Background: Hypothermia is a common complication in the hypovolemic patient. Warm intravenous fluids have proven valuable at preventing this complication during volume replacement. The microwave oven is considered an applicable alternative method for warming fluids but no protocol has been established. Objective: To evaluate the efficacy and affected variables of the microwave oven in warming crystalloid fluids and to determine the appropriate formula for calculating the warming duration. Results: The important variables influencing the operation of the microwave oven include the difference between the crystalloid fluid and room temperature, the microwave oven's capability, variations in microwave irradiation, and fluid shaking. The appropriate formula for calculating warming duration is: Duration (sec) = Volume (cc) x 4.2j.g -1.K-1 x Raised temperature ΔT (K) x 1.1 (Adjusted power)/Microwave power (W) Conclusion: The microwave oven is a safe and practical method for warming crystalloid fluids.