How to work out days' supply, quantity, dose volume and conversions
PTCB topic: Pharmacy calculations, Sig codes and abbreviations
- Days' supply = quantity dispensed ÷ amount used per day.
- Quantity = amount per dose × doses per day × days.
- q12h is 2 doses a day, q8h is 3, q6h is 4.
- Convert first: 1 kg = 2.2 lb, 1 tsp = 5 mL.
- Percent w/v is grams per 100 mL; dilute with C1V1 = C2V2.
Key facts
- Days' supply = quantity dispensed ÷ quantity used per day; for a prn Sig, use the most the Sig allows per day.
- Quantity to dispense = amount per dose × doses per day × number of days.
- q12h is 2 doses a day, q8h is 3, q6h is 4 and q4h is 6.
Show all 7 key factsShow fewer
- Dose volume comes from a proportion: dose (mg) × volume (mL) ÷ strength (mg) of the stock.
- 1 kg = 2.2 lb, 1 tsp = 5 mL, 1 tbsp = 15 mL, 1 fl oz = 30 mL, 1 g = 1,000 mg and 1 mg = 1,000 mcg.
- °F = (°C × 1.8) + 32 and °C = (°F − 32) ÷ 1.8.
- A percent (w/v) strength is grams per 100 mL, and dilutions follow C1V1 = C2V2.
Quick check1 question · 2 more at the end
Rx: metformin 500 mg tablets. Sig: 1 tab PO b.i.d. with meals. Disp: #60. What days' supply should be entered for this fill?
Correct answer: B.
Days' supply = quantity dispensed ÷ quantity the patient uses per day. b.i.d. means twice a day, so the patient uses 2 tablets daily and 60 ÷ 2 = 30 days. CMS billing guidance uses this divide-by-daily-doses method and warns that a wrong days' supply can cause claim rejections.
A Dividing by 4 treats b.i.d. (twice a day) as q.i.d. (four times a day).
B Correct: 60 tablets ÷ 2 tablets a day = 30 days.
C This counts each tablet as one day and ignores the twice-a-day frequency.
D This multiplies the quantity by the doses per day instead of dividing.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Days' supply is the quantity dispensed divided by the amount used per day, and quantity is the amount per dose times doses per day times days. Find a dose volume with a proportion such as mg/mL. Convert first: 1 kg = 2.2 lb, 1 tsp = 5 mL, 1 mg = 1,000 mcg, °F = °C × 1.8 + 32.
1 What does the PTCE expect you to calculate?The 2026 PTCE outline applies pharmacy math to days' supply, quantity, dose, concentration and dilution. 1 min
The 2026 PTCE outline applies pharmacy math to days' supply, quantity, dose, concentration and dilution. It also lists formulas, ratios, proportions and conversions. The arithmetic is simple; setting the problem up correctly is the real skill.
Most errors come from three slips: the wrong number of doses per day, a skipped unit conversion, or a ratio turned upside down.
2 How do you work out days' supply?Days' supply is the quantity dispensed divided by the amount the patient uses each day. 1 min
Days' supply is the quantity dispensed divided by the amount the patient uses each day. CMS billing guidance uses this method, and a wrong days' supply can cause claim rejections.
- Count the daily use from the Sig: tablets per dose × doses per day.
- Divide the quantity by that daily use.
- If the result has a decimal, follow the rounding the question states (for example, drop the partial day).
| Sig | Doses a day |
|---|---|
| daily | 1 |
| b.i.d. or q12h | 2 |
| t.i.d. or q8h | 3 |
| q.i.d. or q6h | 4 |
| q4h | 6 |
Example: 1 tablet b.i.d., #60 → 60 ÷ 2 = 30 days. For a prn (as needed) Sig, CMS says to use the most the Sig allows in a day. For a drug not taken daily, multiply the number of doses by the interval: 4 once-weekly tablets = 28 days.
To check your practice answers, use the free days' supply calculator. It covers tablets, liquids, insulin, drops and inhalers.
