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newSchoolWorks.com › VoTechWorks › Health & Personal Services › Pharmacy Technician › Pharmacy Calculations and Order Entry › Lesson 5
HLT-PHAR41-03 · Lesson 5 · 15 clock hours · theory

Quantity and days-supply calculations

Objective. Calculate quantity to dispense and days' supply for tablets, liquids, inhalers, insulin, eye drops and topicals.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed pharmacist or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Quantity and Days' Supply Matter

Every prescription requires two critical numbers before it can be filled: quantity to dispense and days' supply. The quantity tells the pharmacy how many units to place in the bottle or package. The days' supply tells the patient—and the insurance plan—how long that supply will last. An incorrect days' supply can cause an early-refill rejection, a gap in therapy, or an insurance audit. Getting these numbers right is a core technician skill.

The Master Formula

For almost every dosage form, the same logic applies:

> Quantity = Dose per administration × Administrations per day × Days' supply

Rearranged to find days' supply when quantity is already set:

> Days' supply = Quantity ÷ (Dose per administration × Administrations per day)

Always read the sig (directions) carefully before you calculate.

Tablets and Capsules

Tablets and capsules are the most straightforward. Count the number of dosage units taken per day and divide the dispensed quantity by that number.

Example sig: Take 2 tablets twice daily.

Watch for tapered regimens (e.g., prednisone dose packs), where the dose changes each day. Add up the total tablets consumed day by day to find the days' supply rather than using a simple division.

Oral Liquids

For liquids, convert the dose from the sig into milliliters (mL), then calculate units per day.

Key conversion: 1 teaspoon (tsp) = 5 mL; 1 tablespoon (tbsp) = 15 mL

Example sig: Take 1 teaspoon three times daily.

Always enter the dispensed quantity in mL on the label, not teaspoons.

Metered-Dose and Dry-Powder Inhalers

Inhalers are labeled with total actuations (puffs) per canister. Divide by the number of puffs used per day.

Example sig: Inhale 2 puffs twice daily as needed for wheezing. Canister contains 200 actuations.

If a patient has both a rescue inhaler (albuterol) and a maintenance inhaler, calculate each separately.

Insulin

Insulin is dispensed in vials or prefilled pens. Standard vials contain 10 mL; standard pens contain 3 mL (300 units at U-100 concentration).

Example sig: Inject 30 units subcutaneously twice daily.

Always round up vials so the patient is not left short; then recalculate days' supply for the actual quantity dispensed.

Eye Drops and Ear Drops

One drop ≈ 0.05 mL is the standard pharmacy assumption used for days' supply calculations. Multiply the number of drops per dose by administrations per day to get drops used daily, then convert to mL.

Example sig: Instill 1 drop in each eye twice daily. Bottle contains 5 mL.

Some insurers use 0.05 mL/drop; some accept a range. Use 0.05 mL unless your pharmacy policy specifies otherwise.

Topical Preparations

Creams, ointments, and gels are dispensed in grams. Coverage estimates (fingertip units or BSA-based tables) can guide appropriate quantity, but days' supply is calculated from what is actually dispensed.

Common approach: Determine grams used per application and applications per day.

Example sig: Apply a thin layer to affected area twice daily. 30 g tube dispensed; estimated use is 2 g per application.

Insurance Days' Supply Limits

Most commercial insurance plans and Medicare Part D limit fills to 30 days for a standard fill and 90 days for maintenance medications dispensed through mail order or preferred retail pharmacies. Entering a days' supply greater than the plan allows causes a claim rejection. Always calculate the true days' supply and dispense only what the prescription authorizes.

Rounding Rules Summary

Key terms6

Days' supplyThe number of days a dispensed quantity will last when used exactly as directed by the sig.
ActuationA single measured dose released from a metered-dose or dry-powder inhaler; commonly called a puff.
U-100 insulinThe most common insulin concentration in the United States, containing 100 units of insulin per milliliter.
SigAbbreviated from the Latin signa; the directions for use written on a prescription that tell the patient how to take the medication.
Quantity to dispenseThe total number of dosage units (tablets, mL, grams, inhalers, vials, etc.) placed in the dispensed package.
Fingertip unit (FTU)A practical estimate of topical medication (approximately 0.5 g) squeezed from a standard tube nozzle to the end of an adult index finger; used to estimate how much cream or ointment covers a given body area.

