Days supply calculator
Enter the quantity dispensed, the amount per dose, and how many times a day it's taken. Works for tablets, capsules and liquids.
For studying and practice only. This isn't a clinical tool: always follow the prescriber's order, your pharmacist, and your facility's policies.
How days supply is calculated
Days supply is how long a prescription lasts when it's taken exactly as directed. Pharmacies report it to insurance with every claim, and it decides when a refill is allowed, so getting it right matters.
days supply = quantity ÷ (amount per dose × doses per day), rounded down
Worked example
120 capsules, take 2 capsules by mouth twice daily
- per day = 2 capsules × 2 = 4 capsules
- days = 120 ÷ 4 = 30 days
Worked example
200 mL oral suspension, take 7.5 mL three times daily
- per day = 7.5 mL × 3 = 22.5 mL
- days = 200 ÷ 22.5 = 8.89 → 8 days (rounded down)
Common sig frequencies
Turn the directions into doses per day first:
| Directions | Sig code | Doses per day |
|---|---|---|
| Once daily | QD / daily | 1 |
| Twice daily | BID | 2 |
| Three times daily | TID | 3 |
| Four times daily | QID | 4 |
| Every 12 hours | q12h | 2 |
| Every 8 hours | q8h | 3 |
| Every 6 hours | q6h | 4 |
| Every 4 hours | q4h | 6 |
Some products need an extra conversion before the formula. Eye drops are counted in drops per mL, which varies by product, and insulin is counted in units (U-100 insulin has 100 units per mL). Follow your pharmacy's policy for these.
Practice these calculations the way the PTCB exam asks them
Try free PTCE practice questions on order entry and processing, including weight-based dosing and NDC conversion, with explanations. No signup.
Practice calculation questions freeFrequently asked questions
- What is the formula for days supply?
- Days supply = quantity dispensed ÷ (amount per dose × doses per day). For example, 90 tablets taken 1 tablet three times daily is 90 ÷ 3 = 30 days.
- Do you round days supply up or down?
- Down. A prescription only covers the days it can fully supply, so 100 tablets at 3 a day is 33 days (with 1 tablet left over), not 34.
- How do you calculate days supply for a liquid?
- Use milliliters for both the quantity and the dose: 150 mL of a suspension at 5 mL twice daily is 150 ÷ (5 × 2) = 15 days.
- How is days supply calculated for an as-needed (PRN) prescription?
- Use the maximum daily dose the directions allow. "1 tablet every 6 hours as needed" can be up to 4 tablets a day, so 60 tablets is a 15-day supply.