3 How do you find the quantity to dispense?Quantity is the amount per dose times doses per day times the number of days. 1 min
Quantity is the amount per dose times doses per day times the number of days. For prednisone 40 mg daily for 5 days with 10 mg tablets, each dose is 4 tablets, so 4 × 1 × 5 = 20 tablets.
For a taper, add up each step separately. For insulin pens, turn the package into units first: a 3 mL pen of 100 units/mL holds 300 units.
4 How do you find a dose volume from a concentration?A proportion turns a dose in milligrams into a volume in milliliters. 1 min
A proportion turns a dose in milligrams into a volume in milliliters. Keep the same units in the same position:
400 mg / 5 mL = dose (mg) / x mL
For a 320 mg dose: 400x = 320 × 5, so x = 4 mL. Check that the answer makes sense: a dose smaller than 400 mg must be less than 5 mL.
Weight-based doses use kilograms. Convert pounds to kilograms first (divide by 2.2), multiply by the mg/kg dose, then split the daily total by the number of doses.
5 Which conversions should you know by heart?Memorize this short conversion list, because the PTCE outline lists conversions in this area. 1 min
Memorize this short conversion list, because the PTCE outline lists conversions in this area. Pharmacy problems use the rounded equivalents below, and FDA's food-labeling rule uses the same 5 mL, 15 mL and 30 mL household values.
| From | To |
|---|---|
| 1 kg | 2.2 lb |
| 1 g | 1,000 mg |
| 1 mg | 1,000 mcg |
| 1 L | 1,000 mL |
| 1 teaspoon | 5 mL |
| 1 tablespoon | 15 mL |
| 1 fluid ounce | 30 mL |
Temperature: °F = (°C × 1.8) + 32 and °C = (°F − 32) ÷ 1.8. So 8 °C = 14.4 + 32 = 46.4 °F.
6 How do percent strength and dilution work?A percent weight-in-volume strength means grams of drug in every 100 mL. 1 min
A percent weight-in-volume strength means grams of drug in every 100 mL. So 0.9% sodium chloride has 0.9 g per 100 mL, and 2% lidocaine has 2 g per 100 mL, which is 20 mg/mL.
To dilute a stock, use C1V1 = C2V2 (concentration × volume before = after). To make 500 mL of 2% from a 10% stock: 10 × V1 = 2 × 500, so V1 = 100 mL. Mix 100 mL of stock with 400 mL of diluent.
7 What the technician doesThe technician does the arithmetic and enters the result, and the pharmacist checks it. 1 min
The technician does the arithmetic and enters the result, and the pharmacist checks it.
- Read the whole Sig, including frequency, duration and prn limits.
- Convert units before you calculate.
- Recheck with a quick estimate, then enter the days' supply and quantity.
- Refer any dose that looks unusual to the pharmacist.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Divide pounds by 2.2 to get kilograms, not multiply.
Watch out: q6h means 4 doses a day, not 6 doses a day.
Watch out: 1 teaspoon is 5 mL, not 15 mL (that is a tablespoon).
Watch out: 2% means 2 g per 100 mL (20 mg/mL), not 2 mg/mL.
Watch out: For a once-weekly tablet, multiply doses by 7 days, not count each tablet as one day.
How to remember it
Remember "Convert, Count, Divide" as the order for every problem. Convert units first, count the daily use, then divide.
Self-check:
- How many days does #60 last at 1 tablet b.i.d.?
- How many doses a day does q6h give?
- What is 44 lb in kilograms, and 8 °C in °F?
Show answers
- 30 days.
- b.i.d. means 2 doses a day, so daily use is 1 × 2 = 2 tablets.
- Divide: 60 ÷ 2 = 30 days.
- 4 doses a day.
- q6h means every 6 hours.
- 24 hours ÷ 6 = 4 doses.
- 20 kg and 46.4 °F.
- Pounds to kilograms: divide by 2.2, so 44 ÷ 2.2 = 20 kg.
- Celsius to Fahrenheit: 8 × 1.8 = 14.4.
- Add 32: 14.4 + 32 = 46.4 °F.