New admission, 14 medications, and the prescription uses a mix of abbreviations — QD, BD, TDS, QID, PRN q4h. You need the doses-per-day count for each before entering into the dispensing system.
Dose per admin = Total daily dose ÷ Doses per day
Doses per day: OD=1 · BD=2 · TDS=3 · QID=4 · q4h=6 · q6h=4 · q8h=3 · q12h=2
1 What this calculator does
Converts standard dosing frequency abbreviations to doses per day and dosing interval in hours. Works in both directions: abbreviation to interval, or doses-per-day to standard label instruction. Covers all common pharmacy frequency codes.
2 Formula & professional reasoning
Doses per day = 24 ÷ Dosing interval (hours)
Daily dose = Dose per admin × Doses per day
Frequency abbreviations are a persistent source of dispensing and administration errors. 'QD' (once daily) has been mistaken for 'QID' (four times daily) — a 4× dosing error. 'OD' is ambiguous (once daily or overdose in some contexts). The ISMP and TGA both recommend writing out 'once daily', 'twice daily' etc. in full on prescriptions, but abbreviations remain prevalent in practice.
3 Worked examples
⚠️ Illustrative example only — not clinical or professional instruction.
24 ÷ 2 = 12-hour interval24 ÷ 4 = 6-hour intervalMax: 24 ÷ 4 = 6 doses/day maximum4 Sanity check
5 Common errors
| Error | Cause | Consequence | Fix |
|---|---|---|---|
| QD vs QID confusion | Handwriting or font ambiguity | 4× intended dose — potentially fatal for digoxin, warfarin, opioids | Never abbreviate once-daily as 'QD' — write 'daily' or 'once daily' in full. TGA and ISMP both recommend avoiding QD. |
| OD interpreted as overdose | Context ambiguity | Misinterpretation of instruction | Use 'once daily' or 'mane' instead of OD. In clinical settings OD can mean overdose — avoid ambiguity. |
| Assuming QID means 'during waking hours only' | Patient convenience interpretation | Drug underexposed if 4th dose missed at night (e.g. antibiotics, antiepileptics) | QID means every 6 hours around the clock unless the prescriber specifies otherwise. Clarify for antimicrobials and anti-epileptics specifically. |
| PRN days supply underestimated | Calculating based on expected use not maximum | Insufficient quantity dispensed — patient runs out early | For PRN medications, always calculate days supply using the maximum prescribed frequency |
6 Reference & regulatory links
7 Professional workflow
Common tools used alongside this one: