1. Chapter 7: Methods for Dosage Calculations
Math and Dosage Calculations for Health Care Third Edition Booth & Whaley
Chapter 7: Methods for Dosage Calculations
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2. Learning Outcomes
7.1 Describe how the information on a physician’s order, medication administration record (MAR), or prescription, along with the drug label and package insert, are used to calculate the desired dose.
7.2 Convert the dosage ordered to the desired dose, using fraction proportion, ratio proportion, or dimensional analysis.
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3. Learning Outcomes (cont.)
7.3 Calculate the amount to administer of a drug, using any of the methods of dosage calculation – fraction proportion, ratio proportion, dimensional analysis, or formula.
7.4 Recognize common errors that occur during dose calculations.
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4. Introduction To calculate dosages you will use: Basic math
Information from the physician’s order
Drug labels
Methods of converting quantities from one unit of measurement to another
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5. Dosages and Doses Desired dose
The amount of the drug to be administered at a single time
To find the desired dose, you must have
The dosage ordered
The dose on hand
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6. Dosages and Doses (cont.)
Dosage ordered
The amount of drug ordered and the frequency of administration
Dosage strength
Amount of drug per dosage unit
Medication may be available in different dosage strengths
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7. Dosages and Doses (cont.)
Dose on hand
The amount of drug contained within a dosage unit of medication
If a medication has 250 mg of drug per tablet, the dose on hand is 250 mg.
Dosage unit
Tablet, capsule, mL, etc.
Both can be found on the label
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8. Calculating the Desired Dose
Before calculating the amount to administer, you must first determine the desired dose.
The dosage ordered may be in different units than found on the label.
The dosage ordered must be converted into a desired dose having the same units as the dose on hand.
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9. Dosage Calculation Dosage ordered = O Desired dose = D Dosage unit = Q
Dose on hand = H
Amount to administer = A
Dosage strength = H/Q

10. Calculating the Desired Dose (cont.)
Three methods to calculate desired dose
Fraction proportion
Ratio proportion
Dimensional analysis
Method used is a matter of choice
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11. Calculating the Desired Dose (cont.)
Rule 7 - 1
The unit of measurement for the desired dose must be the same as the unit of measurement of the dose on hand before the amount to administer can be calculated.
The dose ordered must be converted into the same unit of measurement as the dose on hand.
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12. Fraction Proportion Method
Procedure Checklist 3-1
1.Write the conversion factor with the units that you are converting to in the numerator and the units you are converting from in the denominator.
2.Write a fraction with the unknown [?] in the numerator and the number that you need to convert in the denominator.
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13. Fraction Proportion Method (cont.)
3. Set the two fractions up as a proportion. 4. Cancel units. 5. Cross multiply, then solve for the unknown value.
?/B = C/D
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14. Fraction Proportion Method (cont.)
The dosage ordered is mg once a day.
The dosage strength is 100 mcg/tablet.
Find the desired dose.
?/0.2 mg = 1000 mcg/1 mg
?1000 = 0.2
? = 200
Desired dose = 200 mcg
Example

15. Always include the units when performing calculations.
Error Alert!
In a fraction proportion, units from the two fractions can be canceled only when they are the same portion of the fraction.
Units in the denominator of one fraction cannot be canceled with units found in the numerator of the other.
Always include the units when performing calculations.
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16. Ratio Proportion Method
Procedure Checklist 3-2
1. Write the conversion factor as a ratio A : B so that A has the units of the value that you are converting (the dosage ordered) and B has the unit of value of the dose on hand.
2. Write a second C : D so that C is the missing value (desired dose) and D is the number that is being converted (the dosage ordered).
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17. Ratio Proportion Method (cont.)
3. Write the proportion in the form A : B :: C : D.
Note: When using the ratio proportion method to calculate the desired dose, C indicates the unknown value (desired dose).
4. Cancel units.
5. Solve the proportion by multiplying means and extremes.
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18. Ratio Proportion Method (cont.)
The order reads: ASA gr v PO daily. The drug label indicates 325 mg tablets. Find the desired dose using 1 gr = 65 mg 65mg:1gr:: C: 5 gr 1 × C = 65 × 5 C = 325 Desired dose is 325 mg
Example
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19. Always include the units when performing calculations.
Error Alert!
In a ratio proportion, units can be canceled only when they are found in the same part of each of the ratios.
Always include the units when performing calculations.
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20. Dimensional Analysis Procedure Checklist 3-3
1. Determine the units of measure for the answer and place it as the unknown on one side of the equation.
2. On the other side of the equation, write a conversion factor with the units of measure for the answer on top and the units you are converting from on the bottom.
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21. Dimensional Analysis (cont.)
3. Multiply the conversion factor by the number that is being converted over one.
4. Cancel units on the right side of the equation.
The remaining unit of measure on the right side should match the unknown unit of measure on the left side.
5. Solve the equation.
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22. Dimensional Analysis (cont.)
Find the desired dose for the following: Ordered: Nitrostat 800 mcg sublingually PRN chest pain On hand: Nitrostat 0.4 mg sublingual tablets D mg = 1mg/1000mcg x 800mcg/1 D mg = 800/1000 D mg = 0.8 Desired dose is 0.8 mg
Example
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23. Always include the units when performing calculations.
Error Alert!
In dimensional analysis, units can be canceled only when they are found in both the numerator and the denominator of the fraction.
