Drug Dose Calculation Formula

The basic drug doses calculation formula listed below can be used for most drug dosage calculations. These calculations are necessary when a doctor orders a particular desired dose of medication and the medication you have available on hand is in the form of mass/tablet or mass/volume.


Desired Over Have Formula Method

A basic formula or equation to solve for an unknown quantity (x), much like ratio proportion.

D/H × Q = X


  • D = the order amount of medication that the physician prescribed,
  • H = the having amount of drug in a specific unit of measure (what is available; in stock)
  • Q = the unit of measure for the specific dosage strength or quantity of unit
  • X = the desired dose you are trying to calculate (unknown dose)



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?


D = 90 mg

H= 120 mg

Q = 5 ml


X = (90 ÷ 120) × 5 = 3.75 ml


2. Ampicillin 500 mg capsules are supplied. MD orders 1.5 g. How many capsules should be given to the patient?


D = 1.5 g = 1500 mg

H = 500 mg

Q = 1 capsule


X = (1500 ÷ 500) × 1 = 3 capsules


Drug Dosage Based on Body Surface Area (BSA)

BSA is derived from the weight and height of the patient and is expressed as square meters (m2). For certain drugs, particularly chemotherapy drugs and pediatric drugs, the drug dose is based on BSA rather than weight. The average adult is considered to have a BSA of 1.73 m2. A useful equation for calculation of dose based on BSA is:

Patient's Dose = [Patient's BSA (m2) ÷ 1.73 m2] × Drug dose (mg)



If the adult dose for a drug is 70 mg per day. What dose should be given to a child with a BSA of 0.33 m2?

Patient's Dose = [0.33 (m2) ÷ 1.73 m2] × 70 (mg) = 13.4 mg


Calculation of Children’s Doses

Fried’s rule for Infants

[Age (in months) × adult dose] ÷ 150 = dose for infant


Clark’s rule

[Weight (lb) × adult dose] ÷ [150 lb (avg. wt. of adult] = dose for child


Child’s dosage based on body surface area (BSA)

[BSA of child (m2) × adult dose] ÷ [1.73 m2 (avg. adult BSA)] = approximate dose for child


Young’s rule for children 2 years old or older

[Age (in years) × adult dose] ÷ Age (in years) + 12 = dose for child

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