scaling factor for CUFFT

CUFFT has the same behavior as FFTW, it computes unnormalized FFTs. IFFT(FFT(A))=n A where n is the length of the vector. The length n is in number of samples (not floats or bytes). There are some padding differences between FFTW and CUFFT with C2R and R2C that can screw up a simple comparison, but not for C2C. I would double-check your data setup and length calculations, and verify your plan in both FFTW and CUFFT.