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A Guide to Accurate Drug Dosage Calculations for Nursing Professionals

When it comes to medication and dosages, nurses have to keep track of the number of times, and the quantity, when administering drugs to patients. On top of that, performing correct dosage calculations is critical in the nursing profession. Mastering it is a key aspect in a nurse’s practice. If you are already a nurse, or have just stepped into the profession, you should be well aware of the basics related to every drug dosage calculation.

The Abbreviations

Abbreviations rule the medical world. Thus, it is important to know the recognized abbreviations in a clinical setting so you will not be lost when preparing patients’ medications. Here are some:

  • cc- cubic centimeter
  • DD- Desired Dose
  • IM- Intramuscular
  • IO- Intraosseous
  • SL – Sublingual
  • IV- Intravenous
  • IVP- Intravenous Push
  • Kg- Kilogram
  • gm- gram
  • mg- milligram
  • mcg- microgram
  • mEq- milliequivalent
  • L- Liter
  • mL- milliliter
  • µg- microgram
  • gtt – drop
  • µgtt – micro drop
  • tbsp – tablespoon
  • tsp – teaspoon

The Drug Calculation Formulas

Calculating Tablet Dosages

In calculating tablet dosages, the following formula is useful: Desired dosage ÷ stock strength = number of tablets
The desired dosage is the dosage ordered by the physician, while the stock strength is the amount of drug present in each tablet. Stock strength is also known as stock dose.

Example:
  • The patient is ordered to have 2 g of potassium chloride. The drug is available in 500 mg tablets. How many tablets should be given?

    Convert 2 g to mg = 2 x 1,000 = 2,000 mg
    Desired dosage ÷ stock strength = number of tablets
    2,000 mg ÷ 500 mg = 4 tablets

Calculating Mixtures and Solutions

The following formula is useful in calculating mixtures and solutions: Desired dosage ÷ stock strength x stock volume = amount of solution to be given

The desired dosage is the dosage ordered by the physician. The stock strength is the amount of drug present in the preparation, while the stock volume is the amount of the solution where the drug is diluted.

Example:
  • The physician orders 375 mg of cefuroxime for the patient. The drug is available in 750 mg vial. You plan to dilute it in 10 mL of sterile water. How much should you give to your patient?

    [Desired dosage ÷ stock strength] x stock volume = amount of solution to be given
    [375 mg ÷ 750 mg] x 10 mL = amount of solution to be given
    0.5 mg x 10 mL = 5 mL

Calculate IV Rate – mL per hour and minute

It is easy to calculate the running rate of IV fluids in terms of mL per hour or mL per minute: Total IV volume ÷ time (hour or minute) = mL per hour or minute

The total IV volume is the amount of fluid to be infused while the time is the number of running hours or minutes.

Example:
  • You are going to start IV infusion with Plain Lactated Ringer’s Solution 1 L. By regulating it for 11 hours, how much fluid are you going to infuse per minute?

    Total IV volume ÷ time (hour or minute) = mL per minute
    1 L ÷ 11 hours = mL per minute
    [1 L x 1,000] ÷ [11 hours x 60] = mL per minute
    1,000 mL÷ [11 hours x 60] = mL per minute
    1,000 mL ÷ 660 = 1.5 mL per minute

Calculate IV Rate – drops per minute

Calculating for drops per minute is simple with the following formula: [Total IV volume ÷ time (minute)] x drop factor = drops per minute

The total IV volume is the amount of IV fluid to be infused while time is the duration of how long the IV fluid should be infused in terms of minutes.

The drop factor is the "drops per millilite" delivered to the patient and it depends on the macrodrip used for the infusion. The common drop factors used in different hospitals are 10, 15 and 20.

Example:
  • You are going to regulate D5 0.9 NaCl 500 mL solution for 7 hours. The macrodrip used has 15-drop factor. You should regulate the IV fluid for how many drops per minute?

    [Total IV volume ÷ time (minute) ] x drop factor = drops per minute
    [500 mL ÷ 7 hours] x 15 = drops per minute
    [500 mL ÷ {7 hours x 60}] x 15 = drops per minute
    [500 ÷ 420] x 15 = drops per minute
    1.19 x 15 = 17 to 18 drops per minute

Calculate IV Rate – Remaining Time of Infusion

Calculating for the remaining time of infusion for a certain IV fluid is possible with the following formula: [Volume remaining (in mL) ÷ drops per minute] x drop factor = minutes remaining

The volume remaining is the amount of IV fluid remaining for the infusion. The drops per minute is the regulation of the IV infusion. The drop factor can be determined in the macrodrip used in the hospital.

Example:
  • A patient has 350 mL of Plain 0.9 NaCl solution as IV infusion regulated at 20 drops per minute. Drop factor 10 was used for the patient’s macrodrip set. Considering the IV fluid level of the patient, for how many minutes will it run?

    [Volume remaining (in mL) ÷ drops per minute] x drop factor = minutes remaining
    [350 mL ÷ 20] x 10 = minutes remaining
    17.5 x 10 = 175 minutes or 3 hours

The conclusion

If you know the basic drug calculation formulas, you will never be lost in finding the desired dosage for your patient.It’s best to master drug calculations and know the unit conversions. Once you start working in a clinical setting, you will get used to the common formulas used in determining desired dosages or IV regulations. It takes a lot of practice to master drug calculations in a short period of time, nursing pharmacology will then be a piece of cake for you.

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