FFT Resolution Bandwidth Calculator

This tool calculates the Resolution Bandwidth of a Fast Fourier Transform. It also includes the effect of decimation or further downsampling of the digitized signal.


  • NFFT = Length of the FFT
  • Fs = Sampling rate (Samples-per-second or kSPS/MSPS/GSPS)
  • D = Decimation (=1 when the signal is not down-sampled)

FFT Resolution Formula

R = NFFT/(D*Fs)

Example Calculation

For a sampling rate of 1 GSPS and FFT length 4096, the resolution bandwidth is 244 kHz. If the decimation factor is 16, the RBW drops to 15.2 kHz.


The resolution bandwidth is the spacing between consecutive bins in the FFT output spectrum. The smaller the RBW, the easier it is to discern two closely spaced signals.

For a high sampling rate, a larger number of samples (FFT size) are required to get the same RBW as for a lower sampling rate.

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