Drug orders may be written in one unit of measure while the pharmacy may supply the medication in a different unit of measure.  In order to administer the proper dose, the dosage order must be converted into the units of the supply.  Dimensional analysis or ratio and proportion may be used to calculate these conversions. 

Example 1:

Order reads Tylenol gr x for mild pain or fever

Supply: Tylenol 300 mg per tablet

Calculation using dimensional analysis:

Tabs = 1 tab 60 mg gr 10  = 2 tabs
  300 mg 1 gr  


Students often have difficulty converting grains (when measured in fraction) to milligrams.  Here are some tips on dealing with fractions...

To multiply fractions, multiply across the numerator (top numbers), then multiply across the denominator (bottom numbers, then divide the denominator into the numerator to change the fraction into a decimal.

To divide fractions, invert the divisor (the fraction you're dividing into the other fraction) and multiply.

Example 2:

Order reads: Nitroglycerine gr 1/150 sl prn for angina

Supply: Nitroglycerine 0.4 mg tablets

Solution using dimensional analysis:


Tabs =

1 tab 60 mg gr 1/150 = ? tabs
Denominator   0.4 mg (4/10 mg) 1 gr    


Numerator Tabs =  1 X 10 X 60 X 1 = 600 = 1 tablet
Denominator 4 1 150 = 600 

Example 3:

Order reads: Drug 'Q' gr 1/150 IM q3h prn

Supply: Drug 'Q' gr 1/200 per mL

Solution using dimensional analysis:

Numerator mL = 1 mL gr 1/150 = ? mL
Denominator   gr 1/200    
Numerator mL = 1mL  X 200  X 1 = 200 = 1.3 mL
Denominator     1       150 = 150  

For conversions within the metric system, a simple memory "trick" can be used, based on your knowledge of which unit are larger and smaller.

Example 4:

Order reads Synthroid 50 mcg

Supply: Synthroid 0.05 mg


when changing a Larger unit to a Smaller one within the metric system, the decimal point goes three places in the direction from L to S, or to the right.  
Reverse this process to go from a Smaller unit to a Larger one, as seen here...

mcg are the smaller units; move the decimal place 3 places to the left

50.0 mcg = 0.05 mg

Give 1 tablet

For conversion of temperatures from Fahrenheit to Celsius, or Celsius to Fahrenheit, it is necessary to memorize the process.

Fahrenheit to Celsius =  Subtract 32 and divide by 1.8
Celsius to Fahrenheit =  Multiply by 1.8 and add 32


98 F. = (98 - 32) 1.8 = 36.66 or 36.7 C.

38 C. = (38 X 1.8) + 32 = 100.4 F

For an easy way to convert between systems visit: http://www.easyunitconverter.com/

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