How to Calculate Prescription Days Supply for Tablets, Capsules, and Liquids
Last reviewed: July 2026
1. What days supply means
Days supply is a quantity estimate: how many days a dispensed amount may last based on the daily use entered. It is different from a refill date. Use the Medication Days Supply Calculator when the question is duration, and use the Prescription Refill Date Calculator when the question is refill timing.
2. The basic formula
For tablets and capsules, multiply units per dose by doses per day to estimate daily use. Then divide quantity dispensed by daily use. For liquids, multiply mL per dose by doses per day, then divide total mL dispensed by daily mL use.
| Prescription example | Daily use entered | Formula | Estimated days supply |
|---|---|---|---|
| 30 tablets | 1 tablet daily | 30 / 1 | 30 days |
| 60 tablets | 1 tablet twice daily | 60 / 2 | 30 days |
| 45 tablets | 1.5 tablets daily | 45 / 1.5 | 30 days |
| 300 mL liquid | 5 mL twice daily | 300 / 10 | 30 days |
3. Why pharmacy days supply may differ
A pharmacy or insurance system may calculate days supply differently because of label wording, package sizes, as-needed directions, split tablets, coordination rules, or medication forms that require specialized math. Eye drops, inhalers, creams, insulin, injectables, and supplies may need a different calculation method than simple tablets.
If you know the days supplied and need a refill planning date, switch to the refill date calculator. If you need to estimate how long a quantity lasts, stay with the days supply calculator.
4. When to ask a pharmacist
Ask your pharmacist or prescriber if the label directions are unclear, if the medication is used as needed, if the dose changed, or if your estimate does not match the pharmacy label. The calculator is a math helper, not a dosing decision tool.
Comprehensive Reference FAQ
Review common questions about date math and planning. Confirm any pharmacy, insurance, legal, or clinical question with the appropriate professional.
Sources and References
Source publications or reference materials listed by the article.
- U.S. Food and Drug Administration (FDA). Consumer medication information resources.
- Centers for Medicare & Medicaid Services (CMS). Prescription Drug Coverage resources.
- Medicare.gov. Drug Coverage (Part D).
- U.S. Drug Enforcement Administration (DEA). Controlled Substances Act overview.
- U.S. Food and Drug Administration (FDA). Drugs resources.
This article is published as a calculator-first educational guide. It summarizes date math and planning examples only. It does not provide medical, legal, pharmacy, or insurance advice. Confirm final refill availability with your prescriber, pharmacy, insurance plan, medication type, and local rules.