Master formula — works for every drug type
Days Supply = Total Quantity Dispensed ÷ Amount Used Per Day
The only formula you need. The challenge is knowing what counts as "quantity" and "per day" for each drug type.
Tablets & Capsules
DS = Qty ÷ (dose × freq/day)
90 tabs, 1 tab BID → 90 ÷ 2 = 45 days
Oral Liquids
DS = Total mL ÷ mL per day
150 mL, 5 mL BID → 150 ÷ 10 = 15 days
Insulin
DS = (pens × units/pen) ÷ units/day
3 pens, 300 u each, 30 u/day → 900 ÷ 30 = 30 days
Inhalers
DS = Total puffs ÷ puffs per day
200 puffs, 2 puffs BID → 200 ÷ 4 = 50 days
Eye / Ear Drops
DS = (vol × 20 drops/mL) ÷ drops/day
2.5 mL, 1 drop each eye QD → 50 ÷ 2 = 25 days
Topicals & Gels
DS = total grams ÷ grams per day
30 g, apply 1 g BID → 30 ÷ 2 = 15 days
Insulin pen sizes — memorize these
Standard U-100 pen (most) = 300 units (3 mL × 100 u/mL)
Standard vial = 1,000 units (10 mL × 100 u/mL)
Toujeo SoloStar U-300 = 450 units (1.5 mL)
Tresiba / Humalog U-200 = 600 units (3 mL × 200 u/mL)
Always find total units first, then divide by daily dose.
Standard vial = 1,000 units (10 mL × 100 u/mL)
Toujeo SoloStar U-300 = 450 units (1.5 mL)
Tresiba / Humalog U-200 = 600 units (3 mL × 200 u/mL)
Always find total units first, then divide by daily dose.
Common inhaler puff counts
Albuterol HFA (ProAir/Ventolin) = 200 puffs
Flovent HFA 44/110/220 mcg = 120 puffs
Symbicort = 120 puffs
Advair Diskus = 60 doses (not puffs)
Spiriva Respimat = 60 actuations (2 puffs = 1 dose)
PRN inhalers: always use maximum frequency for day supply.
Flovent HFA 44/110/220 mcg = 120 puffs
Symbicort = 120 puffs
Advair Diskus = 60 doses (not puffs)
Spiriva Respimat = 60 actuations (2 puffs = 1 dose)
PRN inhalers: always use maximum frequency for day supply.
Eye drop standard
1 drop = approximately 0.05 mL
Therefore: 20 drops per mL (standard used on PTCB)
"Each eye" = 2 drops per dose, not 1
Latanoprost 2.5 mL = ~50 drops total
Always round down to whole days.
Therefore: 20 drops per mL (standard used on PTCB)
"Each eye" = 2 drops per dose, not 1
Latanoprost 2.5 mL = ~50 drops total
Always round down to whole days.
Frequency quick reference
QD / QAM / QHS = 1×/day
BID = 2×/day | TID = 3×/day
QID = 4×/day | Q6H = 4×/day
Q8H = 3×/day | Q4H = 6×/day
Q12H = 2×/day | Q4-6H PRN = use Q4H (max)
BID = 2×/day | TID = 3×/day
QID = 4×/day | Q6H = 4×/day
Q8H = 3×/day | Q4H = 6×/day
Q12H = 2×/day | Q4-6H PRN = use Q4H (max)
★ PTCB exam strategy: Day supply appears in the Order Entry & Processing domain (22.5% of exam). Expect 3–5 day supply questions. The exam will give you a sig code and a quantity — your job is to translate the sig into doses per day, then divide. Insulin and inhaler day supply are the most commonly missed because students skip the step of finding total units/puffs first.
The 4 most common mistakes:
1. Forgetting to multiply pens × units/pen before dividing insulin — always find total units first
2. Counting "each eye" as 1 drop instead of 2
3. Using doses per administration instead of total doses per day
4. Rounding up instead of down — always round DOWN for day supply
1. Forgetting to multiply pens × units/pen before dividing insulin — always find total units first
2. Counting "each eye" as 1 drop instead of 2
3. Using doses per administration instead of total doses per day
4. Rounding up instead of down — always round DOWN for day supply
Six worked examples — one per drug type. These mirror the style and complexity of real PTCB exam questions. Work through each one yourself before reading the solution.
