Drug dosage calculation.pptx

1. Drug dosage calculation

5. Universal Formula for drug dosage calculation(Toney-Butler, 2021)
In the universal formula (or “desired over have method”), the desired amount (D) is the
dose prescribed and the amount on hand (H) or the amount you “have” is the available
dose or concentration. The quantity (Q) is the form and amount in which the drug is
supplied (i.e. tablet, capsule, liquid). To calculate the dose, take the desired amount and
divide it by the amount on hand, then multiply it by the quantity, like this:

12. 1. Question
• A health care provider orders acetaminophen (Tylenol) supp 1 g pr q 6
hr prn temp > 101. Available: Tylenol supp. 325 mg (scored). How many
supp. will you administer?
ANSWER: 3 supp.
• 1 g is equal to 1000 mg.
• Convert first by dividing 1 g with 325 mg to get 0.003 g.
• Then, use the standard formula.
• Multiply 0.003 g to 1000 mg (desired dose) to get 3.
• Finally, multiply 3 to 1 supp (vehicle) to get 3 supp.

13. 2. Question
• A nurse practitioner prescribes clindamycin (Cleocin) Oral Susp
600 mg po qid; Directions for mixing: Add 100 mL of water and
shake vigorously. Each 2.5 mL will contain 100 mg of Cleocin.
How many tsp of Cleocin will you administer?
Correct answer: 3 tsp.
• Use the standard formula.
• Divide 600 mg (desired dose) by 100 mg (drug on hand) to get 6 mg.
• Then, multiply 6 mg to 2.5 mL (vehicle) to get 15 mL.
• Convert mL to teaspoon; 1 tsp is equal to 5 mL.
• Multiply 15 mL to 1 tsp, then divide it by 5 mL to get 3 tsp.

14. 3. Question
• A health care provider ordered sulfasalazine (Azulfidine) Oral Susp
500 mg q 6 hr; Directions for mixing: Add 125 mL of water and
shake well. Each tbsp will yield 1.5 g of sulfasalazine. How many
mL will you give?
Correct answer: 5 ml.
• Use the standard formula .
• Divide 500 mg (desired dose) with 1.5 g (drug on hand) to get 333.3.
• Multiply 333.3 to 1 g and divide it by 1000 mg to get 0.3 tbsp.
• 1 tbsp is equal to 15 mL.
• Multiply 0.3 to 15 mL to get 4.9 mL.
• Finally, round it off to a whole number to get 5 mL.

15. 4. Question
• A health care provider orders megestrol (Megace) Oral
Suspension 160 mg po bid; Directions for mixing: Add 100 mL of
water and shake vigorously. Each 0.5 mL will contain 10.7 mg of
megestrol. How many tbsp of megestrol will you administer?
Correct answer: 0.5 tbsp.
Use the standard formula.
Divide 160 mg (desired dose) with 10.7 mg (drug on hand) to get 14.9 mg.
Then, multiply 14.9 mg to 0.5 mL (vehicle) to get 7.5 mL.
1 tbsp is equal to 15 mL.
Convert by multiplying 7.5 mL to 1 tbsp, then divide it by 15 mL to get the final answer of 0.5 tbsp or
½ tbsp.

16. 5. Question
• A health care provider prescribes ceftazidime (Fortaz) mg/kg/day p.o. 50
mg/kg p.o. t.i.d. to a child who weighs 25.5 kg. Fortaz is available in an
oral suspension labeled 100 mg/ml. How many ml would the nurse
administer per dose?
Correct answer: 12.8 mL.
• Use the standard formula .
• Multiply 50mg/kg (desired dose) to 25.5 kg (weight of patient) to get 1,275 mg.
• Divide 1,275 mg with 100 mg (drug on hand) to get 12.75 ml.
• Finally, round it off to one decimal place to get 12.8 ml.

17. 6. Question
• A health care provider orders Heparin 7,855 units subq bid; Available:
Heparin 10,000 units per ml. How many mL will you administer?
Correct answer: 0.79 mL.
Use the standard formula .
Divide 7,855 units (desired dose) with 10,000 units (drug on hand), then multiply with 1 ml (vehicle)
to get 0.7855 ml.
Round it of to two decimal places to get 0.79 ml.

18. 7. Question
• A health care provider prescribes meperidine (Demerol) 50 mg IVP q
6 hr prn pain. Available on hand is meperidine 75 mg/1.3 mL. How
many mL will you administer?
Correct answer: 0.87 mL.
Use the standard formula .
Divide 50 mg (desired dose) by 75 mg (drug on hand) to get 0.67 mg.
Multiply 0.67 mg with 1.3 ml (vehicle) to get 0.871 ml.
Finally, round it off to two decimal places to get 0.87 ml.

19. 8. Question
A health care provider orders tobramycin 1.25 mg/kg IM q 12 hr. Available on hand
is tobramycin 0.2 g/mL. How many mL will you administer to a 83.2kg patient for
each dose?
• Correct Answer: 0.5 mL.
• Multiply 83.2 kg with 1.25 mg/kg to get 104 mg.
• Use the standard formula .
• Divide 104 mg (desired dose) with 0.2 g/ml (drug on hand), then divide by 1,000 to get 0.52 ml.
• Finally, round it off to one decimal place to get 0.5 ml.

20. 9. Question
• A 53-year-old female patient with pancreatic cancer was placed under your
care. Her primary care provider orders an administration of 150 mcg
(micrograms) of morphine sulfate per kilogram (kg) every four (4) hours or as
needed for pain management. The available drug on hand is morphine 15
mg/mL. How many mL will you give on the first dose if the patient weighs 90
kg?
Correct answer: 0.9 mL
150 mcg/kg x 90 kg (patient’s weight) = 13,500 mcg
Convert 13,500 mcg to mg by dividing it with 1,000 mcg/mg to get 13.5 mg.
Use the standard formula .
Divide 13.5 mg (desired dose) with 15 mg (drug on hand), then multiply by 1 mL (vehicle) to get 0.9 mL.