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Drug Calculations for Injections and Intravenous Infusion Rates

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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)

Equation: Dose = SunRise/Sunset x volume = StockRequired (12.5) / StockSupplied (50) x volume (2mls) = 0.5mls

 

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

putting medications into IV flask

 

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

 

 

 

 

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Jillean Martyn : last updated October 2009BACK TO TOPHOME