How to calculate days of supply and when to refill

Running out of a medication is one of the most common reasons for missed doses, and one of the easiest to prevent. All it takes is a little arithmetic.

The basic formula

Days of supply = units in the pack ÷ units taken per day

A “unit” is whatever you take: a tablet, a capsule, a puff, a drop.

Pack	Daily dose	Lasts
30 tablets	1 a day	30 days
60 tablets	2 a day	30 days
28 tablets	1 a day	4 weeks
90 tablets	1 a day	about 3 months
180 tablets	1 a day	about 6 months

If the result is not a whole number, round down: the last partial day is not a full dose.

When to order more

Do not wait until the last tablet. Choose a safety margin, such as 7 days, and order when that many days are left:

Refill date = today + (units left ÷ units per day) − safety margin

A week covers weekends, holidays, a busy doctor and a pharmacy that needs to order your medicine.

Our free refill date calculator does this for you, and the days of supply calculator tells you how long a new pack will last.

Doses that change

Some treatments change over time: a starting dose that goes up, a course that tapers down, or a medicine taken only when needed. For these, count week by week, or ask your pharmacist how many packs you will need.

Inhalers, drops and liquids

The same idea works for other forms:

• Inhalers: puffs in the inhaler ÷ puffs per day.
• Liquids: volume in the bottle ÷ volume per dose ÷ doses per day.
• Drops: drops in the bottle are harder to know; check the leaflet or ask the pharmacist.

Let the app count for you

PillRem keeps a count of what is left in each box as you mark doses as taken, so you can see at a glance when a refill is coming up, instead of counting tablets on the kitchen table.

A note on timing

Some prescriptions can only be refilled after a certain date. If you are not sure when you are allowed to collect the next pack, ask your pharmacy.