If you’ve ever looked at a prescription bottle and wondered “is this really a 30-day supply?”, the answer comes down to one simple formula. This guide walks through that formula step by step, with worked examples for the most common dosing schedules.
The 30-Day Supply Formula
The basic formula pharmacies use is:
Quantity Dispensed ÷ Daily Dose = Days Supply
If the result equals 30, the prescription is considered a 30-day supply. Let’s look at how this plays out for different dosing schedules.
Example 1: Once-Daily Medication
You take 1 tablet per day. For a 30-day supply, the pharmacy dispenses:
30 tablets ÷ 1 tablet per day = 30 days
This is the simplest case — the quantity dispensed equals the number of days.
Example 2: Twice-Daily Medication
You take 1 tablet in the morning and 1 tablet at night (2 tablets per day). For a 30-day supply, the pharmacy dispenses:
60 tablets ÷ 2 tablets per day = 30 days
Notice that the quantity doubles, but the days supply stays the same. This is a common point of confusion — a bottle of 60 tablets isn’t automatically a “60-day supply” if you’re taking 2 per day.
Example 3: Fractional (Half-Tablet) Dose
You take half a tablet (0.5) per day. For a 30-day supply, the pharmacy dispenses:
15 tablets ÷ 0.5 tablets per day = 30 days
Fractional doses are common for medications that are tapered or split. The same formula applies — just divide the total quantity by the (possibly fractional) daily dose.
When Can You Refill a 30-Day Supply?
Most insurance plans don’t require you to wait until you’re completely out before refilling. A common guideline is the 75% threshold — once you’ve used about 75% of your supply, a refill is typically allowed. For a 30-day supply, that’s roughly day 22 to 25.
That said, exact refill windows vary by insurance plan, pharmacy, and medication type, so it’s worth confirming your specific refill-eligibility date with your pharmacist or insurance provider rather than assuming.
Try It Yourself
Rather than doing the math by hand every time, you can enter your own quantity, daily dose, and fill date into our prescription refill date calculator to get an exact refill date and recommended refill-by date — including for 60-day, 90-day, and other supply lengths.
If you’re working backwards from a pill count instead — for example, figuring out how many pills you should have left on a given date — see our guide on how many pills you should have left. And if your prescription is a controlled substance, the 75% early-refill guideline above may not apply the same way — see our guide on controlled substance refill rules for details.
FAQ
How do I know if my prescription is a 30-day supply?
Divide the total quantity dispensed by your daily dose. If the result is 30, it’s a 30-day supply. For example, 60 tablets at 2 per day equals a 30-day supply (60 ÷ 2 = 30).
Can I refill before my 30-day supply runs out?
Often yes — many insurance plans allow a refill once roughly 75% of the supply has been used, which is typically around day 22 to 25 of a 30-day supply. However, refill timing depends on your specific insurance plan and pharmacy policy, so confirm with your pharmacist or insurer before assuming an early refill will be approved.
What if my daily dose changes partway through the month?
The simple formula assumes a constant daily dose. If your prescriber changes your dose, the days-supply calculation will change too — ask your pharmacist to recalculate based on the new dose, since the original fill date and quantity may no longer line up with a clean 30-day cycle.
This article is for general informational purposes only and is not medical advice. Always confirm your specific refill timing and dosing instructions with your pharmacist, doctor, or insurance provider.