Two basic formula for drug dose calculation is mentioned below -
Formula 1
D/H × Q = X, [Where, X is the amount to administer]
or
Dose amount to administer = (Desired or Ordered Dose amount/Amount on Hand) x Quantity.
Formula 2
H : Q = D : X
[ Where, D = Desired dose or dose ordered by the primary care provider; H = dose on hand or dose on the label; Q = Quantity; X = amount to administer]
Dose have on hand / Quantity = Desired Amount / X
Dosage Calculation Examples
A provider requests lorazepam 4 Mg IV Push for a patient in severe alcohol withdrawal. The clinician has 2 mg/mL vials on hand. How many milliliters should he or she draw up in a syringe to deliver the desired dose?
Option 1:
[Ordered Dose (4 mg) / Have (2 mg)] x Quantity (1 mL) = Amount wanted to give (2 mL)
Option 2:
Have on hand (2 mg) / Quantity you have (1 ml) = Desired Amount (4 mg)/ X
2 mg/1 mL = 4 mg/x
2x/2 = 4/2
x = 2 mL
Dosage Calculation Practice (Problem with Answer)
Calculation 1
The doctor orders 90 milligrams of liquid cough syrup. The liquid cough syrup has a label that reads 120 milligrams (mg for short) in 5 milliliters (or mL for short). How much cough syrup should the nurse give to the patient?
Here, D = 90 mg, H = 120 mg & Q = 5 mL
So,
X = D/H × Q
X = 90/120 × 5
X = 3.75 mL
Calculation 2
Ampicillin 500 mg capsules are supplied. MD orders 1.5 g. How many capsules should be given to the patient?
Here, D = 1.5 g = 1500 mg, H = 500 mg & Q = 1 capsule
So,
X = D/H × Q
X = 1500 mg/ 500 mg × 1
X = 3 Capsules
Calculation 3
The doctor orders “ibuprofen 600 mg PO BID”. You have 300 mg tablets of ibuprofen on hand. How many tablets should be given to the patient at one time?
Here, D = 600 mg, H = 300 mg & Q = 1 tablet
X = D/H × Q
X = 600 mg/ 300 mg × 1
X = 2 tablets
Calculation 4
The doctor orders “Valporic acid 0.5 g PO TID.” The bottle of Valporic acid on hand says 50 mg per 1 mL. How many milliliters should be given?
Here, D = 0.5 g = 500 mg, H = 50 mg & Q = 1 mL
X = D/H × Q
X = 500 mg/ 50 mg × 1 mL
X = 10 ml
Dosage Calculations for Nursing Students
Q1. How much dextrose is in 4L of 20% solution?
Answer: 4L = 4000 ml × 20% = 800 ml
Q2. A patient needs 0.075g of a medicine that comes in 30 mg tablets. How many tablets will the patient take?
Answer: 0.075 g = 75 mg; 75 mg/30 mg = 2.5 tablets.
Q3. A child weighs 15.5 kg is prescribed 300 µg/ kg of a medication. How much of the medication should be given?
Answer: 15.5 × 300 = 4650 µg = 4.65 mg
Related: Dispensing Pharmacist Interview Questions and Answers
Dosage Calculation Conversions Factors
- 1 kg = 2.2 lb
- 1 lb = 16 oz
- 8 oz = 240 mL = 1 glass
- 1 tsp = 5 mL
- 1 gallon = 4 quart
- 1 L = 1,000 mL
- 1 kg = 1,000 g
- 1 oz = 30 mL = 2 tbsp
- 1 g = 1,000 mg
- 1 inch = 2.54 cm
- 1 mg = 1,000 mcg
- 1 cm = 10 mm
- 1 cup = 8 fl oz
- 1 tbsp = 15 mL
- 1 pint = 2 cups
- 12 inches = 1 foot
- 1 L = 1.057 qt
- 1 tbsp = 3 tsp
- 60 minute = 1 hour
- 1 cc = 1 mL
- 2 pints = 1 qt
- 1 pt = 500 mL = 16 oz
- 1 tsp = 60 gtt
- 1 oz = 30 mL
- 4 oz = 120 mL (Casey, 2018)
Abbreviation Term
- po: by mouth (or orally)
- susp: suspension
- pm: as needed
- tab: tablet
- cap: capsule
- q: every
- bid: twice a day
- tid: three times a day
- qid: four times a day
