This paper describes the prediction of broadband self noise from ducted fans. The source mechanism is assumed to be the interaction of the turbulent boundary layer with the trailing edges of the fan blades. The source levels are obtained from the measurements of self noise from isolated blades by Brooks, Pope and Marcolini and these are corrected to give the in duct sound power from a high solidity fan. It has been found that the blade surface pressures are not uncorrelated on each fan blade and corrections must be included for the scattering from multiple trailing edges. A method is introduced for coupling the modes in a circular duct to the modes of a linear cascade so that the sound power is not singular at the mode cut on frequencies. Results show that the in duct sound power scales with the fifth power of the fan speed at low Mach numbers, but this changes to the sixth power or greater at high Mach numbers. The angle of attack of the blade increases the self noise as 2.4 dB per degree and there are significant increases in low frequency noise when blade stall occurs.