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UCQ

Dosage Calculations

Resources are available for you to understand how to calculate dosages.

Conversions and Calculations for nursing students

One of the important tasks you'll need to do involves converting measurements within the metric system, particularly between grams, milligrams, and micrograms.

TIP: If the units aren't specified, try converting to the smaller units. This way, you'll work with whole numbers instead of decimals, making things easier.

Here are some useful units that you as a nurse will need to know and use in your practice.

Liquid Measurements and Conversions Abbreviations of Units
1 tbsp. = 3 tsp. tbsp. - tablespoon
1 fl.oz. = 2 tbsp. tsp. - teaspoon
1 cup = 8 fl. oz fl. oz. - fluid ounce
1 tsp. = 5 mL. L. - liter
1 tbsp. = 15 mL.  mL. - milliliter
1 cup = 250 mL. cc. - cubic centimeter
1 L. = 1000 mL.  
1 mL. = 1 cc.  
Mass Measurements and Conversions Abbreviations of Units
2.2 lb = 1 kg. lb. - pound
1 lb. = 16 oz oz. = ounces
1 kg. = 1000 g. kg. - kilogram
1 g. = 1000 mg. g. - gram
1 mg. = 1000 mcg. mg. - milligram
  mcg. - microgram

Standard nursing practice have the following direct conversions.

8 fl.oz. = 250 mL = 1 cup    NOT 8 fl. oz. x 30 Ml/fl.oz. = 240 mL

4 fl. oz. 125 mL = 1/2 cup   NOT 4 fl. oz. x 30 Ml/fl.oz. = 120 mL

In ALL OTHER conversions use 1 fl. oz. = 30 mL in your calculations.

EXAMPLE 1

A patient is ordered 0.8g of a medicine. The medication is available in 200mgtablets. How many tablets will you administer?

Step 1: Convert the patient's dosage to milligrams (mg) since the tablet is in milligrams. 1 gram (g) = 1000 milligrams (mg) So, 0.8g * 1000 = 800mg

Step 2: Determine how many tablets are needed based on the available tablet strength. The tablet strength is 200mg each.

800mg (patient's dosage) ÷ 200mg (tablet strength) = 4 tablets

Therefore, the nurse will administer 4 tablets to the patient to meet the prescribed dosage of 0.8g.

EXAMPLE 2

Prescription: The doctor prescribes a medication dosage based on the patient's weight. The prescription is for a patient who weighs 70 kilograms (kg). The medication dosage is 0.1 mg/kg, and the available medication comes in 2 mg tablets. How many tablets will you administer to the patient?

To determine the number of tablets required:

Step 1: Check the patient's weight: 70 kg

Step 2: Calculate the prescribed dosage based on weight:

  • Prescription: 0.1 mg/kg
  • Patient's weight: 70 kg
  • Calculation: 0.1 mg/kg × 70 kg = 7 mg

Step 3: Check the tablet strength: 2 mg per tablet

Step 4: Divide the prescribed dosage by the tablet strength to find out how many tablets are needed.

7 mg (prescribed dosage) ÷ 2 mg (tablet strength) = 3.5 tablets

Since the tablets cannot be divided into fractions, rounding to a whole number is required. In this case, 3.5 tablets would likely be rounded down to 3 tablets. Therefore, the nurse will administer 3 tablets to the patient to approximate the prescribed dosage based on weight.