# Medication Days Supply Calculator

> Work out a prescription’s days supply from the quantity dispensed and the prescribed directions (units per dose and doses per day), with any units left over.

Category: Healthcare operations · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/medication-days-supply-calculator

Days supply is the quantity dispensed divided by the units used per day under the prescriber’s directions. It does not choose or check a dose.

Also called: Day supply calculator, Prescription duration calculator.

## Inputs
- Quantity dispensed (unit units; default 50)
- Units per dose (unit units; default 2)
- Doses per day (default 2)

## Results
- Days supply
- Full days covered
- Units used per day
- Units left after the full days

## Formula

```
Days supply = Quantity dispensed ÷ (Units per dose × Doses per day)
```

Where:
- Quantity dispensed: total units supplied
- Units per dose: tablets, mL, drops or puffs taken each time
- Doses per day: how many times a day, from the directions

## Worked example
1. A 2.5 mL bottle at 20 drops/mL holds 50 drops; one drop in each eye is 2 drops per dose.
2. Daily use = 2 drops × 2 times = 4 drops.
3. Days supply = 50 ÷ 4 = 12.5 days (12 full days, 2 drops left).

**Result:** A 50-drop bottle used one drop in each eye twice a day lasts 12.5 days, the figure in CMS’s pharmacy billing job aid.

## What the result means

Use the result for billing and refill planning. Products dosed on a cycle (weekly patches, rings with a break week) need the cycle length, not a simple count: CMS bills four weekly patches as 28 days.

## Assumptions and limits
- The patient follows the directions exactly, every day.
- The quantity and the dose use the same unit.
- “As needed” directions, tapering doses and cyclical products need manual calculation.
- Drops per mL vary between products; use the manufacturer’s figure.

## Questions

### How do I work out days supply for an inhaler?

Divide the actuations in the inhaler by the puffs used per day. A 120-dose inhaler at 4 puffs a day is a 30-day supply.

### Should I round days supply up or down?

Follow your payer’s rules. Many systems take the whole number of days the supply fully covers.

## Sources
- [CMS: Pharmacy Auditing and Dispensing Job Aid — Billing Other Dosage Forms](https://www.cms.gov/medicare-medicaid-coordination/fraud-prevention/medicaid-integrity-education/downloads/pharmacy-selfaudit-jobaid-billing-other.pdf)

## Related calculators
- [Prescription Refill Date Calculator](https://calculators.geekmidst.com/calculators/prescription-refill-date-calculator)
- [Pharmacy Stock Expiry Calculator](https://calculators.geekmidst.com/calculators/pharmacy-stock-expiry-calculator)
- [Medical Inventory Reorder Calculator](https://calculators.geekmidst.com/calculators/medical-inventory-reorder-calculator)

_This is a general screening estimate for adults, not medical advice or a diagnosis. Talk to a qualified health professional about your health._

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/medication-days-supply-calculator