Medical Dosage Calculations For Dummies
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No matter what initials you have after your name (RN, CNA, PA, and so on), you can bet you’ll see math on a daily basis if you’re going into (or are already in) a career in the medical field.

Grasping some medical math basics — such as how to break down medical dosage problems into steps and use conversion factors — can simplify everyday situations all health care professionals face.

In addition to just knowing math, you’re going to need to know how to read and interpret doctors’ orders, and spot when there’s potential trouble.

Figuring IV flow rate, infusion time, and total volume

Whenever you’re administering intravenous (IV) infusions, you need to know the flow rate, infusion time, and total volume. Fortunately, calculating any one of these three variables is easy to do when you know the other two variables. Use the following equations:

  • flow rate (mL/hr) = total volume (mL) ÷ infusion time (hr)

  • infusion time (hr) = total volume (mL) ÷ flow rate (mL/hr)

  • total volume (mL) = flow rate (mL/hr) × infusion time (hr)

For example, if you must administer 1 L (1,000 mL) of fluid over 4 hours, use the first formula to calculate the flow rate, like so:

flow rate (mL/hr) = total volume (mL) ÷ infusion time (hr)

flow rate (mL/hr) = 1,000 ÷ 4

flow rate (mL/hr) = 250

The flow rate is 250 mL/hr.

Common conversion factors in medical dosage calculations

As a healthcare professional, you have to convert patient weights, fluid volumes, medication weights, and more. Conversion math isn’t hard to do as long as you know the basic conversion factors. Here are the most useful ones:

  • Converting lb to kg and kg to lb

    lb = kg × 2.2

    kg = lb ÷ 2.2

  • Converting mL to L and L to mL

    mL = L × 1,000

    L = mL ÷ 1,000

  • Converting mg to g, g to mg, mg to mcg, and mcg to mg

    mg = g × 1,000

    g = mg ÷ 1,000

    mcg = mg × 1,000

    mg = mcg ÷ 1,000

Unacceptable abbreviations in prescriptions

Making sure that you correctly calculate a dose doesn’t matter much if the medication itself is incorrect or the dosing instructions are unclear. Some abbreviations in prescriptions are unacceptable because they cause ambiguity and confusion (the enemies of patient safety and quality healthcare!). For this reason, you don’t want to see these abbreviations on any medical orders you work with.

Abbreviation Mistaken Meanings Better Choice
DC or D/C Does it mean “discontinue” or
Write discontinue or discharge.
HS Does it mean “half-strength” or “at
Write at bedtime or a designated time.
Also write out the specific dosing strength and/or quantity
QD Does it mean “every day” or “right eye”?
QD looks like OD, which means “right eye.”
(OS means “left eye.”)
Write every day.
QOD Does it mean “every other day” or
Write every other day or daily, according
to patient’s needs.
MSO4 Does it mean “magnesium sulfate” or “morphine
Write magnesium sulfate or morphine sulfate.
U or IU Does it mean “unit” or “zero”? Could it be
mistaken for “0” or “10”?
Write units.
IV Does it mean “intravenous,” “international
units,” or “4”?
IV is an acceptable abbreviation for
“intravenous,” but the doc could write international
or intravenous to be clearer.
Or “4”
SQ or SC Does it mean “subcutaneous” or could it be mistaken
for “5Q” (“5 every”)?
Write Subq, subcut, subcutaneous, or 5 every.
TIW Does it mean “twice a week” or “three times a
week” (the real meaning)?
Write twice a week or three times a week.
cc Does it mean “cubic centimeter” or
“milliliter”? Could it be mistaken for
Write milliliter or mL.
Ug or g Does it mean “microgram” or “Ugh”? Could it
be mistaken for mg?
Write microgram or mcg.
OD Does it mean “once daily” or “right
Write once daily or right eye.

Source: The National Coordinating Council for Medication Error Reporting and Prevention (NCC MERP).

About This Article

This article is from the book:

About the book authors:

Dr. Richard W. Snyder, DO is an osteopathic physician, board certified in both internal medicine and nephrology. He has authored and coauthored several articles in peer-reviewed journals.

Barry Schoenborn is a longtime technical writer and is the coauthor of Technical Math For Dummies.

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