exposure limit directive carries the footnote “milligrams per cubic metre of air at 20 °C and 101,3 KPa”, and HSE’s EH40 spells the constant out as 24.055 26. That is a 1.64% difference in every converted limit value, in the same direction every time: the European mg/m³ figure for a given ppm is always the larger. It is not much, and it is a real fork in the road, and the important consequence is the one people miss — do not convert a limit under one convention and compare it with a measurement reported under the other.

A smaller thing, found while checking. The 24.45 that NIOSH and ACGIH print is not RT/P. Since 2019 the molar gas constant has been exactly 8.314 462 618 J mol⁻¹ K⁻¹, and RT/P at 298.15 K and 101 325 Pa comes to 24.4654 L/mol — which rounds to 24.47, not 24.45. Where does 24.45 come from? Take the 22.4 L/mol that every chemistry class learns as the molar volume at 0 °C and scale it to 25 °C: 22.4 × 298.15/273.15 = 24.4502. Do the same with the real value, 22.413 97, and you get 24.4654 instead. So the American convention carries a rounding made before the gas constant was fixed, and it makes every conversion 0.063% low. It is a twentieth of the regional gap and nobody’s compliance turns on it, but it is a discrepancy rather than a mystery, and the page states it rather than smoothing it over. The dropdown offers both figures, and defaults to the published 24.45, because a published limit value was itself converted with 24.45.

The molar masses here are computed, not copied. Every one comes from the IUPAC abridged standard atomic weights, and the table below carries a check column: the NIOSH Pocket Guide prints its own “Conversion: 1 ppm = x mg/m³” on each substance’s page, and all eighteen compounds here reproduce it to the precision NIOSH prints. That is worth more than agreeing with another conversion chart, because it tests the atomic weights and the molar volume together. If your compound is not on the list, add up its formula’s atomic weights and type the answer in — the page will use it in place of the dropdown.

What the model leaves out. Air and the vapours here are treated as ideal gases, which at ambient conditions is good to a fraction of a per cent — comfortably inside the 1.64% the two regimes differ by. It gets worse near a vapour’s saturation point and at high pressure. Humidity is ignored, and that is safe: both ppm by volume and the molar volume refer to the whole gas mixture, so they move together. For the other side of occupational exposure arithmetic, the noise exposure dose calculator has the same structure of problem — competing regulatory conventions rather than a single right answer. For the conversion this page most resembles in kind, where the answer turns entirely on a lookup, see the cups to grams converter.

Frequently asked questions

How do I convert ppm to mg/m³?

mg/m³ = ppm × molar mass ÷ molar volume. The molar mass is the compound’s, in g/mol, and the molar volume is that of air at whatever reference condition applies to you — 24.45 L/mol at 25 °C in North America, 24.055 L/mol at 20 °C in Britain and the EU. For 25 ppm of benzene that is 25 × 78.114 ÷ 24.45 = 79.87 mg/m³ the American way and 81.18 the European way.

Why does ppm need a molar mass at all?

Because ppm for a gas is a VOLUME ratio, not a mass one — strictly micromoles per mole, one part of gas in a million parts of air by amount of substance. Equal volumes of gas hold equal numbers of molecules, so converting to a mass needs to know what each molecule weighs. That is the opposite of the aqueous case, where ppm is a mass ratio and a litre of dilute solution weighs a kilogram, so 1 ppm is 1 mg/L for everything. The lab utilities section has a page for that; this one is gases only.

Why do NIOSH and the EU use different reference temperatures?

History rather than physics. American practice settled on 25 °C and 760 mmHg, and NIOSH, OSHA and ACGIH all state a molar volume of 24.45 L/mol. European practice settled on 20 °C: every EU occupational exposure limit directive carries the footnote “milligrams per cubic metre of air at 20 °C and 101,3 KPa”, and HSE’s EH40 spells out a molar volume of 24.055 26. Neither is wrong, and the gap is 1.64% in every converted limit. What matters is not converting a limit from one regime and comparing it with a measurement made the other way.

Is the molar volume really 24.45 L/mol at 25 °C?

