Days supply calculation: formula and traps

Days supply is how long a dispensed quantity lasts when the patient takes it exactly as directed. It goes on prescription claims, it decides when a refill is due, and PTCB's 2026 PTCE outline names it directly in area 4.1 ("symbols for days supply, quantity, dose..."). The formula is one line. The work is in reading the sig correctly and getting the units to match.

The formula

Days supply = total quantity dispensed / quantity used per day

Quantity per day = amount per dose x doses per day. Both numbers have to be in the same unit (tablets, mL, units, puffs, drops) before you divide.

Tablets and capsules

Example: #90, sig "1 tab PO tid." Three times a day is 3 tablets a day. 90 / 3 = 30 days.

Example: #60, sig "2 caps PO bid." That's 2 x 2 = 4 capsules a day. 60 / 4 = 15 days.

Example: #4, sig "1 tab PO once weekly." One tablet covers 7 days, so 4 x 7 = 28 days.

Liquids

Convert household measures to mL first: 1 teaspoon = 5 mL, 1 tablespoon = 15 mL.

Example: 150 mL, sig "5 mL PO tid." That's 15 mL a day. 150 / 15 = 10 days.

Example: 100 mL, sig "1 tsp PO bid." One teaspoon is 5 mL, so 10 mL a day. 100 / 10 = 10 days.

Insulin

U-100 insulin has 100 units per mL, so a 10 mL vial holds 1,000 units.

Example: one 10 mL vial of U-100 insulin, sig "30 units subcut daily." 1,000 / 30 = 33.3, so 33 days after rounding down.

Opened insulin also has an in-use expiration set by the manufacturer, which can end before the math runs out. The math gives days supply as prescribed. Whether the patient can use every unit before that expiration is a separate question to flag for the pharmacist.

Inhalers

Divide the total actuations (puffs) in the device by puffs per day.

Example: an inhaler with 200 actuations, sig "2 puffs bid." That's 4 puffs a day. 200 / 4 = 50 days.

Eye drops

Drops are the awkward case, because the number of drops per mL depends on the product and the dropper. So a problem has to give you a drops-per-mL figure, or you use the one your pharmacy works with.

Example (using 20 drops per mL): a 5 mL bottle, sig "1 gtt OU bid" (one drop in each eye, twice a day). The bottle holds 5 x 20 = 100 drops. Use per day = 1 drop x 2 eyes x 2 times = 4 drops. 100 / 4 = 25 days.

The traps

• Misreading frequency. "bid" is twice a day, "tid" three times, "qid" four times, "q6h" every 6 hours, which is 4 times in 24 hours, and "q8h" is 3 times. Mixing up qid (4 times a day) and q4h (6 times a day) turns a 60-tablet supply from 15 days into 10.
• Forgetting the second eye or the second puff. "OU" doubles eye drop use. "2 puffs" doubles inhaler use.
• "As needed" and ranges. For a sig like "1-2 tabs q4-6h prn," calculate from the maximum the directions allow: 2 tablets every 4 hours is 12 a day. #30 / 12 = 2.5, so 2 days. Using the minimum would claim a supply the patient could use up far sooner.
• Rounding up. A result like 33.3 or 2.5 means the patient doesn't have a full day's dose on the last partial day, so round down to whole days.
• Unit mismatch. Dividing mL by teaspoons, or units by mL, gives a number that looks reasonable and is wrong. Convert first.

Checking your work

Run it backward: days supply x daily use should give you back (at most) the quantity dispensed. For the insulin example, 33 x 30 = 990 units, which is within the 1,000 in the vial.

The days supply calculator does the division for any quantity and dose, which makes it handy for checking practice problems. Pharmacy technician math on the PTCE covers the other calculation types, and IV flow rate and drip rate formulas handles the infusion side.

Encodr's PTCB course, with a calculations unit of practice problems, is being built now and isn't live yet. Watch the PTCB exam page for launch.