Days supply calculations for the PTCB exam

How to calculate days supply, worked for the dosage forms that actually catch people out — eye drops, insulin and inhalers — plus the rounding rule and the dimensional-analysis setup that stops the error before the arithmetic.

Test yourself. Practice questions on this topic, each with the reasoning written out.

Days Supply

Days supply = total quantity dispensed divided by the amount used per day. To calculate total quantity needed:

  1. Read the Sig to find how much per dose.
  2. Count doses per 24 hours.
  3. Multiply by the number of days ordered.

Worked example — oral solid, splitting

Amoxicillin 500 mg PO TID × 7 days. Stock: 250 mg capsules.

Worked example — 24-hour hospital supply

Enalapril 5 mg PO BID. Stock: 5 mg tablets. 1 × 2 = 2 tablets / 24 h.

Clindamycin 300 mg PO q6h. Stock: 150 mg capsules. 2 capsules × 4 doses/day = 8 capsules / 24 h.

Eye-Drop Days Supply

A standard workflow the PTCE® likes. Assume 1 mL = 20 drops unless told otherwise.

Tobramycin ophthalmic, 2 drops in each eye six times daily × 7 days. Bottle = 5 mL.

  1. Drops per dose: 4 drops (both eyes).
  2. Drops per day: 4 × 6 = 24 drops.
  3. Total drops: 24 × 7 = 168 drops.
  4. Total mL needed: 168 ÷ 20 = 8.4 mL.
  5. Bottles: 8.4 ÷ 5 = 1.68, round up to 2 bottles.

Always round up on ophthalmic dispensing — bottle loss during administration is expected.

Rounding

Value Tenths Hundredths Thousandths
7.84621 7.8 7.85 7.846
0.67891 0.7 0.68 0.679
15.20494 15.2 15.20 15.205
42.33338 42.3 42.33 42.333

Rule: look one place to the right of where you want to round — if that digit is 5 or greater, round up; if it is 4 or less, round down.

Practical rounding: the number must be measurable. Do not round to 3.847 mL if the dispensing device is a 5 mL oral syringe with 0.5 mL markings. Do not round to 7.3 days when the prescription is written in whole days.

Dimensional Analysis

A calculation method that handles conversions, dose calculations, and IV rates in one chain of fractions.

Five steps

  1. Write the unit you want in the final answer on the far left.
  2. List each given value with its unit.
  3. Identify any conversion factors you need.
  4. Arrange the fractions so that every unit except the target cancels.
  5. Do the arithmetic last.

Worked example — pediatric dose

Diphenhydramine 5 mg/kg/day. Child weighs 35 lb. How many mg/day?

Worked example — drops/min

0.5 L infused over 2 hours; infusion set 10 gtt/mL.

Related

All Order Entry and Processing guides · What is on the PTCE and how it is weighted · The Federal Requirements domain

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