Days’ supply calculations follow the same underlying logic across every dosage form — total quantity dispensed divided by daily use — but the "daily use" side of that equation looks different for tablets, liquids, topicals, insulin, injectables, and tapered regimens. The core formula is: days' supply = quantity dispensed ÷ (units per dose × doses per day).
The Core Formula, and Why Each Form Needs Its Own Version
Every days'-supply calculation is a ratio: how much was dispensed, divided by how much is used per day. What changes by dosage form is how you define a "unit" — a tablet, a milliliter, a gram, an insulin unit, a single-use container — and how you define "per day," which gets more complicated for PRN directions, multi-eye or multi-site applications, and tapered courses that change frequency partway through.
Tablets and Capsules
For scheduled directions: days' supply = quantity dispensed ÷ (tablets per dose × doses per day). A dispensed quantity of 30 tablets at 1 tablet twice daily yields 15 days' supply.
For PRN directions, calculate a maximum-use estimate using the shortest permitted interval: max doses/day = 24 ÷ interval in hours. A drug dosed "1–2 tablets every 4–6 hours PRN" has a maximum of 6 doses/day (24 ÷ 4), which then plugs into the standard formula using the higher end of the per-dose range for a conservative estimate.
Oral Liquids
Days' supply = volume dispensed (mL) ÷ (mL per dose × doses per day). A 120 mL bottle dosed at 5 mL twice daily yields 12 days' supply. Watch for prescriptions written in mg rather than mL — convert using the product's concentration before applying the formula.
Topicals: Creams, Ointments, Gels, and Lotions
Days' supply = grams dispensed ÷ (grams per application × applications per day). The fingertip-unit (FTU) convention is useful for estimating grams per application: roughly 0.5 g per FTU, covering an area about twice the size of a flat adult hand — helpful when a prescription specifies "apply to affected area" without a precise gram amount.
Insulin: Pens, Cartridges, and Vials
U-100 insulin contains 100 units per mL. A standard pen or cartridge (about 3 mL) holds roughly 300 units; a standard vial (about 10 mL) holds roughly 1,000 units. Days' supply = total units dispensed ÷ (units per dose × doses per day) — but always check the current labeled in-use discard period (commonly 28–56 days depending on the product), which can cap usable days independent of remaining units.
Injectables: Single-Dose vs. Multidose
Single-dose vials or prefilled devices are calculated by unit count, not remaining volume — any unused portion is discarded after one use. Multidose vials are calculated by total volume divided by mL per dose. For weekly or interval injectables, convert to a daily-equivalent frequency before applying the standard formula.
Weekly and Interval Dosing
For medications dosed less than daily, calculate days covered rather than a simple ratio: days covered = (quantity dispensed ÷ units per administration) × interval in days between doses. Four once-weekly doses cover 28 days, not 4.
Six-Phase Tapers
Tapered regimens change frequency across phases, so calculate required units for each phase separately, then sum: total required = ∑ (days in phase × doses per day in that phase × unit dose). Compare the total required against the quantity actually dispensed to confirm the package covers the full taper — a common source of underfill errors when a single days'-supply number is estimated instead of a true phase-by-phase total.
Bottom Line
The underlying math is always quantity ÷ daily use — what changes is how "quantity" and "daily use" are defined for each form. Insulin and injectables add an in-use discard consideration; tapers require phase-by-phase summing rather than a single ratio; and PRN directions need a maximum-use estimate rather than an average.
Try the Free Calculator
The Common Prescription Quantity Calculator handles all seven of these scenarios interactively, with a copyable calculation note for documentation. A printable reference PDF is also available with formulas, worksheets, and worked examples.
Frequently Asked Questions
Does the 20 drops/mL convention apply to liquid medications generally? No — that convention is specific to ophthalmic drops. Oral liquids should be measured and dosed in mL directly, not estimated by drop count.
How should I round a days'-supply calculation? Round the dispensed quantity up to the nearest full package when a partial package can't be dispensed, and round the calculated days'-supply estimate down when reporting how long a supply will last, to avoid overstating coverage.
What if the SIG doesn't specify an exact quantity per dose? Clarify with the prescriber before calculating. Ambiguous directions ("apply as needed," "use appropriately") don't have a defensible days'-supply calculation without a specific quantity and frequency.