Tool
Unit converter: µg/m³ ↔ ppb
Converts a gas concentration between µg/m³ and ppb for a chosen substance, temperature and pressure.
- Factors Formaldehyde (CH₂O)BenzeneToluene (C₇H₈)Nitrogen dioxide (NO₂)Ozone (O₃)Carbon monoxide (CO)
Standards & bodies: World Health Organization (WHO)Methods: Exposure assessmentLow-cost sensors
Formula
c(µg/m³) ≈ φ(ppb) × M(g/mol) ÷ Vm(L/mol), where Vm = R·T ÷ p
Why two units exist for one measurement
A gas concentration can be written as how much of the substance is in a cubic metre of air (mass per volume, µg/m³ or mg/m³) or as how large a share of the air’s molecules it is (amount-of-substance fraction, ppb or ppm). The first depends on how tightly the air is packed and so on temperature and pressure; the second does not. Instruments, guidelines and product labels use both, often for the same substance, which is why a limit can look twice as strict as another when the two are identical.
The bridge between them is the molar volume: at 25 °C and 101.325 kPa a mole of any ideal gas occupies about 24.47 L, so one ppb of formaldehyde — molar mass 30.026 g/mol — is about 1.23 µg/m³. Warm the air and the same molecules spread through a larger volume, so the mass in a cubic metre falls while the fraction stays put.
Reading the result
The output is rounded to three significant figures and carries “≈”, because the conversion rests on the ideal-gas relation and on a temperature and pressure you have stated rather than measured. Report the unit you measured in and give the converted value as a conversion, never as a second measurement.
The abbreviations ppb and ppm are convenient but they are not units in the sense of the SI, and the IUPAC Green Book discourages them precisely because their meaning depends on whether the fraction is taken by volume, by mass or by amount of substance. On this site ppb always means the amount-of-substance fraction in air, which for a gas is what a volume fraction amounts to.
Assumptions
- The gas behaves as an ideal gas — accurate to better than 1 % for trace gases at indoor temperature and pressure.
- Temperature and pressure are those at which the concentration is reported, not necessarily those in the room.
- The substance is a single compound of known molar mass, so the conversion is defined.
Outside these assumptions the number is still arithmetic, but it stops describing the room.
What this tool does not do
- It cannot convert particulate matter. PM₂.₅ and PM₁₀ are mixtures of solid and liquid particles with no molar mass, so µg/m³ is the only meaningful unit for them; ppb has no definition here.
- It cannot convert a mixture reported as one number, such as total volatile organic compounds, unless a reference compound and its molar mass are stated.
- It does not tell you which unit a limit is written in — that is on the limits table, and the two are frequently different for the same substance.
- It does not know the conditions of your own measurement; a value converted at 25 °C and quoted as if measured at 0 °C is out by about 9 %.
Frequently asked questions
Why does the same limit appear as 0.1 mg/m³ in one place and 80 ppb in another?
Because they are two units for one quantity. The WHO short-term guideline for formaldehyde is 0.1 mg/m³, which at 25 °C and 101.325 kPa is about 81 ppb. Neither number is stricter than the other; they are the same air.
Which temperature should I use?
The one the source states. European product-emission work is usually reported at 23 °C, WHO guidelines and much of the North American literature at 25 °C, and reference-method reporting is sometimes normalised to 0 °C or 20 °C. The difference between 0 °C and 25 °C is about 9 %, which is smaller than the spread of most measurements but large enough to matter when a value sits on a limit.
Can I convert PM₂.₅ to ppb?
No. Particles are not a single substance and have no molar mass, so the conversion is undefined. Anything that offers it is producing a number without meaning.
Sources
- Quantities, Units and Symbols in Physical Chemistry (IUPAC Green Book), Third Edition (2007). IUPAC / RSC Publishing. doi:10.1039/9781847557889 CKB 1705
- Quantities, Units and Symbols in Physical Chemistry (IUPAC Green Book), 4th Edition (2023). IUPAC. CKB 1706
- International vocabulary of metrology - Basic and general concepts and associated terms (2012). BIPM / JCGM. doi:10.59161/JCGM200-2012 CKB 1704
- Bureau International des Poids et Mesures: The International System of Units (SI Brochure), 9th edition, 2019.
CKB = document number in the Coridum Knowledge Base, our curated library of peer-reviewed studies. Follow the DOI for the original paper.
Cite this page
Coridum Research. Unit converter: µg/m³ ↔ ppb. Coridum Knowledge T-0001, 6 September 2026. https://coridum.com/k/T-0001/