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Days Supply Calculation Practice

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Days supply is how long a dispensed quantity lasts, which is what insurance billing and refill timing are calculated from. Eye and ear drops, tablet and capsule counts, and metered-dose inhalers are the high-yield cases, each converting a total quantity into a per-day usage.

Formulasdays = total quantity (drops, tablets, or puffs) ÷ amount used per dayeye drops assume 20 drops/mL  •  always round DOWN
Worked example

A 5 mL eye drop bottle (20 drops/mL = 100 drops), 1 drop in each eye twice daily = 4 drops/day. 100 ÷ 4 = 25 days.

Round down. You can't bill for a partial day. Inhalers count total actuations (puffs), not mL.

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Days Supply
An ipratropium/albuterol Respimat inhaler contains 60 actuations (puffs). The sig is 1 puff four times daily (QID). What is the days supply? Round down to the nearest whole number.
days

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How to set up a days supply calculation

Days supply is how many days a dispensed quantity lasts at the prescribed sig. Insurance billing, refill-too-soon rejections and controlled-substance limits are all computed from it, so it's the most common calculation in a community pharmacy and a fixture on the exam. The method is one division, and the two skills are counting the total quantity in the right unit and counting the daily use in that same unit.

days supply = total quantity ÷ quantity used per day, rounded DOWN

Rounding down is not optional. A 33.3-day supply is 33 days, because on day 34 there isn't a full dose left. Billing a partial day as a whole one is a claim for medication the patient doesn't have.

Tablets and capsules

100 tablets, 1 tablet three times daily.

per day = 1 × 3 = 3 tablets

days = 100 ÷ 3 = 33.33, round down → 33 days

When the sig is written as a frequency (q8h, q12h) rather than "times daily", convert: q8h is 3 doses a day, q6h is 4, q12h is 2. "Every 4 hours while awake" is conventionally 6 doses. A PRN sig is billed on its maximum: "1 to 2 tablets every 6 hours as needed" is 8 tablets a day for days-supply purposes.

Eye and ear drops

A 10 mL ophthalmic bottle, 1 drop in each eye three times daily. Assume 20 drops per mL.

total drops = 10 × 20 = 200 drops

per day = 1 × 2 × 3 = 6 drops

days = 200 ÷ 6 = 33.33, round down → 33 days

The drops-per-mL figure is a convention, and the question gives it; 20 is the common one, but use whatever number the stem states. "Each eye" doubles the daily count and is the detail most often missed. In practice most opened ophthalmic bottles carry a 28-day discard-after-opening limit, so a calculated 33 days may be capped at 28 at the bench; the practice problems here say to assume the bottle is used until empty.

Inhalers

An albuterol HFA inhaler labelled 200 actuations, 2 puffs four times daily.

per day = 2 × 4 = 8 puffs

days = 200 ÷ 8 = 25 days

Inhalers are counted in actuations, never in mL or grams, and the number of actuations is on the label. A rescue inhaler prescribed as needed is billed on the maximum daily puffs the sig allows.

Oral liquids

A 150 mL bottle of amoxicillin 400 mg/5 mL, 7.5 mL twice daily.

per day = 7.5 × 2 = 15 mL

days = 150 ÷ 15 = 10 days

Work in mL. The strength only matters when the sig is written in milligrams, in which case convert the dose to mL first: a 600 mg dose of the 400 mg/5 mL suspension is

dose volume = 600 ÷ 400 × 5 = 7.5 mL

and then the calculation is the same.

Where students lose the points

10 Days Supply Calculation practice problems with worked solutions

Every problem below is worked start to finish. Try it yourself first, then check the steps. For a fresh problem every time, use the practice box above. It generates unlimited variations of these same question types.

Problem 1: Days Supply

An ophthalmic solution comes in a 10 mL bottle (assume 20 drops/mL). Sig: 1 drop in the left eye (OS) four times daily (QID). Assume the bottle is used until empty. What is the days supply? Round down to the nearest whole number.

Answer 50 days

Solution
  1. Total drops: 10 mL × 20 drops/mL = 200 drops.
  2. Per day: 1 drop/dose × 4 doses/day = 4 drops/day.
  3. 200 drops ÷ 4 drops/day = 50 → round down = 50 days.
  4. Rounded as the question asks: 50 days.

Drops are the classic days-supply case: total volume × drops/mL, ÷ daily drops, rounded down. In practice most opened ophthalmics carry a 28-day discard-after-opening limit, so a very long calculated supply is capped at the bedside even when the arithmetic says otherwise.

Problem 2: Days Supply

A 237 mL bottle of acetaminophen 160 mg/5 mL suspension is dispensed. Sig: 10 mL by mouth four times daily (QID). What is the days supply? Round down to the nearest whole number.

Answer 5 days

Solution
  1. Volume per day: 10 mL/dose × 4 doses/day = 40 mL/day.
  2. Days supply: 237 mL ÷ 40 mL/day ≈ 5.92 → round down = 5 days.
  3. Rounded as the question asks: 5 days.

For liquids, work in mL. The mg/5 mL strength is not needed unless the sig is written in mg. Always divide the dispensed volume by the daily volume, then round down.

Problem 3: Days Supply

An ophthalmic solution comes in a 2.5 mL bottle (assume 20 drops/mL). Sig: 1 drop in the left eye (OS) three times daily (TID). Assume the bottle is used until empty. What is the days supply? Round down to the nearest whole number.