Always include the units when performing calculations.
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24. Practice Determine the desired dose. Ordered: Penicillin VK 0.25 g
On hand: Penicillin VK 500 mg/tablet
Desired dose: 250 mg
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25. Calculating the Amount to Administer
Once the desired dose is calculated, the amount to administer must be calculated.
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26. Calculating the Amount to Administer (cont.)
Rule 7–2 To calculate the amount of medication to administer (A), the following information must be known:
The desired dose (D) or the amount of drug to be given at a single time. This is the dosage ordered converted to the same units as the dose on hand.
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27. Calculating the Amount to Administer (cont.)
Rule (cont.)
The dosage strength or the dose on hand (H) per the dosage unit (Q)
The dose on hand (H) is the amount of drug contained in a dosage unit.
The dosage unit (Q) is the unit by which you will measure the medication-tablets, capsules, milliliters, teaspoons, etc. (Find this on the medication label).
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28. Calculating the Amount to Administer (cont.)
Procedure 7-1 Calculating the Amount to Administer by Fraction Proportion:
1. Set up the proportion as follows:
dosage unit amount to administer
dose on hand desired dose
2. Cancel units.
3. Cross multiply, then solve for the unknown value.
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29. Calculating the Amount to Administer (cont.)
Ordered: 250 mg IM Dosage strength available: g/mL
Find the amount to administer.
Convert 0.5 gm to milligrams
500 x A = 1mL x 250
Example
A 500 = 250mL
Amount to administer= 0.5mL
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30. Calculating the Amount to Administer (cont.)
Procedure 7-2 Calculating the Amount to Administer by Ratio Proportion:
1. The proportion will be set up as follows:
dosage unit : dose on hand :: amount to administer : desired dose or
Q : H :: A : D
2. Cancel units.
3. Multiply the means and extremes then solve for the missing value.
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31. Calculating the Amount to Administer (cont.)
The dosage ordered is Erythromycin 500 mg PO q 6h
On hand: Erythromycin 250 mg tabs
Find the amount to administer.
D = 500mg Q = 1 tablet H = 250mg
1 tablet : 250mg :: A : 500mg
A = 1 tablet x 2
Example
Amount to administer = 2 tablets
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32. Calculating the Amount to Administer (cont.)
Procedure 7-3 Calculating the Amount to Administer by Dimensional Analysis:
With dimensional analysis you will not need to calculate the desired dose and amount to administer separately.
You will place your unknown (amount to administer) on one side of the equation then multiply a series of factors on the right side of the equation.
Canceling units will help you determine the equation has been set up correctly.
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33. Calculating the Amount to Administer (cont.)
Procedure 7-3 (cont.):
1. Determine the units of measure for the answer and place it as the unknown on one side of the equation.
2. On the right side of the equation, write a conversion factor with the unit of measure for the desired dose on top and the unit of measure for the dosage ordered on the bottom.
If units are different than the dose on hand.
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34. Calculating the Amount to Administer (cont.)
Procedure 7-3 (cont.):
3. Multiply the conversion factor by a second factor – the dosage unit over the dose on hand.
4. Multiply by a third factor - dose ordered over the number one.
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35. Calculating the Amount to Administer (cont.)
Procedure 7-3 (cont.):
5. Cancel units on the right side of the equation.
The remaining unit of measure on the right side of the equation should match the unknown unit of measure on the left side of the equation.
6. Solve the equation.
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36. Calculating the Amount to Administer (cont.)
The order reads: Prozac Liquid 40mg daily On hand: Prozac 20 mg/5 mL Find the amount to administer. D = 40 mg Q = 5 mL H = 20 mg A mL = 200/20 Amount to administer = 10 mL
Example
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37. Calculating the Amount to Administer (cont.)
Procedure 7-4 Calculating Amount to Administer Using the Formula Method:
1. Determine the desired dose. Determine the dose on hand (H) and dosage unit (Q)
2. Fill the formula
D for the desired dose
H for the dose on hand
Q for the dosage unit in the formula
A for the unknown or the amount to administer
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38. Calculating the Amount to Administer (cont.)
Procedure 7-4 (cont.):
3. Cancel the units.
4. Solve for the unknown.
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39. Calculating the Amount to Administer (cont.)
The dosage ordered is Famvir mg PO q 8 h
On hand: Famvir 250 mg/tabs
Find the amount to administer.
D = 500 mg H = 250 mg Q = tablet
2 x 1 tablet = A
2 tablets = Amount to administer
Example
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40. Apply Your Knowledge
The amount of the drug to be administered at a single time is called the:
a. dosage ordered
b. dosage unit
c. desired dose
d. dosage strength
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41. Apply Your Knowledge True or False Answer True
If a medication has 300 mg of drug per tablet, the dose on hand is 300 mg.
Answer True
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42. Apply Your Knowledge The MAR reads Glucotrol 10 mg PO qd.
The desired dose is ____ mg.
Answer mg
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43. Apply Your Knowledge Calculate the amount to administer.
Ordered: Prednisone 10 mg PO qid
On hand: Prednisone 5 mg tablets
Answer (Using ratio proportion)
5 mg : 1 tablet : : 10 mg : ? tablet
1 x 10 = 5 x ?
? = 2
Amount to administer: 2 tablets
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44. End of Chapter 7
Each problem that I solved became a rule, which served afterwards to solve other problems.
-- Rene Descartes
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