---
title: "Days Supply Calculator — Tablets, Liquids, Inhalers, Insulin"
description: "Days supply calculator that shows the work: tablets, liquids, inhalers, insulin, and eye drops, with the formula, worked examples, and unlimited practice problems."
updated: 2026-08-03
canonical: https://ptcbready.com/pharmacy-tech-math/days-supply-calculator
source-hash: 4b236740f35ba7fc
sources:
  - "PTCE 2026 content outline (Order Entry & Processing calculations)": https://www.ptcb.org/credentials/certified-pharmacy-technician
---

# Days Supply Calculator — Tablets, Liquids, Inhalers, Insulin

> Days supply calculator that shows the work: tablets, liquids, inhalers, insulin, and eye drops, with the formula, worked examples, and unlimited practice problems.

## The formula

**Days supply = quantity dispensed ÷ (dose per administration × administrations per day)**

## Method

1. Read the sig: how much per administration, how many administrations per day.
2. Multiply the two for the daily usage.
3. Divide the quantity dispensed by daily usage. Round DOWN for insurance billing — a partial day is not a covered day.

Special forms: liquids convert the dose to mL first; insulin converts units/day into mL/day (U-100 = 100 units/mL), floors the result, and caps at the product's beyond-use date; inhalers divide total puffs by puffs/day.

## Worked example (computed)

**Problem.** A prescription for lisinopril 10 mg is filled with 120 tablets. The sig reads: "Take 2 tablets PO once daily." What is the days supply?

1. 2 tablets × 1 time/day = 2/day — Multiply the dose per administration by administrations per day to get daily usage.
2. 120 ÷ 2 = 60 — Divide the quantity dispensed by daily usage to get the days supply.

**Answer: 60 days.**

## Common traps

- Dividing by frequency only and ignoring the per-dose quantity (or vice versa).
- Rounding UP — days supply always floors.
- Forgetting the beyond-use-date cap on insulin and reconstituted liquids.

The interactive calculator shows this work for every input: https://ptcbready.com/pharmacy-tech-math/days-supply-calculator

---
Maintained by PTCB Ready (https://ptcbready.com) — not affiliated with or endorsed by PTCB®.
Cite as: PTCB Ready, updated 2026-08-03. Interactive version: https://ptcbready.com/pharmacy-tech-math/days-supply-calculator