The Planck constant and its units

•Cycle or rad angle units should be included in expressing physical values.•In particular, the derived SI unit hertz (Hz) should be defined as Hz = cycle/s.•Quantity calculus yields h = ℏ for Planck’s constant and ν=ω for frequencies.•Recommended units for h, ℏ, ν, and ω are J s/cycle, J s/rad, cycle/s or rad/s.•We urge that the SI system be extended to allow for explicit angular units. [Display omitted] We urge spectroscopists to use the units cycle s−1 or radian s−1, with an appropriate prefix such as kilo as needed, when expressing the value of a transition frequency; the hertz (Hz) should be explicitly defined as being cycle s−1. Currently, in The International System of Units (SI), the unit of angular measure (cycle or radian) is omitted and transition frequencies simply have the unit s−1 which is called the hertz. The use of the symbols ν (or f) and ω is supposed to carry the information as to which angular unit obtains. The omission of the unit of angular measure can lead to an error of 2π. In using the rules of quantity calculus to express the value of a transition frequency, and of the Planck constant, it is necessary to include the unit of angular measure. One result of doing this shows that h and ℏ each represent the value of the Planck constant, but with the units J s cycle−1 and J s radian−1, respectively.