Monday 9 April 2012

Formulas for Drug Calculations

Calculating Tablets

How much tablets should I give?


Formula:            Dose to be given
       ----------------------------  =  Number of tablets
       Stock strength / Stock dose

  • Dose to be given = amount of tablets you need …e.g. doctor prescribed 1g of the tablet to be administered.
  • Stock strength / Stock dose = strength of one tablet …e.g. each tablet contains 500mg of the drug.
  • Convert to same unit of weight… 1g = 1000mg
  • Calculate ! 1000mg ÷ 500mg = 2 tablets


Calculating Mixtures, Solutions, Fluid for injection

How much medication should I give?

Formula:

        Dose to be given                Stock Volume
----------------------------  X  ---------------  =  Amount of solution to be given
Stock strength / Stock dose                   1

  • Dose to be given = amount of medication you need …e.g. doctor prescribed 2g to be administered.
  • Check the “strength per volume” of the medication …e.g. 500mg/10mls
  • Stock strength / Stock dose = strength of the medication …500mg
  • Stock volume = the amount of medication containing that strength …10mls
  • Convert to same unit of weight… 2g = 2000mg
  • Calculate! (2000mg ÷ 500mg) X (10mls)
  • = 4 X 10
  • = 40mls
  

Calculating IV rates: mls per hours (& minute)

How many mls per hour do I set?

Formula for hours:        Total volume to be given (mls)
---------------------------------  =  mls per hour
 Time (hours)

E.g. 1200mls ÷ 10hrs = 120mls/hr 


Formula for minutes:    Total volume to be given (mls)
---------------------------------  =  mls per minute
         Time (hours) X 60

E.g. 1200mls ÷ (10hrs X 60)
     =1200mls ÷ 600
     =2mls/min
     

 
Calculating IV rates: drops per minute (dpm)

How many drops of medication are given per minute?

 
Formula:
Total volume to be given (mls)        Drop factor
-------------------------------  X  ------------  =  mls per hour
            Time (mins)                               1



Calculating IV rates: time remaining

How much time is remaining in this infusion?

Formula:

Volume remaining in the bag (mls)       Drop factor
----------------------------------  X  ------------  =  mls per hour
       Drops per minute (mls/hr)                    1


*See administration set, usually 15, 20, 60 drops/ml

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