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Days supply is the total quantity dispensed divided by the quantity used per day. The difficulty is never the division; it is reading the Sig correctly to find the daily use.
A calculation the exam likes because it tests two things at once: arithmetic and Sig comprehension.
The formula
Total quantity dispensed, divided by quantity used per day. That is all of it.
Everything hard about these questions happens before the division.
Reading the Sig
| Sig | Doses per day |
|---|---|
| BID | 2 |
| TID | 3 |
| QID | 4 |
| Q6H | 4 |
| Q8H | 3 |
| QD or daily | 1 |
Note that Q6H and QID both give four. Questions use the hourly form specifically because it requires a conversion the abbreviation does not.
Worked
Ninety tablets, one tablet TID. Three per day, so 90 divided by 3 is 30 days.
Sixty tablets, two tablets BID. That is four per day, not two, so 60 divided by 4 is 15 days.
The second example is where candidates fail. BID is two doses; two tablets BID is four tablets. Multiplying the quantity per dose by the frequency is a step people skip when the quantity per dose is not one, and the distractor for 30 days will be waiting.
Liquids
Same method with a volume. A 150 mL bottle, 5 mL TID, is 15 mL a day, so 10 days.
Where the Sig is in teaspoons, convert first: one teaspoon is 5 mL.
Insulin and inhalers
These are the hard variants. Insulin is calculated in units against the units per mL in the vial, and an inhaler in actuations against actuations per day.
The method does not change. What changes is that you have to find the total quantity in the right unit before dividing, and that unit is often on the packaging rather than in the Sig.
Why it matters beyond the exam
Days supply drives refill timing and insurance claims. A wrong figure is a rejected claim or an early refill, which is why it is examined rather than assumed.
Common questions
How do I calculate days supply?
Total quantity dispensed divided by the quantity used per day.
What is the common mistake?
Forgetting to multiply the quantity per dose by the number of doses. Two tablets BID is four a day.
How many doses is Q6H?
Four a day, the same as QID.
How does it work for liquids?
The same way, using volume. Convert teaspoons to mL first.
Why does days supply matter?
It drives refill timing and insurance claims, so a wrong figure causes rejections or early refills.