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Days' supply calculator

Work out how many days a prescription lasts. Pick the product, enter the quantity and the directions, and get whole days with each step shown.

For study only. Not for patient care or clinical decisions.

Use it to check your homework and exam practice. In practice, follow the order, your facility's policy and the product label.

tablets
mL
Packages dispensed (U-100)

U-100 = 100 units per mL: a 3 mL pen holds 300 units, a 10 mL vial 1,000 units.

Bottle
mL
drops/mL

Use the drops-per-mL number the question or product gives.

Inhalers dispensed
inhalers
puffs each

The number of puffs (actuations) is on the label.

Directions
tablet(s)
times a day

For "as needed" directions, use the most allowed in a day.

Eyes or ears
units/day
priming/day

Add priming units only if the question includes them. An opened pen or vial is often good for only 28 days (check the label), which can limit the days' supply.

Days' supply

45 days

90 tablets ÷ 2 a day

  1. Daily use: 1 tablet × 2 times a day = 2 tablets a day.
  2. Divide: 90 ÷ 2 = 45 days.

No partial day to drop.

The example (90 tablets, 1 tablet twice a day) is filled in. Change any number to recalculate.

How to calculate days' supply by hand

Days' supply = quantity dispensed ÷ amount used per day
  1. Convert the package and the dose into the same unit: tablets, mL, units, drops or puffs. 1 tsp = 5 mL, 1 tbsp = 15 mL.
  2. Count the daily use: amount per dose × times a day. For "as needed," use the most the directions allow.
  3. Divide, then drop any partial day if the question wants whole days.

Worked example: amoxicillin 400 mg/5 mL, 150 mL, "1 tsp twice a day." 1 tsp = 5 mL, so daily use is 10 mL. 150 ÷ 10 = 15 days.

Read the pharmacy calculations lesson

Frequently asked questions

How do you calculate days' supply?
Divide the quantity dispensed by the amount the patient uses in one day. 90 tablets taken 1 tablet twice a day is 90 ÷ 2 = 45 days.
Do you round days' supply up or down?
Follow the rounding the question gives. When it asks for whole days, drop the partial day, because the patient does not have enough for that last full day: 100 ÷ 3 = 33.3, so 33 days.
How do you calculate days' supply for insulin?
Change the package into units first. U-100 insulin has 100 units per mL, so a 10 mL vial holds 1,000 units and a 3 mL pen holds 300 units. Then divide total units by the units used per day.
How do you calculate days' supply for a PRN prescription?
Use the most the directions allow in a day. "1 tablet every 6 hours as needed" allows up to 4 tablets a day, so 30 tablets is 30 ÷ 4 = 7.5, or 7 whole days.
How do you calculate days' supply for eye drops?
Turn the bottle into drops: mL × drops per mL. Then divide by drops per day, counting both eyes if the directions say "each eye." Use the drops-per-mL number the question or product gives.
Is days' supply on the PTCB exam?
Yes. The 2026 PTCE content outline lists pharmacy calculations, including days' supply, quantity and dose, under Order Entry and Processing.

Keep going

Pharmacy calculations lessonDays' supply, dose and conversionsOrder entry practice testFree PTCB questions with answersTop 200 drugsBrand names, classes and schedules

Sources

All accessed October 27, 2026.

  1. PTCB: PTCE Content Outline (effective January 6, 2026)
  2. CMS Medicaid Program Integrity, Pharmacy Auditing and Dispensing Job Aid: Billing Oral Products
  3. FDA, Humalog (insulin lispro) prescribing information: storage after opening
  4. eCFR 21 CFR 101.9(b)(5)(viii): household measure equivalents

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