OBJECTIVE : to develop a point-of-care ultrasound-derived formula to estimate urinary bladder volume noninvasively in dogs and cats. DESIGN : Prospective case series. SETTING : Private 24-hour veterinary emergency center. ANIMALS : Client-owned dogs and cats requiring urethral catheterization. INTERVENTIONS : Ultrasound measurements of length, width, and height of balloons filled with known water volumes were used to develop a formula to estimate urinary bladder volume using linear regression. The formula was then applied to point-of-care ultrasound-derived cysto-colic view measurements, and calculations were compared to total aspirated urine volume. MEASUREMENTS AND MAIN RESULTS : Fifteen balloons with known volumes (median, 126 mL [range, 27–689 mL]) were used to identify length × width × height (cm) × 0.2 × π as the best formula to estimate urinary bladder volume in milliliters. Fourteen cats and 14 dogs were used for comparison of formula-derived volume estimate to actual urinary bladder volume. Median aspirated urine volume, bias (formula-derived minus actual aspirated), and percentage difference were 80 mL, –4.1 mL, and –6.6% for cats, respectively. For dogs, the results were 78 mL, 3.4 mL, and 3.6%, respectively. CONCLUSIONS AND CLINICAL RELEVANCE : The point-of-care ultrasound-derived formula may be useful to estimate urine volume noninvasively in dogs and cats.