What is the maximum PBS days supply?

For most PBS items the maximum is 30 days. Some restricted benefit items, Section 100 items and authority prescriptions have different maximum quantities. Always check the PBS schedule for the specific medicine.

How do I calculate days supply for a 'when required' (PRN) medicine?

PRN days supply must be based on the maximum stated frequency in the prescription. For example, 'up to 4 times daily as needed' — calculate as if the patient uses it 4 times every day.

What is a dispense number for Schedule 8 drugs?

In Australia, Schedule 8 (controlled) drugs have state-regulated maximum supply quantities (usually 1 month). Some states require permits for ongoing supply. Check your state health department's controlled drug guidelines.

Days Supply Calculator — Free Pharmacy Calculator

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PBS prescription in hand, the patient is 5 minutes from closing and asks if this will last until their next appointment in 28 days. You need the answer in seconds.

Days Supply Calculator

Dispensing

Quantity dispensed

Total tablets, mL, patches etc.

Dose per administration

e.g. 1 tablet, 5 mL

Doses per day

OD=1 · BD=2 · TDS=3 · QID=4

Days supply = (Qty ÷ Dose per admin) ÷ Doses per day Example: 60 tablets, 1 tablet BD → (60÷1)÷2 = 30 days
Liquid: 200 mL, 5 mL TDS → (200÷5)÷3 = 13.3 days

💡 Accurate days supply is required for PBS claims and continuity-of-care records.

Reference: PBS — Pharmaceutical Benefits Scheme · Pharmacy Board of Australia

⚕️ Clinical safety: 🇦🇺 Verify with facility drug formulary and senior clinician · Meets AHPRA/ACSQHC standards

1 What this calculator does

Calculates days supply from the quantity dispensed, the dose per administration, and the dosing frequency. Works for tablets, liquids, patches, inhalers and all standard dosage forms. Also reverse-calculates the quantity needed to fill a prescribed number of days.

2 Formula & professional reasoning

Days supply = Quantity dispensed ÷ (Dose per admin × Doses per day)  |  Quantity needed = Doses per day × Dose per admin × Days

Days supply is fundamental to dispensing accuracy, PBS compliance, adherence monitoring and Medicines Reconciliation. Incorrect days supply causes early refill requests, adherence gaps and potential regulatory issues with controlled drugs. The calculation must account for the actual number of doses per day, not just frequency abbreviations.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Simple daily tablet

Given: Qty: 30 tablets · Dose: 1 tablet · Frequency: Once daily (1/day)

Working: 30 ÷ (1 × 1)

Answer: 30 days supply

💡 Standard PBS maximum supply. Matches 28-day month with 2 tablets spare.

Standard

TDS liquid antibiotic

Given: Qty: 150 mL · Dose: 5 mL · Frequency: TDS (3/day)

Working: 150 ÷ (5 × 3) = 150 ÷ 15

Answer: 10 days supply

💡 Standard antibiotic course. Check: 5 mL × 3 × 10 = 150 mL. ✓

Advanced

BD inhaler — dose count

Given: Qty: 1 inhaler × 120 doses · Dose: 2 puffs · Frequency: BD (2/day)

Working: 120 puffs ÷ (2 puffs × 2 times/day) = 120 ÷ 4

Answer: 30 days supply

💡 Confirm inhaler dose counter — some inhalers have extra 'tail-off' doses that shouldn't be counted.

4 Sanity check

PBS maximum supply

Usually 30 days (authority items may differ)

Check PBS schedule for specific item maximum quantity. Some items have 5-day or 4-day maximum.

Controlled drug supply

Often restricted to 28 days maximum

Schedule 8 items: check state-specific maximum supply quantities.

Days supply vs quantity check

Qty = Doses/day × Dose × Days

Cross-multiply to verify: if Days supply = 30, dose = 1 tab, BD → Qty must = 60 tablets.

Liquid rounding

Round down for critical medications

For antibiotics and antivirals, do not round up — partial days are incomplete courses.

5 Common errors

Error	Cause	Consequence	Fix
Using frequency abbreviation without confirming doses/day	QID assumed as 4/day, but may mean 'four times during waking hours' (3× if patient sleeps 8 hrs)	Days supply off by ~25% for QID medications	Clarify with prescriber if QID means true every-6-hour dosing or 4 times during waking hours
Not accounting for dose changes	Previous dispensing record used	Days supply based on old dose	Always base the calculation on the CURRENT prescription dose, not dispensing history
Confusing units for liquids	Entering mL when mg required or vice versa	Completely wrong result	For liquids: quantity in mL, dose in mL. For tablets: quantity in tablets, dose in tablets.
Counting inhaler priming doses	Some inhalers require 3–4 priming puffs when new	Days supply overestimated by 1–2 days	Subtract priming doses from total dose count before calculating days supply