Prescription Quantity Calculator

What Is a Prescription Quantity Calculator?

A prescription quantity calculator helps you determine how many pills, tablets, capsules, milliliters, patches, or injection pens you need for a given treatment duration. Whether you're requesting a 30-day, 90-day, or yearly supply, getting the right quantity avoids running out of medication or overpaying for excess pills. The calculation is simple: Quantity = Dose per administration × Doses per day × Number of days. However, with varied dosing schedules (every other day, twice weekly, q8h), the math can get tricky — that's where this calculator helps.

How Many Pills for 90 Days?

A 90-day supply is the most common long-term prescription quantity, especially for chronic medications like blood pressure pills, statins, thyroid medication, and antidepressants. For a simple once-daily medication, you need 90 tablets. For twice daily, you need 180. But if you take 1.5 tablets twice daily, you need 270 — and your pharmacy may dispense 270 or round to 300. Always confirm with your pharmacist what pack sizes are available and what your insurance covers.

Standard Pack Sizes Explained

Medications come in standard pack sizes that align with common durations. In the US, the most common sizes are 30 (1-month supply), 60 (1-month BID), 90 (3-month QD), and 100 (institutional/bulk). Some medications, especially birth control and specialty drugs, come in 28-day or 84-day packs. When your calculated quantity falls between standard sizes, pharmacies typically round up to the next available size — you'll receive the extra tablets. Insurance plans may have preferred quantities; check your formulary.

Liquid Medications & Bottle Sizes

Liquid medications are measured in milliliters (mL) and come in standard bottle sizes: 100 mL, 150 mL, 200 mL, 250 mL, and 500 mL. To calculate how much you need, multiply the mL per dose by the number of doses per day and the number of days. For example, 5 mL three times daily for 10 days = 150 mL, which fits perfectly in a 150 mL bottle. The calculator automatically suggests the nearest bottle size for your needs.

Patches and Injections

Transdermal patches (fentanyl, estrogen, nicotine) are changed at specific intervals — every 72 hours, weekly, or twice weekly. The number of patches needed equals the treatment days divided by the change interval, rounded up. For injections (insulin pens, biologics like adalimumab), the calculation involves total dose needed divided by the content per pen or vial. The calculator handles both types and suggests quantities with a built-in safety buffer.

Key References

• Academy of Managed Care Pharmacy (AMCP). Prescription Dispensing Quantity Guidelines. 2023.
• FDA. Approved Drug Products with Therapeutic Equivalence Evaluations (Orange Book). Updated 2024.
• Centers for Medicare & Medicaid Services (CMS). Part D Prescription Drug Benefit Manual. Chapter 6: Part D Drugs and Formulary Requirements.