EasyRetailPTCB style
Patient at the counter picks up metformin 500 mg. Rx reads: Take 1 tablet by mouth twice daily. Quantity dispensed: 60 tablets.
How many days will 60 tablets last at 1 tab BID?
Step 1 — Translate BID → doses per day:
BID = 2 times per day → 1 tab × 2 = 2 tablets/day
Step 2 — Apply the formula:
60 tabs ÷ 2 tabs/day = 30 days
✓ Day supply: 30 days
EasyRetailPTCB style
Pediatric Rx: amoxicillin 250 mg/5 mL suspension. Sig: 5 mL by mouth twice daily. Quantity: 150 mL bottle.
How many days will 150 mL last at 5 mL BID?
Step 1 — Daily volume:
5 mL × 2 (BID) = 10 mL/day
Step 2 — Day supply:
150 mL ÷ 10 mL/day = 15 days
✓ Day supply: 15 days
MediumRetailPTCB style
Patient picks up insulin glargine (Lantus) SoloStar pens. Sig: inject 30 units subcutaneously every night at bedtime. Quantity: 1 box of 5 pens (each pen = 300 units).
How many days will 1 box of 5 pens last at 30 units nightly?
Step 1 — Find total units (critical first step):
5 pens × 300 units/pen = 1,500 units total
Step 2 — Day supply:
1,500 units ÷ 30 units/day = 50 days
✓ Day supply: 50 days
The #1 insulin trap: Dividing 5 (pens) by 30 (units/day) gives 0.17 — obviously wrong. You must calculate total units first. Every insulin day supply problem starts with: pens × units/pen.
MediumRetailPTCB style
Patient picks up albuterol HFA (ProAir MDI). Sig: inhale 2 puffs every 4–6 hours as needed for shortness of breath. Canister contains 200 puffs. Use maximum PRN frequency for day supply.
At max PRN use (Q4H = 6 times/day), what is the day supply for 1 inhaler?
Step 1 — Puffs per day at max frequency (Q4H = 6×/day):
2 puffs × 6 = 12 puffs/day
Step 2 — Day supply (round down):
200 ÷ 12 = 16.67 → 16 days
✓ Day supply: 16 days — always round down
MediumRetailPTCB style
Patient picks up latanoprost 0.005% ophthalmic solution. Sig: instill 1 drop in each eye every evening. Bottle size: 2.5 mL.
How many days will the 2.5 mL bottle last?
Step 1 — Total drops in bottle (20 drops per mL):
2.5 mL × 20 = 50 drops total
Step 2 — Drops per day ("each eye" = both eyes = 2 drops):
1 drop × 2 eyes × 1 (QHS) = 2 drops/day
Step 3 — Day supply:
50 drops ÷ 2 drops/day = 25 days
✓ Day supply: 25 days
"Each eye" means 2: Every time you see "each eye" or "both eyes," multiply drops per dose by 2. Entering 1 drop per day instead of 2 doubles the day supply — a common insurance audit flag.
HardPatient scenarioPTCB style
Mrs. Torres has type 2 diabetes. Rx: insulin lispro (Humalog), inject 10 units with each meal and 5 units with bedtime snack. She eats 3 meals per day. Quantity: 2 vials (1,000 units each).
How many days will 2 vials last for Mrs. Torres?
Step 1 — Mealtime units per day:
10 units × 3 meals = 30 units
Step 2 — Add bedtime dose:
30 + 5 = 35 units/day
Step 3 — Total units in 2 vials:
2 × 1,000 = 2,000 units
Step 4 — Day supply (round down):
2,000 ÷ 35 = 57.1 → 57 days
✓ Day supply: 57 days
How this quiz works: Choose your question count and difficulty, then start. After you pick an answer, click Reveal explanation to see the full step-by-step solution — answers never show automatically. Your score and a breakdown by drug type appear at the end so you know exactly where to focus.
ⓘ This lesson has 44 questions in the bank. A 10-question quiz draws a random subset. Run multiple sets to see all 44 over time.
ⓘ This lesson has 44 questions in the bank. A 10-question quiz draws a random subset. Run multiple sets to see all 44 over time.
How many questions?
Question difficulty
Options
Randomize question order
Shuffle every time you start
Timed mode
45-second countdown per question
Number of sets
Each set draws a fresh question pool. Great for drilling.
1
Question 1 of 10
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Score
Quiz complete!
Lesson 3 · Day Supply Calculations · AlabamaPharmTech.com
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