Dosage Calculations
Dosing Considerations
- Sex
- Weight
- Age
- Physical condition
- Other drugs that the patient is taking
Accurate Dosing
- Dependent on:
- The prescriber who orders the drug
- The pharmacist who dispenses the drug
- The nurse who administers the drug
Measuring Systems
- Metric system
- Apothecary system
- Household system
- Avoirdupois system
Metric System
- Solids: gram (g)
- 1 milligram (mg) = 0.001 g
- 1 microgram (mcg) = 0.000001 g
- 1 kilogram (kg) = 1000 g
- Liquids: liter (L)
- 1 milliliter (mL) = 0.001 L
- 1 mL = 1 cubic centimeter = 1 cc
Apothecary System
- Solids: grain (gr)
- 60 gr = 1 dram (dr)
- 8 dr = 1 ounce (oz)
- Liquids: minim (min)
- 60 minim = 1 fluidram (f dr)
- 8 f dr = 1 fluidounce (f oz)
Household System
- Solids: pound (lb)
- 1 lb = 16 ounces (oz)
- Liquids: pint (pt)
- 2 pt = 1 quart (qt)
- 4 qt = 1 gallon (gal)
- 16 oz = 1 pt = 2 cups (c)
- 32 tablespoons (tbsp) = 1 pt
- 3 teaspoons (tsp) = 1 tbsp
- 60 drops (gtt) = 1 tsp
Conversion Table

Conversion Between Systems
amount of drug available = amount of drug prescribed
one tablet or capsule # of tablets or capsules to give
amount of drug available = amount of drug prescribed
volume available volume to administer
Solving for X
amount of drug available = amount of drug prescribed
one tablet or capsule # of tablets or capsules to give
10 mg = 40 mg
1 x
Examples
1 oz = 6 oz
30 mL X
1 oz x X = 6 oz x 30 mL
1(oz)X = 180 (oz) (mL)
X = 180 (oz) (mL)
1 oz
X = 180 mL
Calculating IV Drip Rate
drops/min = mL of solution prescribed per hr x drops delivered per mL
60 min/hr
Difference Between Children and Adults
- Children absorb, distribute, metabolize, and excrete drugs differently than adults
- Children’s organs are not as developed as adults’ organs
Pediatric Dosage Calculation
- Fried’s rule
- Young’s rule
- Clark’s rule
- Surface area calculation
Fried’s Rule
This rule assumes that an adult dose would be appropriate for a child who is 12.5 years (150 months) of age
child’s dose (age <1 year) = infant’s age (in months)
150 months x average adult dose
Young’s Rule
child’s dose (age 1 to 12 years) =
child’s age in years
child’s age in years + 12 x average adult dose
Clark’s Rule
Uses the child’s weight to calculate the appropriate dose and assumes the adult dose is based on a 150-lb person
child’s dose = weight of child in pounds
150 pounds x average adult dose
Surface Area Calculation
- Determine the child’s surface area with the use of a nomogram (the height and weight of the child are taken into consideration in this chart)
child’s dose = surface area in square meters
1.73 x average adult dose
Nomogram
