A detailed investigation into the determination of the delta ferrite content in duplex and super duplex stainless weld metals was carried out. The ferrite number (FN) was predicted using the WRC-92 diagram and measured by magnetic instrument. The predicted and measured ferrite values were compared and the correlation between them was analysed. The data were also examined using the Q factor. It was found that, in general, the predicted and measured FN had a good agreement, but this was influenced by the weld metal nitrogen content. Tungsten content below 1% was found to have no detectable effect on the accuracy of prediction.