Which formula is used to calculate IV infusion rate?

• A. Flow rate (mL/hr) = Time (hr) / Volume to be infused (mL)
• B. Flow rate (mL/hr) = Volume to be infused (mL) / Time (hr)
• C. Flow rate (mL/hr) = Volume infused (mL) × Time (hr)
• D. Flow rate (mL/hr) = Time (hr) × Volume to be infused (mL)

Answer: (B)

Infusion rate is the speed at which fluid is delivered, expressed as milliliters per hour. To determine it, take the total volume to be infused and divide it by the time over which it should be infused. This yields flow rate in mL/hr, since mL divided by hr gives mL/hr. For example, delivering 1000 mL over 8 hours gives 1000/8 = 125 mL/hr. If you used time divided by volume, you’d get hours per milliliter, not a rate in mL/hr; multiplying volume by time would produce a volume–time unit, not a rate. So the correct approach is to divide the volume to be infused by the time.