How to Calculate Days Supply
Updated
Days supply is the number of days a dispensed quantity will last, and it is one of the most predictable calculations on the PTCE. The formula never changes:
days supply = total quantity ÷ amount used per day
Almost nobody gets this wrong because of the division. They get it wrong because the container and the sig are in different units, so the real skill is converting the container into whatever the sig counts: drops, units, actuations or grams. Then you divide, then you round down.
The four steps
- Convert the container into the unit the sig uses.
- Work out how much the patient uses in one day.
- Divide the total by the daily use.
- Round down to a whole day, since a partial day is not a full day of therapy.
Worked example: tablets
The simplest case
Lisinopril 10 mg, quantity 60. Sig: 1 tablet by mouth twice daily.
- Step 1. The container and the sig are already in the same unit: tablets. No conversion needed, so the total is 60 tablets.
- Step 2. Twice daily means 2 tablets per day.
- Step 3. 60 ÷ 2 = 30.
Answer: 30 days
The trap: Reading BID as once daily. Translate the sig into plain English before you calculate anything.
Worked example: eye drops
Eye drops are the classic days-supply question because the bottle is labelled in millilitres but the sig counts drops. The standard conversion is 20 drops per millilitre.
Both eyes, twice daily
Timolol 0.5% ophthalmic solution, 5 mL bottle. Sig: 1 drop in each eye twice daily.
- Step 1. Convert the bottle to drops: 5 mL × 20 drops/mL = 100 drops.
- Step 2. One drop in each eye is 2 drops per dose, and twice daily makes 4 drops per day.
- Step 3. 100 ÷ 4 = 25.
Answer: 25 days
The trap: Forgetting that 'each eye' doubles the dose. A sig reading one drop in each eye uses two drops, not one.
Worked example: insulin
Insulin is counted in units, and the concentration tells you how many units the container holds. U-100 insulin contains 100 units per millilitre, so a 10 mL vial holds 1,000 units and a 3 mL pen holds 300 units.
A vial of U-100
Insulin glargine U-100, one 10 mL vial. Sig: inject 42 units subcutaneously at bedtime.
- Step 1. Convert the vial to units: 10 mL × 100 units/mL = 1,000 units.
- Step 2. The patient uses 42 units per day.
- Step 3. 1,000 ÷ 42 = 23.8.
Answer: 23 days
The trap: Rounding 23.8 up to 24. Days supply rounds down, because the patient does not have enough insulin for a complete 24th day.
Worked example: inhalers
Inhalers are labelled with a number of actuations (puffs). Use that number, not the weight in grams printed beside it.
A rescue inhaler at maximum use
Albuterol HFA inhaler, 200 actuations. Sig: 2 puffs by inhalation every 6 hours.
- Step 1. The inhaler holds 200 actuations.
- Step 2. Every 6 hours is 4 doses a day, and 2 puffs per dose makes 8 puffs per day.
- Step 3. 200 ÷ 8 = 25.
Answer: 25 days
The trap: Using the canister weight in grams instead of the actuation count, or reading 'every 6 hours' as 6 times a day rather than 4.
Worked example: creams and ointments
Topicals are measured in grams
Hydrocortisone 1% cream, 30 g tube. Sig: apply 1 g to the affected area twice daily.
- Step 1. The tube holds 30 g.
- Step 2. 1 g twice daily is 2 g per day.
- Step 3. 30 ÷ 2 = 15.
Answer: 15 days
The trap: Topical sigs often say 'apply sparingly' with no quantity, in which case days supply cannot be calculated from the sig alone and the pharmacy uses a standard estimate.
Quick reference conversions
| Dosage form | Convert using | Example |
|---|---|---|
| Eye and ear drops | 20 drops per mL | 5 mL = 100 drops |
| U-100 insulin | 100 units per mL | 10 mL vial = 1,000 units |
| Insulin pen | 3 mL per pen | 1 pen = 300 units; box of 5 = 1,500 units |
| Inhalers | Actuations on the label | 200 actuations |
| Creams and ointments | Grams | 30 g tube |
Frequently asked questions
How do you calculate days supply?
Divide the total quantity dispensed by the amount the patient uses per day: days supply = total quantity divided by daily use. The step people miss is converting the container into the same unit the sig uses, such as turning millilitres into drops or a vial into units, before dividing.
How do you calculate days supply for eye drops?
Convert the bottle volume to drops using the standard 20 drops per millilitre, work out the drops used per day, then divide. Remember that a sig reading one drop in each eye uses two drops per dose, not one.
How do you calculate insulin days supply?
U-100 insulin contains 100 units per millilitre, so a 10 mL vial holds 1,000 units and a 3 mL pen holds 300 units. Divide the total units by the units injected per day. A box of five 3 mL pens holds 1,500 units.
How do you calculate days supply for an inhaler?
Use the number of actuations (puffs) printed on the inhaler, not its weight in grams. Divide the total actuations by the puffs used per day. A rescue inhaler dosed two puffs every six hours uses eight puffs a day.
Do you round days supply up or down?
Round down to a whole day. A partial day is not a full day of therapy, so a calculation giving 23.8 days is dispensed and billed as 23 days.
Practise this
Days supply sits in PTCB Order Entry & Processing, the domain that carries most of the exam's calculations. Drill it with the pharmacy math practice test and the order entry quiz. Sig translation comes first in every one of these problems, so keep the pharmacy abbreviations reference nearby.
PTCB Quiz Prep is an independent study resource, not affiliated with the Pharmacy Technician Certification Board. The 20 drops per mL figure is the standard pharmacy convention used for calculations; actual drop size varies by dropper and solution.