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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.
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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.
Write the units beside each number. If the bottle contains tablets, the daily-use figure must be tablets per day. If the bottle contains millilitres, the daily-use figure must be millilitres per day. Dividing tablets by millilitres per day is an error even if the arithmetic looks tidy.
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.
Show the units in the working
| What you know | Convert to daily use | Days supply |
|---|---|---|
| 90 tablets; one tablet TID | One × three = three tablets per day | 90 ÷ three = 30 days |
| 60 tablets; two tablets BID | Two × two = four tablets per day | 60 ÷ four = 15 days |
| 150 mL; 5 mL TID | 5 × three = 15 mL per day | 150 ÷ 15 = 10 days |
The middle column is the point of the exercise. If you skip it and divide by the frequency alone, the result can look plausible. On the second row, using only BID produces 30 days, exactly twice the correct supply. Work out how much leaves the container in a full day before touching the total quantity.
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.
Read the dispensed label carefully. A direction in teaspoons and a bottle size in mL require a volume conversion before the daily-use calculation. A direction in mL with a tablet quantity, on the other hand, signals that something in the question has been misread; do not force the units to cancel.
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.
For insulin, start by reading the concentration on the exact product and the total volume dispensed. Multiply those to get total units, then divide by the units used each day. For an inhaler, use the stated actuations in the dispensed device and the number of actuations prescribed per day. These examples test whether you recognize what the package contains; do not assume every vial or inhaler has the same size.
When the directions are not a fixed daily dose
A PRN instruction says the medicine is used as needed. 'Up to' a stated number of doses gives a maximum frequency, not a promise that the patient uses that amount every day. Follow the exact question's assumptions and the dispensing rules that apply to the product; if the real prescription is unclear, a technician should refer it to the pharmacist rather than invent a frequency. The same caution applies to a taper, a starter pack, or a schedule that changes by week. Calculate each period separately and add the quantities before estimating the supply.
A quick error check
- Underline the total amount actually dispensed and its unit.
- Translate the Sig into amount per dose and doses per day.
- Multiply those amounts to get daily use in the same unit as the dispensed amount.
- Divide the total by daily use and ask whether the result makes sense for the package size.
If the answer doubles when the instruction changes from one tablet to two tablets per dose, the working is inverted: more medicine taken each day should shorten the supply, not extend it. This sanity check catches many wrong options before you commit to one.
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.
What if the Sig says as needed?
Do not assume a fixed daily use unless the question supplies one. Use the stated maximum only when that is the requested basis, and refer an unclear real prescription to the pharmacist.