How Many Days Will My Prescription Last?
Divide the number of pills by how many you take in a day. That single number, the days supply, answers most "how long will this last?" questions, and it's the same figure your pharmacy records. This prescription days supply calculator does the division, rounds it the way pharmacies do, and shows the date the supply runs out, so it doubles as a simple medication or prescription supply calculator for any bottle.
The Days Supply Formula
Days Supply = Quantity ÷ (Amount per Dose × Doses per Day)
Work out how much you take in a full day first, then divide the quantity by it. Round down to whole days, because a partial day doesn't cover all of that day's doses.
Worked Examples by Frequency
| Directions | Per day | Quantity | Days supply |
|---|---|---|---|
| 1 tablet once daily | 1 | 30 | 30 days |
| 1 capsule twice daily | 2 | 28 | 14 days |
| 2 tablets three times daily | 6 | 180 | 30 days |
| 1 tablet four times daily | 4 | 40 | 10 days |
| ½ tablet once daily | 0.5 | 45 | 90 days |
| 1 tablet every other day | 0.5 | 15 | 30 days |
| 1 tablet once a week | 1/7 | 4 | 28 days |
The calculator also gives a quick reverse view: at your dose, how many units you'd need for 7, 14, 30, 60 and 90 days.
Special Cases Pharmacies Handle Differently
As-needed (PRN) prescriptions
For "take 1 tablet every 6 hours as needed," the pharmacy usually records days supply at the maximum allowed: 4 tablets a day. Ninety tablets is then a 22-day supply on paper, even if it lasts you three months. Your plan uses that recorded number for refill timing.
Antibiotics and short courses
A 10-day antibiotic course is a 10-day supply, even if a few tablets are left over because of package size. Finish the course as directed unless your prescriber tells you otherwise.
Things this calculator doesn't cover
Inhalers, insulin, injectable pens, eye drops, creams and patches use different units such as puffs, units, drops or grams, and the days supply depends on the product. Ask your pharmacist for those. For liquids measured in mL, use the liquid days supply calculator.
Days Supply vs. How Long the Bottle Lasts
The days supply is fixed when the prescription is filled. How long the bottle lasts you depends on what actually happens: missed doses, dropped tablets, a dose change. If you want dates based on what's really in the bottle, use the pills remaining calculator or enter your count in the refill date calculator.
Why the Number Matters for Insurance
Plans often limit how many days you can get at once: commonly 30 days at a retail pharmacy and 90 days for maintenance medications or through mail order. They also use the days supply on each claim to time your next fill. If the recorded days supply doesn't match how you actually take the medication, for example after a dose change, ask your prescriber to send a new prescription so the record is right.