Hz is a unit of Frequency. dB or deciBel is used to express the ratio of two power values on a logarithmic scale.
Therefore in general, Hz cannot be converted to dB as there is no equivalence between frequency and amplitude. However the ratio of two frequencies can be expressed on a logarithmic scale as dB-Hz.
This tool converts Hertz (Hz) to deciBel-Hertz (dB-Hz)
Hz is a unit of frequency and is equivalent to cycles per second.
deciBel or dB represents a ratio of two values. Typically the two quantities are Amplitude or Power Levels. The units are the same and hence dB is simply a ratio.
A linear number can be converted to its dB equivalent using this calculator. the formula is PdB = 10*Log10(Plinear).
dB-Hz = 10*Log10(Hz)
dB has a logarithmic nature. The benefit of using dB is that a very large (or very small) ratio can be conveniently represented by a small number. For example, 1000000000 Hz = 90 dB-Hz.
100 Hz can be represented as 20 dB-Hz.
1000 Hz is equivalent to 30 dB-Hz.
 dB-Hz should not be confused with dBm/Hz which is the power spectral density. PSD can be converted to dBm.
 Carrier-to-Noise Density ratio also has the units dB-Hz. Carrier power is expressed in dBm while Noise Density has units dBm/Hz.
Interested in learning more about Electronics? We recommend checking out the Art of Electronics.