USING A MATHEMATICAL FORMULA TO CALCULATE DRUG DOSES FOR LIQUID MEDICATIONS AND INTRAVENOUS (IV)
INFUSION RATES
Liquid Medications (oral syrups or injection): Calculating volume dose
Formula: Strength required x Volume
Strength in stock 1
Drug Prescription : IM chlorpromazine for a 25kg child if the desired dose was 0.5mg per kg?
Chlorpromazine injection comes in vials that hold 50mg of the drug in 2mL of solution. The dose for children is 0.5mg for each kg of body weight. How much solution must be drawn up into a syringe for a child who weighs 25kg? 
Remember the acronym of SR (Sun Rise)
= Stock Required and SS (Sun Set) = Stock Supplied
Stock Required (in this case = 12.5 mg since it is 25 kg × 0.5 mg)
Stock Supplied (in this case 50 mg in the ampoule)
To calculate hourly rate of IV therapy
Formula: Total Volume to be infused (mls) = mls per hour
Time in Hours
Example: Calculate the hourly rate: 1 litre over 8 hours
1000 mls = 125 mls per hour : this is the rate that is set on the infusion pump
8 hrs 
To calculate drops per minute if the infusion is to be administered without a volumetric pump
Formula: Total Volume (ml) x drop factor = drops per minute
Total Time in Minutes
The drop factor will always be stated. The drop factor is the number of drops per ml
that
the equipment delivers.
The most common set delivers 20 drops per ml
Calculate the flow rate in drops per minute with a drop factor of 20
1 litre over 8 hours
1000 x 20
8 x 60
= 41.6
= 42 drops per minute 
Example: 500ml of 5%dextrose solution is to be given intravenously. The infusion giving set has a drop
factor of 20 drops per ml and you are instructed to ensure that the patient receives this volume in 12
hours. What should the drop rate (drops per minute) be?
Answer:
Drop rate =
1000 x 20 = 13.89 drops per minute
12 x 60
Rounding off, the answer would be
14 drops per minute
