This paper describes an in-fiber standardization method by Solid Phase Microextraction (SPME) for the determination of a sample volume. After loading a specific amount of standard, the volumetric standard, on a PDMS-coated fiber (n₀), the fiber was exposed in the headspace of sample vials containing different volumes of water. The amount of standard that remained on the fiber after equilibrium (n_f), which was determined with gas chromatography/mass spectrometry (GC/MS), depends on the volume of the sample. Naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, 1-ethylnaphthalene, and 2-ethylnaphthalene were chosen as volumetric standards based on theoretical calculations. The effect of loading time, exposure time, and exposure temperature were investigated. The effect of the matrix was also studied, analyzing both water and wine samples. Precision and accuracy of the method were obtained for each standard in both water and wine. The partition coefficients of the compounds between the fiber and the sample (K_fs) and between the headspace and the sample (K_hs) were obtained by plotting n₀/n_fversus sample volume.