Study Objectives: Dehydration in children is a common diagnosis in US emergency departments (ED) with 179 million cases of acute gastroenteritis occurring annually, resulting in some 600,000 hospitalizations and an estimated 5,000 deaths. Current methods for identifying hypovolemia in children all have poor sensitivity and specificity. In adults, point-of-care Ultrasonography (POCUS) has emerged as an objective, noninvasive, rapid, inexpensive and validated tool to assess a patient's intravascular volume status. However, in pediatrics there is mixed support of POCUS to assess intravascular volume. The primary objective of this study was to assess collapsibility index (CI) as measured by respiratory variation to establish reference values for the inferior vena cava (IVC) and aorta (IVC/Ao Ratio) in longitudinal and transverse views by age, weight, height and Broselow-Luten color groups. A secondary objective was to determine which method of sonographic measurement was more reliable. Methods: This was a single-center, prospective, convenience study of patients who presented to the ED between January-December 2016 for complaints other than dehydration. Patients 4 months to 8 years of age, whose parent/legal guardian provided informed consent, were enrolled. Patients were recruited based on their color grouping on the Broselow-Luten tape, with a goal of at least 28 patients per group. Ultrasound examinations measured the IVC diameter in both a long and transverse axis during one respiratory cycle and the transverse aorta was measured at its maximal diameter. Results: There were 255 patients enrolled with fifteen excluded for inadequate data, leaving 240 for analysis. Fifty percent were female, most were African American (AA, 86%) followed by White (W, 7%), which was higher than our ED population of (AA, 65%; W, 30%), mean age was 2.67 years. Distribution over the color zones: (14) Pink, (28) Red, (33) Purple, (35) Yellow, (42) White, (31) Blue, (34), Orange, (23) Green. Goals for enrollment were met or exceeded in all groups, expect pink and green. Summary statistics of sonographic measurement as per age, height and weight were done and the bi-variate correlations between IVC long min-max, IVC Cross min-max, and aorta min-max with the set of variables age, height, weight and color zone, were all moderate or strong positive correlations (Pearson's correlation coefficients ranging from 0.56 to 0.75, all p<.001). Conclusions: The IVC and aorta longitudinal and transverse measurements show a positive correlation with age, weight, height and Broselow-Luten color group, indicating that as the patient grows the IVC and aorta increase in a predictable manner. The strongest association was with the Broselow-Luten color groups. Transverse measurements of the IVC and aorta proved more reliable, especially in the smaller-sized children due trunk movement with normal respiration in the long axis plane.