Pharmacy Math: Mastering Day Supply Calculations

Why Day Supply Matters

Day supply is the number of days a dispensed quantity of medication will last based on the prescribed directions. Correct day supply calculations are essential for insurance adjudication, refill timing, and PTCE exam success. An incorrect day supply can trigger insurance rejections or allow patients to run out of medication too early.

Basic Formula

Day Supply = Quantity Dispensed / Daily Dose

For tablets and capsules, this is straightforward. Dispense 90 tablets with directions to take 1 tablet three times daily: 90 / 3 = 30 days.

Liquid Medications

For liquids, convert the quantity to the dose unit. Example: Amoxicillin 250mg/5mL, dispense 150mL, take 5mL three times daily. Daily consumption = 5mL x 3 = 15mL. Day supply = 150mL / 15mL = 10 days.

Inhalers

Metered dose inhalers (MDIs) list the total number of actuations (puffs). Example: Albuterol 90mcg MDI, 200 actuations, use 2 puffs every 4 to 6 hours as needed. For insurance purposes, assume maximum use: 2 puffs x 6 times daily = 12 puffs per day. Day supply = 200 / 12 = 16.67, rounded down to 16 days. Some insurers use a standard 30-day supply for rescue inhalers.

Insulin Pens

Each insulin pen contains a set number of units (usually 300 units per pen for standard pens). Example: Lantus SoloStar, dispense 5 pens, inject 40 units at bedtime. Each pen lasts 300 / 40 = 7.5 days. Five pens last 7.5 x 5 = 37.5, rounded down to 37 days.

Eye and Ear Drops

Ophthalmic drops typically have approximately 20 drops per mL (varies by manufacturer). Example: Latanoprost 2.5mL bottle, use 1 drop in each eye at bedtime. Daily use = 2 drops. Total drops = 2.5mL x 20 = 50 drops. Day supply = 50 / 2 = 25 days.

Practice Tips

Always read the directions carefully. "As needed" (PRN) directions require using the maximum frequency to calculate day supply. Practice these calculations daily using PTCB practice problems until they become second nature.