Answer 16 days

Solution
  1. Total drops: 2.5 mL × 20 drops/mL = 50 drops.
  2. Per day: 1 drop/dose × 3 doses/day = 3 drops/day.
  3. 50 drops ÷ 3 drops/day ≈ 16.67 → round down = 16 days.
  4. Rounded as the question asks: 16 days.

Drops are the classic days-supply case: total volume × drops/mL, ÷ daily drops, rounded down. In practice most opened ophthalmics carry a 28-day discard-after-opening limit, so a very long calculated supply is capped at the bedside even when the arithmetic says otherwise.

Problem 4: Days Supply

A 120 mL bottle of prednisolone 15 mg/5 mL solution is dispensed. Sig: 15 mL by mouth twice daily (BID). What is the days supply? Round down to the nearest whole number.

Answer 4 days

Solution
  1. Volume per day: 15 mL/dose × 2 doses/day = 30 mL/day.
  2. Days supply: 120 mL ÷ 30 mL/day = 4 → round down = 4 days.
  3. Rounded as the question asks: 4 days.

For liquids, work in mL. The mg/5 mL strength is not needed unless the sig is written in mg. Always divide the dispensed volume by the daily volume, then round down.

Problem 5: Days Supply

A prescription for 180 tablets of metformin 500 mg is written to take 1 tablet twice daily (BID). What is the days supply? Round down to the nearest whole number.

Answer 90 days

Solution
  1. Tablets per day: 1 × 2 = 2 tablets/day.
  2. Days supply: 180 tablets ÷ 2 tablets/day = 90 → round down = 90 days.
  3. Rounded as the question asks: 90 days.

The workhorse case: total quantity ÷ (amount per dose × doses per day), rounded down.

Problem 6: Days Supply

A 60 mL bottle of cefdinir 250 mg/5 mL suspension is dispensed. Sig: 5 mL by mouth once daily. What is the days supply? Round down to the nearest whole number.

Answer 12 days

Solution
  1. Volume per day: 5 mL/dose × 1 doses/day = 5 mL/day.
  2. Days supply: 60 mL ÷ 5 mL/day = 12 → round down = 12 days.
  3. Rounded as the question asks: 12 days.

For liquids, work in mL. The mg/5 mL strength is not needed unless the sig is written in mg. Always divide the dispensed volume by the daily volume, then round down.

Problem 7: Days Supply

A 240 mL bottle of prednisolone 15 mg/5 mL solution is dispensed. Sig: 15 mL by mouth twice daily (BID). What is the days supply? Round down to the nearest whole number.

Answer 8 days

Solution
  1. Volume per day: 15 mL/dose × 2 doses/day = 30 mL/day.
  2. Days supply: 240 mL ÷ 30 mL/day = 8 → round down = 8 days.
  3. Rounded as the question asks: 8 days.

For liquids, work in mL. The mg/5 mL strength is not needed unless the sig is written in mg. Always divide the dispensed volume by the daily volume, then round down.

Problem 8: Days Supply

A fluticasone/salmeterol HFA inhaler contains 120 actuations (puffs). The sig is 2 puffs twice daily (BID). What is the days supply? Round down to the nearest whole number.

Answer 30 days

Solution
  1. Puffs per day: 2 puffs/dose × 2 doses/day = 4 puffs/day.
  2. Days supply: 120 actuations ÷ 4 actuations/day = 30 → round down = 30 days.
  3. Rounded as the question asks: 30 days.

Inhaler days supply uses total actuations (not mL). Rescue/PRN inhalers are billed on maximum daily puffs.

Problem 9: Days Supply

An ophthalmic solution comes in a 5 mL bottle (assume 20 drops/mL). Sig: 1 drop in the right eye (OD) three times daily (TID). Assume the bottle is used until empty. What is the days supply? Round down to the nearest whole number.

Answer 33 days

Solution
  1. Total drops: 5 mL × 20 drops/mL = 100 drops.
  2. Per day: 1 drop/dose × 3 doses/day = 3 drops/day.
  3. 100 drops ÷ 3 drops/day ≈ 33.33 → round down = 33 days.
  4. Rounded as the question asks: 33 days.

Drops are the classic days-supply case: total volume × drops/mL, ÷ daily drops, rounded down. In practice most opened ophthalmics carry a 28-day discard-after-opening limit, so a very long calculated supply is capped at the bedside even when the arithmetic says otherwise.

Problem 10: Days Supply

A 100 mL bottle of amoxicillin 400 mg/5 mL suspension is dispensed. Sig: 5 mL by mouth three times daily (TID). What is the days supply? Round down to the nearest whole number.

Answer 6 days

Solution
  1. Volume per day: 5 mL/dose × 3 doses/day = 15 mL/day.
  2. Days supply: 100 mL ÷ 15 mL/day ≈ 6.67 → round down = 6 days.
  3. Rounded as the question asks: 6 days.

For liquids, work in mL. The mg/5 mL strength is not needed unless the sig is written in mg. Always divide the dispensed volume by the daily volume, then round down.

Sources and how far they go

Where a convention on this page comes from a published standard, the standard is named. Where it comes from a textbook or from ordinary practice instead, the entry says so. That difference matters when a preceptor or an exam disagrees with you.

Citations are to the source of the method. They are not a review of this page, and none of these bodies has reviewed or endorsed it.

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