A new formula for predicting the average crack widths in reinforced and partially prestressed concrete beams has been developed incorporating four governing parameters. The performance of the proposed formula is checked using the authors' test results which includes crack spacing and crack width measurements from 18 reinforced and 12 partially prestressed concrete beams. Also included in the comparison are published data on 76 beams from other laboratory investigations. The comparative study indicates that the predictions are accurate. The performance of the proposed formula is also compared with three major code formulas, viz those recommended in the ACI Building Code, the British Standard and the Eurocode. It is concluded that better correlation with test data is achieved by the proposed formula.