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:
Numerator 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
LMNOPQRS
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 
Examples:
98 º F. = (98  32) ÷ 1.8 = 36.66 or 36.7º C.
38 º C. = (38 X 1.8) + 32 = 100.4 º F
For an easy way to convert between systems visit: http://www.easyunitconverter.com/
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