Background: Accurate electric‐field modeling for transcranial direct‐current stimulation (tDCS) relies on tissue segmentation, and is especially sensitive to how cerebrospinal fluid (CSF) is labelled. We assessed the impact of two CSF segmentation approaches on tDCS simulation results. Methods: T1‐weighted MRIs from 47 healthy adults (26–69 years) were processed with (i) the default CHARM pipeline and (ii) the same pipeline but substituting its CSF mask with one from a commercial pipeline optimized for grey and white matter volumetry. Using SimNIBS 4.1, we ran a 1 mA frontal montage (anode F3, cathode F4, 5×5 cm electrodes, 4 mm thick). From cortical grey matter, the E 99.9 \% peak (E99) and the volume experiencing ≥ 75 \% of that peak were extracted. Paired differences failed normality (Shapiro–Wilk), so we applied the Wilcoxon signed‐rank test and report p-values and effect sizes (r). Results: The second segmentation resulted in larger CSF volume. E99 changed minimally, from 0.236 ± 0.058 V/m (CHARM) to 0.224 ± 0.041 V/m (commercial software), median Δ=-0.004 V/m (p=0.071, r=0.29). In contrast, focality was more strongly affected: the volume of cortex experiencing ≥ 75 \% of the peak decreased from 10.9 ± 2.3 cm³ to 6.43 ± 2.34 cm³ (median Δ=-4.16 cm³ , ≈ 39, p = 3.6 ×10⁻¹⁴) with a large effect size (r = 0.87). Conclusion: As expected, the segmentation with the larger CSF volume, results in a larger spread of the electrical current. This results in lower focality, whereas the strength of the electrical field remains similar.