In this work stress concentration factors K_t for a flat bar with fillets are considered on the basis of exact solutions now available for special cases and accurate numerical results. Then, a convenient K_t formula useful for any dimensions of the fillet is proposed. The conclusions can be summarized as follows.

1. For the limiting cases of deep (d) and shallow (s) fillets, the body force method is used to calculate the K_t values. Then, the formulas are obtained as K_td and K_ts.

2. On the one hand, upon comparison of K_t and K_td, it is found that K_t is nearly equal to K_td if the fillet is deep or blunt.

3. On the other hand, if the fillet is sharp or shallow, K_t is controlled mainly by K_ts and the fillet depth.

4. The fillet shape is classified into several groups according to the fillet radius and fillet depth. Then, the least-squares method is applied for calculation of K_t/K_td and K_t/K_ts.

5. Finally, convenient formulas are proposed that are useful for any dimensions of fillet in a flat bar. The formulas give SCFs with less than about 1 % error in most cases for any dimensions of fillet under tension and bending.