How do I calculate an antibiotic dose by weight?

Multiply the weight in kilograms by the prescribed mg/kg/day to get the daily dose, then divide by the number of doses per day. Example: a 15 kg child prescribed amoxicillin 45 mg/kg/day divided every 12 hours needs 15 × 45 = 675 mg/day, or 337.5 mg per dose. With 250 mg/5 mL suspension (50 mg/mL) that is 337.5 ÷ 50 = 6.75 mL per dose. If the order is in mg/kg/dose, multiply weight by the per-dose rate instead.

What is the difference between mg/kg/day and mg/kg/dose?

mg/kg/day is the total for 24 hours, which is then divided into the doses (45 mg/kg/day ÷ 2 = 22.5 mg/kg per dose). mg/kg/dose is the amount given each time, so the daily total is that amount times the number of doses. Confusing the two causes two- to four-fold dosing errors, so check which one the prescription uses.

What are typical pediatric amoxicillin doses?

The FDA label for children over 3 months and under 40 kg gives 25 mg/kg/day divided every 12 hours (or 20 mg/kg/day every 8 hours) for mild to moderate infections and 45 mg/kg/day every 12 hours (or 40 mg/kg/day every 8 hours) for severe or lower respiratory infections. The AAP otitis media guideline uses a high dose of 80–90 mg/kg/day in 2 divided doses, and IDSA strep throat guidance uses 50 mg/kg once daily (max 1,000 mg) or 25 mg/kg twice daily (max 500 mg per dose). Children of 40 kg or more are dosed as adults. Your prescriber chooses the regimen.

Is there a maximum dose for children?

Yes, most antibiotics have one. Examples from product labels: cefdinir 600 mg/day, azithromycin for strep throat 500 mg/day, and for most drugs a child at or above 40 kg receives the adult dose rather than more mg/kg. A weight-based calculation for a large child can exceed these limits, which is why the calculator flags caps. Ask the prescriber or pharmacist before giving more than a labelled maximum.

How do I convert mg to mL for an oral suspension?

Read the strength on the bottle, written as X mg per Y mL (for example 400 mg/5 mL = 80 mg/mL). mL per dose = mg per dose × Y ÷ X. The same drug comes in several strengths (amoxicillin 125, 200, 250 and 400 mg/5 mL), so always use the bottle you actually have; using the wrong strength is a common cause of overdosing.

Should I round the dose to the syringe markings?

Pharmacists usually round to a volume the oral syringe can measure, such as the nearest 0.1 mL on a 1–3 mL syringe or 0.2–0.5 mL on a 5–10 mL syringe. The calculator shows the rounded volume and the milligrams it actually delivers. Follow the volume written on the pharmacy label if it differs.

Can I use a kitchen spoon to measure liquid antibiotics?

No. FDA guidance and the AAP recommend dosing in millilitres with the oral syringe, dropper or cup supplied with the medicine. Household teaspoons vary widely and cause dosing errors.

Is this calculator medical advice?

No. It performs arithmetic on an instruction you copy from a prescription and shows published reference ranges for comparison. It does not diagnose, choose an antibiotic or set a dose, and it does not account for allergies, kidney function, age limits or the infection being treated. Always confirm doses with the prescriber or pharmacist.

Antibiotic Dose by Weight Calculator

Enter the weight and the prescribed mg/kg/day (or mg/kg/dose) and doses per day to get mg per dose, mg per day and mL from the bottle strength. Example: 15 kg at 45 mg/kg/day ÷ 2 = 337.5 mg per dose = 6.75 mL of 250 mg/5 mL. Optionally compare with label ranges. Not medical advice; confirm with the prescriber or pharmacist.

Prescription Arithmetic Inputs

Copy the exact weight and dosing instructions from the prescriber or pharmacist label.

Patient Weight *

Prescription Instruction Format *

Prescribed Rate (mg/kg/day) *

Doses per Day *

Prescribed Daily Maximum (Optional)

mg / day

If the prescriber or label specifies an absolute daily cap.

