Objective: This study aimed to compare the pre-voiding bladder and post-voiding residual [BV, post-void residual (PVR)] volumes measured by the portable ultrasonic scanner (PUS) in standing and supine positions. Materials and Methods: This study included 436 children. Two groups were composed (group-1: PUS vs. volume by catheter and group-2: PUS vs. infused volume during the urodynamic study) to evaluate the agreement of PUS measurements with true bladder volume. Additionally, the third group (group-3) was created to analyze the correlation between PUS measurements in different positions. In groups 1 and 2, PUS measurement agreements were evaluated using the paired sample T or Wilcoxon signed-rank tests. Following the agreement, correlations were analyzed using Pearson’s or Spearman’s coefficients depending on whether variables were distributed normally or not, respectively. Coefficients were interpreted as 0.90-1.00 (very high correlation) and 0.70-0.90 (high correlation). Results: The catheter and PUS measurements were similar in group-1 (Wilcoxon signed-rank test, p=0.976) and were highly correlated (r=0.873). The measurements of volumes infused by urodynamic device and PUS were similar in group-2 that revealed the agreement of PUS measurements on different volumes and highly correlated at the 25th and very highly correlated at the 50th, 75th, and 100th percentiles of the estimated bladder capacity related to age. The BV and PVR measurements by PUS in standing and supine positions in group-3 were highly correlated, revealing that PUS can be used in both positions. Conclusion: Measurements of BV before uroflowmetry or PVR volume by PUS in standing position gave similar results with those in the supine position.