The units used for rotational constants depend on the type of measurement. With infrared spectra in the wavenumber scale (), the unit is usually the inverse centimeter, written as cm, which is literally the number of waves in one centimeter, or the reciprocal of the wavelength in centimeters (). On the other hand, for microwave spectra in the frequency scale (), the unit is usually the gigahertz. The relationship between these two units is derived from the expression
where ν is a frequency, λ is a wavelength and c is the velocity of light. It follows that 
As 1 GHz = 10 Hz, the numerical conversion can be expressed as