How this instrument works
Amoxicillin dosing in children scales with body weight, but the target rate — how many milligrams per kilogram per day — depends on what's being treated. This calculator covers three published regimens: the standard dose for acute otitis media (ear infection), a high-dose regimen used for otitis media when resistant bacteria are more likely, and a once-daily regimen for streptococcal pharyngitis (strep throat). Selecting a regimen determines both the mg/kg/day rate and how many times a day the total is split.
For acute otitis media, the AAP/AAFP clinical practice guideline gives a standard dose of 40-45 mg/kg/day divided twice daily (BID), and a high-dose alternative of 80-90 mg/kg/day, also BID, favored when the child is at higher risk for a resistant infection — for example, recent antibiotic use, daycare attendance, or a more severe presentation. This calculator uses the midpoint of each published range: 42.5 mg/kg/day for standard-dose and 85 mg/kg/day for high-dose.
For streptococcal pharyngitis, the IDSA guideline gives a once-daily regimen of 50 mg/kg/day, capped at a flat 1000 mg/day regardless of weight — a simpler schedule intended partly to improve adherence over multiple daily doses for a full 10-day course. Because it's once daily rather than split, the per-dose amount and the daily total are the same number in that regimen.
- Enter the child's weight in kilograms.
- Select the regimen: standard-dose otitis media, high-dose otitis media, or once-daily strep throat.
- Read the total daily dose in mg/day and how many doses per day that regimen uses.
- Read the amount per dose in mg, and confirm it against the child's actual prescription.
Worked example — 15 kg child, standard vs. high-dose otitis media
A 15 kg child on the standard otitis media regimen (42.5 mg/kg/day) gets a total daily dose of 15 × 42.5 = 637.5 mg/day, split into two doses of 318.75 mg each, roughly 12 hours apart.
The same 15 kg child on the high-dose regimen (85 mg/kg/day), used when resistant bacteria are more of a concern, gets 15 × 85 = 1275 mg/day — nearly double — split into two doses of 637.5 mg each.
For strep throat, a 25 kg child on the once-daily 50 mg/kg/day regimen has a raw weight-based total of 25 × 50 = 1250 mg/day, but the regimen's 1000 mg/day cap applies, so the actual dose is capped at 1000 mg, given as a single daily dose.
Questions
Is this calculator a substitute for the child's actual prescription?
No. Always follow the prescription and pharmacy label exactly. A prescriber may adjust the dose or regimen for reasons this calculator doesn't account for — kidney function, a documented penicillin allergy history, weight that's changed since the last visit, or a different indication entirely. Use this as a reference to sanity-check a prescription, and call the prescriber or pharmacist with any discrepancy rather than adjusting the dose yourself.
Why does the high-dose regimen use almost double the standard dose?
The high-dose regimen (80-90 mg/kg/day) is designed to overcome amoxicillin-resistant strains of Streptococcus pneumoniae, the most common bacterial cause of acute otitis media, by achieving higher drug concentrations. The AAP/AAFP guideline favors it for children at higher risk of a resistant infection — for example, those with recent antibiotic exposure, daycare attendance, or more severe symptoms — while reserving the standard dose for lower-risk cases.
Why is the strep throat regimen once daily instead of twice daily?
The IDSA guideline includes a once-daily 50 mg/kg/day (up to 1000 mg) amoxicillin regimen for group A streptococcal pharyngitis as an option alongside twice- or three-times-daily dosing, largely because once-daily dosing over the required 10-day course tends to improve how reliably children actually complete treatment.
Why is the strep throat dose capped at 1000 mg/day but the otitis media doses aren't?
The 1000 mg/day cap is specific to the once-daily strep throat regimen as published by the IDSA guideline. The otitis media regimens in this calculator follow the AAP/AAFP guideline's mg/kg/day ranges without an equivalent flat daily cap, though in practice a prescriber would still weigh a very high calculated total against standard adult dosing ranges for a heavier child or adolescent.
What if my child is allergic to penicillin?
Amoxicillin is a penicillin-class antibiotic and is not appropriate for a child with a true penicillin allergy — this calculator does not screen for allergy history. Tell the prescriber about any prior reaction to penicillin or related antibiotics before amoxicillin is prescribed; alternative antibiotics exist for both otitis media and strep throat.
Does the mg/kg/day rate stay the same as a child gets older or heavier?
The mg/kg/day rate for a given regimen doesn't change with age within the pediatric range this calculator targets — it's weight that scales the total. But a prescriber may choose a different regimen (or transition toward fixed adult dosing) as a child grows, which is a clinical judgment this calculator doesn't make.
References
- Lieberthal AS, Carroll AE, Chonmaitree T, et al. — The Diagnosis and Management of Acute Otitis Media (AAP/AAFP), Pediatrics. 2013;131(3):e964-e999
- Shulman ST, Bisno AL, Clegg HW, et al. — Clinical Practice Guideline for Group A Streptococcal Pharyngitis (IDSA), Clin Infect Dis. 2012;55(10):e86-e102
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.