Free tool

Days Supply Calculator

Work out the days supply for tablets and capsules, oral liquids, or insulin. Enter what was dispensed and the directions, and it shows the math and the rounded-down whole-day answer.

Built for pharmacy technician (PTCE) exam prep

The prescription

Days supply

Enter the quantity and directions to see the days supply.

Days supply is one of the calculations the PTCE keeps coming back to - Encodr uses spaced repetition so the conversions are automatic by exam day.

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The days supply formula

days supply = quantity dispensed ÷ quantity used per day, where quantity per day is the amount per dose times the number of doses per day. Put both in the same unit first: for liquids that means converting the dose to mL (1 teaspoon = 5 mL, 1 tablespoon = 15 mL), and for insulin it means converting vials or pens to total units.

Round down to a whole number of days. A partial day at the end isn't a full day of therapy - the patient runs out partway through it - so reporting it as a full day would overstate the supply. That is also how exam answer keys expect it.

Worked examples: 90 tablets, 1 tablet three times a day is 90 ÷ 3 = 30 days. 150 mL of a suspension at 1 teaspoon three times a day is 150 ÷ (5 × 3) = 150 ÷ 15 = 10 days. One 1,000-unit vial at 20 units twice a day is 1,000 ÷ 40 = 25 days. Five 300-unit pens at 10 units three times a day with 2 priming units per injection is 1,500 ÷ ((10 + 2) × 3) = 1,500 ÷ 36 = 41.67, which rounds down to 41 days.

For insulin, also check the product's in-use dating once opened - if the vial or pen must be discarded before the calculated supply runs out, that limit is the practical days supply. More pharmacy tech prep: the PTCB exam deck and the exam pass score calculator.

From the blog

Days supply calculation: formula and traps

The days supply formula for tablets, liquids, insulin, inhalers and eye drops, with worked examples and the traps behind most wrong answers.

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Pharmacy technician math on the PTCE

The calculation types behind the PTCE outline (conversions, doses, days supply, percent strength, dilutions, alligation, IV rates) with worked examples.

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