Frequency bin (bin index)

One output value $X_k$ of a DFT, indexed by a slot number $k$ , not a frequency in hertz on its own.

Each output $X_k$ of an $N$ -point DFT is called the $k$ -th frequency bin; $k$ itself is the bin index, a slot number, not a frequency in hertz on its own. Converting a bin index $k$ to an actual frequency requires the sampling rate $f_s$ that produced the samples: bin $k$ corresponds to $kf_s/N$ Hz.

Examples

For $N=100$ samples taken at $f_s=1000$ Hz, bin $k=5$ corresponds to the actual frequency $5\times1000/100=50$ Hz.