In the present paper, stress intensity factors of cracks emanating from side notches of rectangular plates in tension are calculated by the method of the superposition of analytic and finite-element solutions. Empirical formulas are proposed for stress intensity factors on the basis of these numerical results and physical considerations. In the case of shallow cracks, the stress intensity factor is significantly influenced by notches, and the method for estimating stress intensity factors is discussed in detail. It is clarified that if the crack is longer than a certain length, the notch effect on stress intensity factors becomes insignificant. The empirical formulas proposed herein can be effectively employed for fatigue crack propagation tests.