Prescription Refill Calculator (run-out date & earliest refill)

Understanding the prescription refill calculator — concept and principle

This calculator works out when your medicine supply will run out, the earliest date your pharmacy or insurer is likely to allow a refill, and when to order so you do not miss doses. It uses the fill date and the days' supply on the pharmacy label.

Fill date + days supply: with the default inputs the result is 20 Nov (run-out · order by 15 Nov).

How it is calculated

Run-out date = fill date + days' supply

Earliest refill date = fill date + days' supply × refill threshold %, rounded up to a whole day

Days of supply left today = days' supply − days since the fill date

Order by = run-out date − reminder days

Following refill due = fill date + 2 × days' supply

The fill day counts as day 1 of use, so the next supply is needed on the run-out date.

The principle

Days' supply is the quantity dispensed divided by the amount used per day. Counting forward from the fill date gives the date the last dose is taken. Insurers and pharmacies often allow an early refill once a set share of the supply, such as 75–80%, should have been used, to avoid stockpiling while giving a buffer for delivery and holidays.

Worked example

Default inputs: filled 21 Oct 2026, 30-day supply, refill allowed after 80%, reminder 5 days (today 10 Nov 2026).

Run-out = 21 Oct + 30 = Fri 20 Nov 2026.
Earliest refill = 21 Oct + 30 × 0.8 = 21 Oct + 24 = Sat 14 Nov.
Days left today = 30 − 20 = 10 days.
Order by = 20 Nov − 5 = Sun 15 Nov.
Following refill due = 21 Oct + 60 = Sun 20 Dec.

How to read the result

Ordering between the earliest refill date and the order-by date leaves a buffer for pharmacy delays. If you take extra doses as directed or the dose changes, the days' supply changes and the dates move earlier.

Common mistakes

Using the date the prescription was written instead of the fill date.
Forgetting that controlled medicines often have stricter refill rules.
Not updating the days' supply after a dose change.

Keep in mind: This calculator only does date arithmetic. Refill rules vary by plan, pharmacy and country; never change how you take a medicine to make it last longer without asking a pharmacist or doctor.

Frequently asked questions

What does days' supply mean?

How many days the dispensed quantity lasts at the prescribed dose.

Why can't I refill earlier?

Plans limit early refills to prevent overuse and waste; ask your pharmacist about travel or emergency exceptions.

What if I run out before the refill date?

Contact your pharmacist or prescriber; some medicines should not be stopped suddenly.

Sources: Calendar arithmetic on the days' supply printed on the pharmacy label; early-refill thresholds are set by each insurer or pharmacy.

Checked 9 Nov 2026. Calendar arithmetic on the days’ supply written on the pharmacy label; early-refill thresholds vary by plan and must be taken from your insurer or pharmacy.

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