Converting VOC Limits: ppm, mg/m³ and g/h, With a Table for 16 Common Solvents

By the NextAir Systems engineering team. Last reviewed October 2026.

A permit says 50 mg/Nm³. A gas detector says 12 ppm. A supplier’s brochure says 500 to 2,000 ppm inlet. All three can describe the same exhaust, and the arithmetic to move between them is simple, but only if you know which temperature the milligrams are measured at.

This page gives the formulas, a conversion table for 16 solvents we see in paint, printing and chemical exhausts, and the traps that cause factor-of-ten mistakes.

The short version

• mg/m³ = ppm × molecular weight ÷ molar volume. The molar volume is 22.414 L/mol at 0 °C and 24.465 L/mol at 25 °C, both at 101.325 kPa.
• The same permit limit in ppm differs by about 9% between a 0 °C and a 25 °C reference. Always check the stated reference condition.
• Mass emission rate (g/h) = concentration (mg/m³) × airflow (m³/h) ÷ 1,000. Use the airflow at the same reference condition as the concentration.
• EPA’s range of 500 to 2,000 ppm for carbon adsorber inlet streams equals about 1,880 to 7,530 mg/m³ for toluene at 25 °C.^[1]

The formulas

For an ideal gas, one mole occupies 22.414 litres at 0 °C and 101.325 kPa, and 24.465 litres at 25 °C and the same pressure. A concentration in parts per million by volume converts to milligrams per cubic metre like this:

• mg/m³ = ppm × MW ÷ 24.465 (at 25 °C)
• mg/m³ = ppm × MW ÷ 22.414 (at 0 °C, the common basis for European permits and normal cubic metres)
• ppm = mg/m³ × molar volume ÷ MW

MW is the molecular weight in g/mol. For another temperature, scale the molar volume by T/273.15, using absolute temperature in kelvin.

The table

Each row gives the molecular weight, the mg/m³ equivalent of 1 ppm at both reference temperatures, what a 50 mg/m³ limit means in ppm on the 0 °C basis, and what a 100 ppm reading means in mg/m³ at 25 °C.

Solvent	MW (g/mol)	1 ppm = mg/m³ at 25 °C	1 ppm = mg/m³ at 0 °C	50 mg/m³ = ppm (0 °C basis)	100 ppm = mg/m³ (25 °C basis)
Methanol	32.04	1.31	1.43	35.0	131
Ethanol	46.07	1.88	2.06	24.3	188
Acetone	58.08	2.37	2.59	19.3	237
Isopropanol	60.10	2.46	2.68	18.6	246
Methyl ethyl ketone (MEK)	72.11	2.95	3.22	15.5	295
n-Butanol	74.12	3.03	3.31	15.1	303
Benzene	78.11	3.19	3.48	14.3	319
Cyclohexane	84.16	3.44	3.75	13.3	344
Dichloromethane	84.93	3.47	3.79	13.2	347
n-Hexane	86.18	3.52	3.84	13.0	352
Ethyl acetate	88.11	3.60	3.93	12.7	360
Toluene	92.14	3.77	4.11	12.2	377
Methyl isobutyl ketone (MIBK)	100.16	4.09	4.47	11.2	409
Styrene	104.15	4.26	4.65	10.8	426
Xylene (mixed isomers)	106.17	4.34	4.74	10.6	434
Butyl acetate	116.16	4.75	5.18	9.6	475

A light solvent such as methanol is a large number of ppm for a small mass. A heavy one such as butyl acetate is the reverse. This is why a permit written in mg/m³ treats solvents more evenly than one written in ppm, and why a detector calibrated to one solvent can misread another.

A worked example

Worked example. A coating line vents 8,000 m³/h and the permit allows 50 mg/Nm³ of toluene. On the 0 °C basis that is 50 × 22.414 ÷ 92.14 = 12.2 ppm. The mass limit is 50 × 8,000 ÷ 1,000 = 400 g/h. If the stream arrives at 120 ppm, that is 493 mg/Nm³, so the abatement must remove 90% to comply. EPA reports 95 to 99% removal from well-run carbon adsorbers, and above 98% for dilute streams, so a 90% requirement leaves comfortable margin.^[1]

Percent by volume and the flammable range

One percent by volume is 10,000 ppm. The 500 to 2,000 ppm that EPA cites for carbon adsorber inlets is therefore 0.05 to 0.2% by volume.^[1] That is a small fraction of the concentration at which solvent vapor in air can burn, but process exhausts can climb. EPA’s adsorber inspection material lists keeping the inlet concentration at or below 25% of the lower explosive limit among the checks for a carbon adsorber.^[2] Keep both scales in mind: the permit is in mg/m³ or ppm, the safety case is in percent of the lower explosive limit, and they are linked by the same molar-volume arithmetic as above.

Traps that cause factor errors

• Reference temperature. Normal cubic metres are usually at 0 °C. A stack measurement at 80 °C is a different number of actual cubic metres, and the airflow must be converted the same way as the concentration.
• Wet or dry basis. Many European limits are stated on a dry gas basis. A humid stream has fewer VOC molecules per cubic metre of actual gas than a dry one.
• Oxygen correction. Combustion processes correct to a stated oxygen percentage. Adsorbers on ordinary air normally do not, but check what your permit says.
• Total versus individual. Total VOC as carbon (mg C/m³) is a different unit from a compound’s own mass concentration. Convert with the carbon fraction of the molecule.
• Parts per million by volume or by mass. Gas ppm is nearly always by volume. If a figure came from a liquid analysis it may be by mass.

If you are converting a permit limit to the inlet concentration an adsorber will see, pair this table with our guide to sizing the bed and the bed life calculation, which both start from mass per hour.

Where this fits with NextAir

Send our dry VOC adsorber manufacturer team your exhaust airflow, the compounds and their measured concentration, and the permit limit with its reference condition, and we will confirm the required removal and the bed size. Our dry VOC adsorber overview explains how the technology works, and the chemical and pharmaceutical page covers streams with several compounds.

Sources

1. US EPA, Office of Air Quality Planning and Standards. EPA Air Pollution Control Cost Manual, Chapter 1: Carbon Adsorbers (7th edition).
2. EPA / Air Pollution Training Institute (airknowledge.gov). PERM140 course slides: VOC controls.

Figures and tables on this page are calculated or compiled by NextAir Systems and you are welcome to reuse them with a link back to this article.

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