IV drip rate calculations: the 3 formulas you need
IV therapy has 3 core calculations. Getting any one of them wrong can under-treat a patient or cause fluid overload. Here's each one, plain.
mL/hr for volumetric pumps
Rate (mL/hr) = Volume (mL) / Time (hours). 1000mL over 8 hours = 125 mL/hr. Program that number into the pump. This is the most common calculation in clinical practice and the basis for all the others.
Drops per minute for gravity sets
Drops/min = (Volume x Drop factor) / Time in minutes. The drop factor is on the giving set packaging. Standard macrodrip = 20 gtt/mL. Blood sets = 15 gtt/mL. Paediatric microdrip burettes = 60 gtt/mL. Getting the drop factor wrong changes the rate by 33-300%. Always check the packaging.
500mL over 4 hours with a 20 gtt/mL set: (500 x 20) / 240 = 41.7, so 42 drops per minute. Count drops for 15 seconds and multiply by 4 to verify.
mcg/kg/min for critical care concentration infusions
Rate (mL/hr) = (Dose in mcg/kg/min x Weight in kg x 60) / Concentration in mcg/mL. The concentration comes from the drug amount added to the bag divided by the bag volume. Dopamine 400mg in 250mL = 1600mcg/mL. At 5mcg/kg/min for a 70kg patient: (5 x 70 x 60) / 1600 = 13.1 mL/hr.
Always double-check the preparation concentration before entering it. A 10-fold error here becomes a 10-fold error in the patient's dose. For patients also needing dose adjustment from kidney disease, use the Renal Dose Adjustment Calculator before finalising the rate. For dose calculations before IV administration, use the Weight-Based Dosage Calculator.
Infusion time from volume and rate
Time (hours) = Volume (mL) / Rate (mL/hr). Useful for checking how long a bag will last before the next bag needs to be prepared. 500mL at 83 mL/hr runs for 6 hours. Worth knowing before the night shift starts.