In this work, stress concentration factors (SCFs) Kt of a flat bar with double circular-arc notches or double V-shaped notches are considered on the basis of exact solutions now available for special cases and accurate numerical results. First, for the limitting cases of deep and shallow notches, the body force method is used to calculate the SCFs ; then the formulas are obtained as Ktd and Kts. On the one hand, upon comparison of Kt and Ktd, it is found that Kt is nearly equal to Ktd if the notch is deep or blunt. On the other hand, if the notch is sharp or shallow, Kt is mainly controlled by Kts and the notch depth. The notch shape is classified into several groups according as the notch radius and depth ; then, the least squares method is applied for calculation of Kt/Ktd and Kt/Kts. Finally, a set of convenient formulas useful for any shape of notch in a flat test specimen are proposed. The formulas yield SCFs With less than 1% error for any shape of notch.