Worked examples4

1. Rx: Amoxicillin 250 mg/5 mL suspension. Sig: Give 1 teaspoon by mouth three times daily for 10 days. How many milliliters should be dispensed?
  1. Step 1 – Convert dose to mL: 1 teaspoon = 5 mL per dose.
  2. Step 2 – Calculate mL per day: 5 mL × 3 doses/day = 15 mL/day.
  3. Step 3 – Calculate total quantity: 15 mL/day × 10 days = 150 mL.
  4. Step 4 – Verify: 150 mL dispensed ÷ 15 mL/day = 10 days. Correct.
Answer: Dispense 150 mL; days' supply = 10 days.
2. Rx: Albuterol HFA inhaler. Sig: Inhale 2 puffs four times daily as needed. The inhaler contains 200 actuations. What is the days' supply using maximum use?
  1. Step 1 – Determine maximum puffs per day: 2 puffs × 4 times/day = 8 puffs/day.
  2. Step 2 – Calculate days' supply: 200 actuations ÷ 8 puffs/day = 25 days.
  3. Step 3 – Verify: 25 days × 8 puffs/day = 200 actuations. Correct.
Answer: Days' supply = 25 days.
3. Rx: Insulin glargine U-100. Sig: Inject 40 units subcutaneously once daily. Dispense a 30-day supply. How many 10 mL vials are needed, and what is the actual days' supply?
  1. Step 1 – Calculate total units needed: 40 units/day × 30 days = 1,200 units.
  2. Step 2 – Calculate units per vial: 10 mL × 100 units/mL = 1,000 units/vial.
  3. Step 3 – Determine vials needed: 1,200 ÷ 1,000 = 1.2 vials → round up to 2 vials.
  4. Step 4 – Calculate actual days' supply: 2 vials × 1,000 units/vial = 2,000 units total; 2,000 ÷ 40 units/day = 50 days.
Answer: Dispense 2 vials; actual days' supply = 50 days.
4. Rx: Timolol 0.5% ophthalmic solution 10 mL. Sig: Instill 1 drop in each eye once daily. What is the days' supply?
  1. Step 1 – Determine drops per day: 1 drop × 2 eyes × 1 time/day = 2 drops/day.
  2. Step 2 – Convert drops to mL: 2 drops × 0.05 mL/drop = 0.10 mL/day.
  3. Step 3 – Calculate days' supply: 10 mL ÷ 0.10 mL/day = 100 days.
  4. Step 4 – Verify: 100 days × 0.10 mL/day = 10 mL. Correct.
Answer: Days' supply = 100 days.

The math in this lesson2

Learners compute quantity and days supply, e.g., 5 mL three times daily for 10 days = 150 mL.
6.RP.3N-Q.1
Brush up in the math halls at newSchoolWorks.com:
In class: Days-supply problem sets · Insurance days-supply limits · Eye-drop and insulin-pen cases
Materials: Product packages, calculators
Check: Quiz ≥ 90%

Practice check18

Answer each question, then check it. Nothing is recorded; this is your own practice.

Order Entry and Processing
1. Sig: 1 tablet once daily. Quantity dispensed: 30 tablets. What is the days' supply?
Order Entry and Processing
2. Sig: 5 mL by mouth twice daily. How many days will a 100 mL bottle last?
Order Entry and Processing
3. Sig: 1 tablet twice daily. Quantity dispensed: 28 tablets. What is the days' supply?
Order Entry and Processing
4. Sig: 2.5 mL by mouth four times daily. How many days will a 100 mL bottle last?
Order Entry and Processing
5. Sig: 5 mL by mouth three times daily. How many days will a 150 mL bottle last?
Order Entry and Processing
6. Sig: 1 tablet three times daily. Quantity dispensed: 45 tablets. What is the days' supply?
Order Entry and Processing
7. Sig: 10 mL by mouth twice daily. How many days will a 200 mL bottle last?
Order Entry and Processing
8. Sig: 1 tablet three times daily. Quantity dispensed: 90 tablets. What is the days' supply?
Order Entry and Processing
9. Sig: 7.5 mL by mouth twice daily. How many days will a 120 mL bottle last?
Order Entry and Processing
10. Sig: 1 tablet four times daily. Quantity dispensed: 120 tablets. What is the days' supply?
Order Entry and Processing
11. A prescription reads: Take 1 tablet by mouth twice daily. The pharmacist dispenses 60 tablets. What is the days' supply?
Order Entry and Processing
12. A prescription for prednisone 10 mg tablets has a tapering schedule: 4 tablets daily for 3 days, then 3 tablets daily for 3 days, then 2 tablets daily for 3 days, then 1 tablet daily for 3 days. How many total tablets are needed?
Order Entry and Processing
13. Which volume is equivalent to one tablespoon?
Order Entry and Processing
14. A metered-dose inhaler contains 120 actuations. The sig reads: Inhale 2 puffs twice daily. What is the days' supply?
Medications
15. A patient uses insulin lispro U-100. The sig reads: Inject 25 units subcutaneously three times daily. How many units does the patient use per day?
Order Entry and Processing
16. A patient brings in a prescription for latanoprost 0.005% ophthalmic solution 2.5 mL. The sig reads: Instill 1 drop in the left eye once daily at bedtime. Using 0.05 mL per drop, what is the days' supply?
Order Entry and Processing
17. A prescription for triamcinolone 0.1% cream is dispensed as a 60 g tube. The sig reads: Apply to affected area twice daily. The patient uses approximately 3 g per application. What is the days' supply?
Patient Safety and Quality Assurance
18. A pharmacy technician enters a days' supply that is longer than the patient's insurance plan allows. What is the most likely immediate consequence?
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