Background: Tumor volume in Wilms tumor (WT) guides risk stratification under the International Society of Pediatric Oncology (SIOP) protocol. While the radiological ellipsoid formula is the current protocol-specified reference method, modalities such as planimetry, intraoperative specimen weight, and pathological volume are often used interchangeably in practice without systematic evaluation. Methods: We reviewed 60 pediatric WT patients. Three measurements were compared against the SIOP-standard radiological ellipsoid volume: radiological planimetry, intraoperative specimen weight, and pathological tumor volume. Pearson and Spearman correlations and Bland–Altman analysis were performed. Results: Radiological planimetry showed near-perfect correlation with the SIOP ellipsoid (Pearson r = 0.993, p < 0.001; mean bias +21.1 cm³, 95% limits of agreement −82.8 to +124.9 cm³, percentage bias +6.1%). Intraoperative specimen weight (r = 0.730) and pathological tumor volume (r = 0.594) showed moderate-to-good correlations but wide limits of agreement and systematic biases of +46.6% and −34.1%, respectively. At the critical 500 cm³ treatment threshold, ellipsoid and planimetry showed 100% concordance. However, using pathology volume would have produced discordant (lower) risk categorization for 47% of ellipsoid-defined high-risk patients, a discordance whose clinical treatment implications were not directly assessed in this study. Conversely, relying on intraoperative weight would have upstaged 9.1% of patients to a higher risk category, which could correspond to a more intensive treatment category, including doxorubicin, under the SIOP protocol; whether this would translate into unnecessary treatment in practice was not directly assessed in this study. Conclusions: Radiological planimetry is a highly reliable surrogate for the SIOP ellipsoid formula across all stages and histologies. Intraoperative weight and pathological volume exhibit significant systematic biases and should not be used to revise radiological risk groups. These findings support continued anchoring of SIOP risk stratification to preoperative radiological volumetry, consistent with the currently validated protocol threshold, to avoid unintended deviations in treatment intensity.