Calculating Drug Dosages
Calculating Doses from PreparedStrength Liquids, Tablets, and Capsules
Calculating With Proportions
Steps:
1. Convert to a consistent unit of measure.
2. Set up a proportion:
Original dose = Desired dose
Per amount Per amount
3. Crossmultiply and solve for the unknown (x).
Example 1: Tablets
How many tablets are needed to deliver 500 mg of Tylenol?
Original dose = 250 mg
Per amount = 1 tablet
Desired dose = 500 mg
Per amount = unknown (x)
1. Set up a proportion:
250 mg = 500 mg
1 tab x tab
2. Crossmultiply and solve for the unknown (x):
250x = 500
x = 500/250 = 2
Answer: 2 tablets are needed to deliver 500 mg of Tylenol
Example 2: Solutions
How many milliliters of elixir are needed to deliver 15 mg of Phenobarbital?
Original dose = 120 mg
Per amount = 30 ml
Desired dose = 15 mg
Per amount = x ml
1. Set up a proportion:
120 mg = 15 mg
30 ml x ml
2. Crossmultiply and solve for the unknown (x):
120x = 450
x = 450/120 = 3.75 ml
Answer: 3.75 ml are needed to deliver 15 mg of the elixir

Drug Amounts in Units
Some drugs are manufactured in units (U) rather than in grams or milligrams. Note: A unit does not have a standard metric equivalent. The metric equivalent (milligrams, etc.) varies for each drug. Examples are penicillin, insulin and heparin. Solving dose problems for these drugs is exactly the same as for other dosage units except that unit (U) is substituted for milligrams (mg).
Example 3: Units
How many milliliters are needed to deliver 500 U of heparin?
Original dose = 120 mg
Per amount = 30 ml
Desired dose = 15 mg
Per amount = x ml
1. Set up a proportion:
1000 U = 500 U
1 ml x ml
2. Crossmultiply and solve for the unknown (x):
1000x = 500
x = 500/1000 = 0.5 ml
Answer: 0.5 ml are needed to deliver 500 units of heparin

Calculations With a Dosage Schedule
There are times when the dose of a drug must be obtained from a schedule, which may be based on the size of the person. The dose may be presented in mg/kg of body weight. This means that the dose must first be calculated after the body weight is obtained, and then the amount of the drug preparation needed can be calculated.
Steps:

Calculate the dose needed

Convert to a consistent unit of measure

Set up a proportion
Original dose = Desired dose
Per amount Per amount
4. Crossmultiply and solve for the unknown (x)
Example 4: Dosage Schedule
Using a dosage schedule of 0.1 mg/kg for albuterol syrup, and a preparedstrength mixture of 2mg / 5 ml, how much of the syrup is needed for a 30 kg child?

Calculate the dose needed:
Dose = 0.1 mg/kg x 20 kg = 3.0 mg

Set up a proportion:
Original dose = 2.0 mg
Per amount = 5 ml
Desired dose = 3.0 mg
Per amount = x ml
3. Set up a proportion, crossmultiply and solve for the unknown (x):
2.0 mg = 3.0 mg
5 ml x ml
2x = 15
x = 7.5
Answer: 7.5 ml are needed to deliver 3.0 mg of albuterol syrup to this child

Calculating Doses From PercentageStrength Solutions

Terms:

Solute – the active ingredient (solid or liquid)

Solvent – the liquid that dissolves the solute (sterile H2O, NS)

Solution – a solute which is dissolved in a solvent

Strength – the percentage of solute to total solvent and solute

Percentage – parts of the active ingredient (solute) in a preparations contained in 100 parts of the total preparation (solute and solvent)

Types of Percentage Preparations:

Weight to Weight: Percent in weight (W/W) expresses the number of grams of a drug or active ingredient in 100 ml of a mixture:

W/W: Grams per 100 g of mixture

Weight to Volume (most common): Percent may be expressed for the number of grams of a drug or active ingredient in 100 ml of a mixture:

W/V: Grams per 100 ml of mixture

Volume to Volume: Percent expresses the number of milliliters of drug or active ingredient in 100 ml of a mixture:

V/V: Milliliters per 100 ml of a mixture

Calculating with PercentStrength Solutions
Steps:

Convert the percent solution to metric units (mg/1ml)
Useful Trick: Percent Solutions can be quickly converted to milligrams per 1 milliliter by moving the decimal one place to the right:
Example: 2% solution, move the decimal one place to the right,
2
.0. mg/1 ml = 20 mg/1 ml

Set up a proportion and substitute knowns
Original dose = Desired dose
Per amount Per amount

Crossmultiply and solve for the unknown (x)

Express answer in the units requested
Example 5: PercentStrength Solution
How many milligrams of active ingredient are in 3 ml of a 0.083% of albuterol (unit dose)?

Convert to metric units (mg/1 ml):
0.083% = 0.083 g = 83 mg = 0.83 mg
100 ml 100 ml 1 ml
2. Set up a proportion:
0.83 mg = x
1 ml 3 ml
3. Crossmultiply and solve for the unknown (x):
x = 0.83 * 3
x = 2.49 mg
Answer: There are 2.5 mg of active ingredient in a unit dose of albuterol.

Solutions By Ratio

Frequently when diluting a medication for use in an aerosol treatment, a solute to solvent ratio is given (1:200, 1:1000)
Example 5: Solutions by Ratio
How many milligrams of active ingredient are in 2 ml of a 1:200 solution of isoproterenol?
1. Convert to metric units (mg/1 ml):

Convert to % solution

1:200 = 1/200 = 0.005 x 100 = 0.5%

Then, convert to metric units (mg/1 ml)

0.5% = 0.5 g = 500 mg = 5 mg
100 ml 100 ml 1 ml
2. Alternate method to convert to metric units (mg/1 ml)
a. 1:200 = 1 g/200 ml = 1000 mg/200 ml = 10 mg/2ml = 5 mg/1ml
3. Set up a proportion:
5 mg = x
1 ml 2 ml
4. Crossmultiply and solve for the unknown (x):
x = 5 * 2
x = 10 mg
Answer: There are 10 mg of active ingredient in a 1:200 solution of isoproterenol.

Solving Dilutions With a Dilute Solute

When the active ingredient, or solute is already diluted and less than pure, the following equation is used:
C_{1} * V_{1} = C_{2} * V_{2}
Solving for V_{2}:
V_{2} = C_{1} * V_{1} C_{2}
Where:
C_{1} = desired concentration (in decimals)
V_{1} = desired total volume (mL)
C_{2} = concentration of solution on hand
V_{2} = volume of solution needed for dilution
Steps:

Substitute known variables:

Concentration of the solution as a decimal

20% solution = 20/100 = 0.20

Volume (ml)

Solve for the unknown variable (V_{2})

Determine the amount of diluent (normal saline, sterile H_{2}O) needed

Total solution amount – V_{2}
Example: Dilutions with a Dilute Solute
How much 20% Mucomyst is needed to prepare 5 ml of a 10% Mucomyst?

Substitute the known variables:
C_{1} = 0.10
V_{1} = 5 ml
C_{2} = 0.20
V_{2 }= unknown
2. Solve for the unknown variable (V_{2}):
V_{2} = 0.10 * 5 = 0.5 = 2.5 ml
0.20 0.2
3. Determine the amount of diluent needed:
Total solution amount – V_{2} = amount of diluent needed
5 ml – 2.5 ml = 2.5 ml diluent
Answer: 2.5 ml of 20% Mucomyst and 2.5 ml normal saline are mixed to prepare 5 ml of a 10% solution of Mucomyst
***Note: Students are not responsible for calculating intravenous infusion rates. 