It is what NIOSH and ACGIH print, but it is not RT/P. Since the 2019 SI redefinition the gas constant is exactly 8.314 462 618 J/(mol K), and RT/P at 298.15 K and 101 325 Pa is 24.4654 L/mol. The published 24.45 is 22.4 × 298.15/273.15 = 24.4502 — the rounded 22.4 L/mol from the schoolroom, scaled up; scaling the real 22.413 97 the same way gives 24.4654 instead. Using 24.45 makes a conversion 0.063% low, which is negligible against a limit value, and this page offers both and defaults to the published one, because that is the number the regulator’s own limits were converted with.

Does air pressure change the answer?

Yes, and more than the temperature does in practice. The molar volume is proportional to absolute temperature and inversely proportional to pressure, so a 5 °C shift is 1.7% while a 3 kPa weather swing is 3.0% and Denver’s 84 kPa is 21%. Exposure limits are nonetheless defined AT the reference condition, not at the condition of your sample, so for compliance you convert the limit at 25 °C or 20 °C whatever the weather was. The last option in the reference dropdown is there for the times you genuinely want the concentration at conditions of your own.

Is 1 ppm of carbon monoxide dangerous?

This page does not answer that; it converts units. What it will tell you is that 1 ppm of CO is 1.15 mg/m³ at the American reference condition and 1.16 mg/m³ at the European one. The exposure limits themselves are set by the authority that governs your workplace, and they differ between jurisdictions by far more than 1.64%.

What about water vapour, or a gas that is not ideal?

The ideal gas law is the model here and it is good to a fraction of a per cent for the gases and vapours on this page at ambient conditions — well inside the regional disagreement the page is mostly about. It degrades near a vapour’s saturation point and at high pressure. Humidity has almost no effect on the conversion, because ppm by volume and the molar volume both refer to the whole gas mixture and both move together.

Related calculators

References

National Institute for Occupational Safety and Health. NIOSH Pocket Guide to Chemical Hazards, cdc.gov. A work of the United States Government. Each substance page prints its own “Conversion: 1 ppm = x mg/m³” at 25 °C and 760 mmHg, and every one of the eighteen compounds in the table below reproduces from its molar mass divided by 24.45 to the precision NIOSH prints — which is the check that both the molar masses and the 24.45 are being used the way NIOSH uses them.
Commission on Isotopic Abundances and Atomic Weights (IUPAC). Abridged Standard Atomic Weights, ciaaw.org. Every molar mass on this page is computed from this table — H 1.0080, C 12.011, N 14.007, O 15.999, F 18.998, S 32.06, Cl 35.45, Ca 40.078 — and none is copied from a conversion chart.
Health and Safety Executive. EH40/2005 Workplace exposure limits, paragraph 68. Gives the British and European conversion explicitly: WEL in mg·m^-3 = WEL in ppm × molecular weight ÷ 24.055 26, “at 20 ºC and 1 atmosphere pressure (760 mm mercury, 101325 Pa, 1.01325 bar)”. Note the pressure: 101 325 Pa, not the 101.3 kPa the directives write.
Commission Directive 2000/39/EC establishing a first list of indicative occupational exposure limit values, Annex, footnote 6: “mg/m³: milligrams per cubic metre of air at 20 °C and 101,3 KPa.” This is the wording every later EU OEL directive repeats, and it is where the 20 °C reference condition comes from.
Canadian Centre for Occupational Health and Safety. Converting Occupational Exposure Limits from mg/m³ to ppm, ccohs.ca, reproducing the conversion in the ACGIH TLV booklet. States the North American reference condition: 25 °C, 760 torr, molar volume 24.45 L/mol.
Bureau International des Poids et Mesures. The International System of Units (SI), 9th edition 2019. Since the 2019 redefinition the molar gas constant is exactly 8.314 462 618… J mol^-1 K^-1 and the molar volume of an ideal gas at 0 °C and 101.325 kPa is 22.413 969 54… L/mol. Every molar volume on this page is RT/P computed from that constant, which is why the page can say where the published 24.45 came from.