Dedicated hardware accelerators enable energy-efficient implementations of radio and imaging basebands. Multistandard, multi-mode radio basebands require an on-the-fly reconfigurable fast Fourier transform (FFT) accelerator that implements many different FFT sizes. An instance of a runtime-reconfigurable 2n3m5k FFT accelerator was generated by a custom hardware generator to meet the requirements of common wireless standards (Wi-Fi, LTE). The accelerator is integrated with a RISC-V processor, and the measured 16nm FinFET chip runs up to 940MHz and consumes 0.46 to 22.6mW of power when running FFT benchmarks for Wi-Fi and LTE symbol lengths.