Stacking fault energy (SFE) is considered an important parameter to predict the prevalent plastic deformation mechanism in face-centered cubic (fcc) alloys. Experimental methods for determining SFE presuppose that SFE is positive. Density functional theory (DFT) is a promising tool to predict alloy compositions with low SFEs and desirable mechanical properties. For metastable fcc alloys, DFT predicts negative SFE values, which cannot be validated by the existing experimental procedures. In this contribution, it is demonstrated that experimental procedures to assess SFE values only provide an apparent value that needs correction. The suggested correction relies on the critical resolved shear stress for twinning, which is grain size-dependent, just like the apparent SFE. The correction provides SFE values that are independent of grain size. Accordingly, negative SFEs predicted by DFT can be experimentally validated.