Educational tool · Last reviewed: March 2026
This tool provides weight-based dosing estimates for azithromycin in paediatric patients.
⚠️ QT prolongation: Azithromycin prolongs the QT interval. Before prescribing, review the patient's cardiac history, baseline QTc interval, and all concomitant QT-prolonging medications.
Dosing ranges are educational. Always verify with local antimicrobial guidelines, the treating clinician, and product labelling for specific indications.
Azithromycin is a macrolide antibiotic with activity against a broad range of gram-positive and atypical organisms (Mycoplasma, Chlamydophila, Legionella). Its uniquely long tissue half-life (2–4 days) and intracellular accumulation allow for short, convenient dosing regimens — most commonly a 5-day course for respiratory infections.
In pediatrics, azithromycin is used as a second-line or alternative first-line antibiotic for community-acquired pneumonia (particularly atypical), pharyngitis (in penicillin-allergic patients), pertussis prophylaxis and treatment, otitis media (in penicillin allergy), and chlamydial infections.
The drug's Z-pack reputation for convenience has made it one of the most commonly prescribed antibiotics worldwide — but it is also associated with significant antimicrobial stewardship concerns due to overuse, increasing macrolide resistance among respiratory pathogens, and its cardiac side effect profile.
Enter the patient's weight (in kg, g, or lb), age (years and months), and select a dosing regimen. Each preset encodes the full dosing schedule — Day 1 loading dose and Days 2–5 maintenance doses — aligned with the clinical indication.
The calculator returns the per-dose amount in mg, the volume of suspension (for liquid formulations), and a day-by-day dosing schedule summarising each dose and its maximum. Results are displayed with inline warnings for relevant safety concerns.
QT prolongation warning is prominently displayed for all calculations, with a persistent alert reminding clinicians to screen for QT risk factors before prescribing.
Age warnings alert when patients are under 6 months — azithromycin is approved for paediatric use from 6 months for most indications, and its use in younger infants requires specialist guidance and has been associated with pyloric stenosis in neonates.
Volume results can be rounded to the nearest 0.5 mL increment to align with standard oral syringe graduations, reducing practical measurement errors.
Community-acquired pneumonia (5-day course): Day 1: 10 mg/kg (max 500 mg) as a loading dose; Days 2–5: 5 mg/kg/day (max 250 mg). This regimen achieves therapeutic tissue concentrations for atypical and typical respiratory pathogens and is the most common outpatient course.
Pharyngitis / tonsilitis (5-day course, penicillin allergy): 12 mg/kg/day (max 500 mg) for 5 days as a once-daily dose. This regimen delivers higher tissue levels appropriate for Streptococcus pyogenes eradication.
Pertussis treatment and post-exposure prophylaxis: 10 mg/kg (max 500 mg) on Day 1; 5 mg/kg/day (max 250 mg) on Days 2–5. The 5-day course is standard for both treatment and prophylaxis in contacts.
Single-dose chlamydia treatment (adult/adolescent): 1 g as a single dose — this regimen is not weight-based and is used in patients ≥ 15 years or ≥ 45 kg who can take adult doses.
Azithromycin blocks the hERG potassium channel, which delays ventricular repolarisation and prolongs the QT interval on the ECG. In susceptible individuals, this can precipitate Torsade de Pointes — a potentially fatal ventricular arrhythmia.
Risk factors for azithromycin-related cardiac events include: female sex, pre-existing prolonged QT, hypokalaemia, hypomagnesaemia, bradycardia, congenital long QT syndrome, and concurrent use of other QT-prolonging drugs (e.g., antipsychotics, fluoroquinolones, ondansetron, some antifungals).
Regulatory agencies have issued warnings about azithromycin's cardiac risk. In 2013, the FDA updated labelling to include information about QT prolongation and fatal arrhythmias. For most healthy children at standard doses, the absolute risk is low — but screening for risk factors is mandatory before prescribing.
Azithromycin tablets and capsules should be taken on an empty stomach — food significantly reduces the absorption of capsules, though the interaction is less pronounced for tablets and suspensions.
Azithromycin oral suspension can be taken with or without food. Shake the bottle well before each use. Reconstituted suspension should be stored at room temperature or refrigerated and used within 10 days.
The long tissue half-life of azithromycin means that a 5-day course provides tissue concentrations that persist for up to 10 days after the last dose — which is why shorter courses can be as effective as longer courses of other antibiotics for the same indications.
Day 1 loading dose (10 mg/kg, max 500 mg) and Days 2–5 maintenance dose (5 mg/kg, max 250 mg). Volume calculated for 200 mg/5 mL suspension.
| Weight (kg) | Day 1 dose (mg) | Day 1 vol (mL) | Days 2–5 dose (mg) | Days 2–5 vol (mL) |
|---|---|---|---|---|
| 5 kg | 50 mg | 1.25 mL | 25 mg | 0.6 mL |
| 8 kg | 80 mg | 2 mL | 40 mg | 1 mL |
| 10 kg | 100 mg | 2.5 mL | 50 mg | 1.25 mL |
| 12 kg | 120 mg | 3 mL | 60 mg | 1.5 mL |
| 15 kg | 150 mg | 3.75 mL | 75 mg | 1.9 mL |
| 18 kg | 180 mg | 4.5 mL | 90 mg | 2.25 mL |
| 20 kg | 200 mg | 5 mL | 100 mg | 2.5 mL |
| 25 kg | 250 mg | 6.25 mL | 125 mg | 3.1 mL |
| 30 kg | 300 mg | 7.5 mL | 150 mg | 3.75 mL |
| 40 kg | 400 mg | 10 mL | 200 mg | 5 mL |
| 50 kg | 500 mg | 12.5 mL | 250 mg | 6.25 mL |