What the R-numbers mean
The rule for single compounds
For halocarbons the designation is built from the molecular formula. Reading the number from the right: the last digit is the number of fluorine atoms, the next is the number of hydrogen atoms plus one, and the next is the number of carbon atoms minus one. A leading zero is dropped.
Take R-22. Written in full it is R-022. Two fluorines, one hydrogen (2 minus 1), one carbon (0 plus 1). That gives CHClF₂, and the remaining bond is filled by chlorine. The chlorine is why R-22 depletes ozone, and the scheme tells you it is there without naming it.
R-134a: four fluorines, two hydrogens (3 minus 1), two carbons (1 plus 1), giving C₂H₂F₄. The trailing letter distinguishes isomers, molecules with the same atoms arranged differently. R-134 and R-134a are not the same substance.
What the first digit tells you at a glance
Any number below 100 is a single compound in the methane, ethane or propane series. The higher series are categories rather than formulas.
400 series: zeotropic blends. Components with different boiling points, so the mixture glides. R-404A, R-407C, R-410A, R-448A. Numbers are allocated in order of registration, so R-407 and R-410 tell you nothing about composition. The letter suffix distinguishes different proportions of the same components: R-407A, R-407C and R-407F all contain R-32, R-125 and R-134a, mixed differently. They are not interchangeable.
500 series: azeotropic blends. Mixtures that behave as a single fluid, with no glide. R-508B is the one we stock, an R-23 and R-116 azeotrope for ultra-low-temperature duty.
600 series: organic compounds that are not halocarbons. R-600a is isobutane, the hydrocarbon in most domestic refrigerators.
700 series: inorganic compounds, numbered by molecular weight. R-717 is ammonia, molecular weight 17. R-744 is carbon dioxide, molecular weight 44. R-718 is water.
The prefixes
You will see the same fluid written several ways. R-32, HFC-32 and difluoromethane are one substance. The prefix names the chemical family rather than changing the identification: CFC, HCFC, HFC, HFO and HC for hydrocarbon.
Safety data sheets and regulations tend to use the family prefix because the regulatory treatment follows the family. The trade uses the R-number. See the gas families guide.
Why this is worth knowing
Two practical reasons. First, the letter suffix is not decoration, and treating R-407A and R-407C as the same thing puts the wrong fluid in a machine. Second, the series number tells you immediately whether you are handling something that glides and must be liquid-charged.
A 400-series number is a warning label. It means blend, it means glide, and it means do not vapour charge.
Quick answers
What does the R in R-22 stand for?
Refrigerant. The number that follows is a code derived from the molecule under the ASHRAE Standard 34 designation system.
What does the letter after a refrigerant number mean?
For blends it distinguishes different proportions of the same components, as in R-407A and R-407C. For single compounds it distinguishes isomers, as in R-134 and R-134a.
What is the difference between the 400 and 500 series?
The 400 series is zeotropic blends, whose components boil at different temperatures so the mixture glides. The 500 series is azeotropic blends, which behave as a single fluid.
Why is ammonia called R-717?
The 700 series covers inorganic compounds, numbered by molecular weight. Ammonia has a molecular weight of 17, so it is R-717.
- ASHRAE Standard 34 · number designation and safety classification of refrigerants · ashrae.org.
- AHRI Standard 700 · refrigerant specifications and blend designation · ahrinet.org.
- UNEP Ozone Secretariat · Montreal Protocol annexes and substance families · ozone.unep.org.
This guide is general information compiled from the cited sources — not legal, safety or engineering advice. Read our full disclaimer.
Related: Refrigerant glossary · CFC, HCFC, HFC and HFO · Blend compositions · Zeotropic vs azeotropic