The oldest method to do drug dosage calculations is the Ratio and Proportion formula.  It is set up like a algebraic equation where you have to solve for the unknown or X.  In this lesson the lower case "x" means multiplication. 

The formula is

H  :  V  ::  D  :  X    You will multiply V times D and H times the unknown X and then solve for X

H is the drug on hand or available from the pharmacy

V is the vehicle or drug form (tablet, capsule, liquid)

D is the desired dose that is ordered

X is the unknown or the amount of the drug that you have to give

Another way to work this problem is to set up a Fractional equation and cross multiply

 V x D = H x X

Let's work an easy problem

Amoxicillin (Amoxil) 100 mg PO q. i. d. is ordered

What is on hand from the pharmacy is Amoxil 250 mg in 5 ml

using the formula

H is the drug on hand is Amoxil 250 mg 

V is the vehicle or drug form is in 5 ml of a liquid

D is the desired dose that is ordered is Amoxil 100 mg

X is the unknown or the amount of the drug that you have to give

Think about this the drug to give (100 ml of Amoxil) is a lesser amount than the 250 mg Amoxil in 5 ml, therefore you answer will be less than 5 ml.

V      x   D           =   H         x  X  

5 ml   x  100 mg   =  250 mg x  X   

Multiply V x D = H x  X    and solve for X

5 x 100 = 250 X 

500      = 250 X

500      =  250 X       Divide both sides of the equation by 250

250          250           to get the unknown "X" to one side of the equation.   

500   ₌     X

250

      2  =  X 

or  2 ml of the liquid will provide 100 mg of Amoxil

Now let's work a harder problem

The order is for Aspirin/ASA 10 grains every 4 hours as needed for fever

What is available from the pharmacy is Aspirin/ASA 325 mg tablets

Grains and milligrams are not the same measures, therefore a conversion of grains to milligrams is necessary because what is available from the pharmacy is in milligrams,  Before you can proceed with the calculation, grains will need to be converted to milligrams.

From the conversions lesson you learned that 1 grain equal 65 mg

To convert grains to milligrams you will need to set up a fractional equation.

If 1 grain equals 65 mg then 10 grains equals X

1 grain : 65 mg :: 10 grains : X

65 x 10   =  1 x X 

      650   =  X

Now that you know that 10 grains equal 650 mg you can now solve the problem

Remember the formula

V x D = H x X

H is the drug on hand or available from the pharmacy or 325 mg ASA

V is the vehicle or drug form (tablet, capsule, liquid) 1 tablet

D is the desired dose that is ordered or 10 grains which is 650 mg ASA

X is amount of the drug that you have to give or the unknown 

Cross multiply

1 x 650 = 325 x X

1 x 650 =  325 X

650    =    325 X          Remember to get the unknown to one side of the equation

325          325               you must divide both sides by 325        

   650    =      X

   325          

   2 tablets  =   X

To give the correct dose of 650 you will give two tablets of 325 mg Aspirin/ASA

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