The ratio: \[f_{\text{r}}=\frac{A_{\text{r}}}{A_{\text{g}}}\] where Ar is the real (true, actual) surface (interface) area and Ag is the geometric surface (interface) area.
Source:
PAC, 1986, 58, 437. 'Interphases in systems of conducting phases (Recommendations 1985)' on page 439 (https://doi.org/10.1351/pac198658030437)