Liquid Suspension Concentration (Optional)

Copy the active concentration exactly as printed on the pharmacy bottle (e.g. 250 mg in 5 mL).

Amount in mg

In volume (mL)

Common strengths (check your bottle):

Round volume to syringe markings

Compare with a reference regimen (optional)

Not medical advice. This calculator performs purely mathematical arithmetic. It does not select antibiotics, verify clinical appropriateness, or replace professional judgment. Always verify the calculation, dosing device markings, and rounding increments with a licensed pharmacist or prescriber.

No calculations to display

Enter patient weight, prescribed dosing rate, and doses per day to calculate the mathematical dose.

Active Dose Arithmetic (mg)

Dose per Administration

mg / dose

Total Daily Dose

mg / day

Dosing Schedule

Calculated Weight Basis

Arithmetic Standards & Unit Factors

Mass conversion: 1 kg = 2.2046226218 lb (unrounded precision preserved during calculation).
Volume conversion: Liquid volume (mL) = mg dose × (label mL / label mg).
Volume unit: Expressed strictly in millilitres (mL) per FDA safe-use guidance.

Published Pediatric Oral Antibiotic Regimens

For checking a prescription only. Indication, age, allergies and kidney function change the right dose. Not medical advice; the prescriber and pharmacist decide.

Drug and use	Regimen	Maximum	Source
Amoxicillin FDA label, over 3 months and under 40 kg	25 mg/kg/day ÷ 12 h or 20 mg/kg/day ÷ 8 h (mild–moderate); 45 mg/kg/day ÷ 12 h or 40 mg/kg/day ÷ 8 h (severe, lower respiratory)	40 kg or more: adult dose (up to 875 mg every 12 h)	FDA label (Amoxil)
Amoxicillin high dose, acute otitis media (AAP)	80–90 mg/kg/day in 2 divided doses	Not set in the guideline; prescriber decides	AAP AOM guideline 2013
Amoxicillin strep throat, once daily (IDSA)	50 mg/kg once daily for 10 days	1,000 mg per day	IDSA GAS pharyngitis 2012
Amoxicillin strep throat, twice daily (IDSA)	25 mg/kg twice daily for 10 days	500 mg per dose	IDSA GAS pharyngitis 2012
Amoxicillin-clavulanate ES-600 acute otitis media (label)	90 mg/kg/day (amoxicillin) ÷ 12 h for 10 days; 600 mg/42.9 mg per 5 mL	Not studied at 40 kg or more	FDA label (Augmentin ES-600)
Azithromycin otitis media, 3-day course (label)	10 mg/kg once daily for 3 days (alternatives: 30 mg/kg once, max 1,500 mg; or 10 mg/kg day 1 then 5 mg/kg days 2–5)	500 mg per day	FDA label (Zithromax)
Azithromycin strep throat, 2 years and older (label)	12 mg/kg once daily for 5 days	500 mg per day	FDA label (Zithromax)
Cephalexin most infections, over 1 year (label)	25–50 mg/kg/day in divided doses; 50–100 mg/kg/day for severe infections; otitis media 75–100 mg/kg/day ÷ 6 h	Adult maximum 4 g/day	FDA label (Keflex)
Cefdinir 6 months to 12 years (label)	7 mg/kg every 12 h or 14 mg/kg every 24 h	600 mg per day	FDA label (Omnicef)
Clindamycin palmitate oral solution (label)	8–12 (serious), 13–16 (severe) or 17–25 (more severe) mg/kg/day in 3 or 4 equal doses; 75 mg/5 mL	At 10 kg or less, 37.5 mg three times daily is the minimum	FDA label (Cleocin Pediatric)
Trimethoprim-sulfamethoxazole UTI or otitis media, 2 months and older (label)	8 mg/kg/day trimethoprim (40 mg/kg/day sulfamethoxazole) ÷ 12 h for 10 days; dose by the TMP component, 40 mg TMP per 5 mL	Adult dose 160 mg TMP every 12 h	FDA label (Bactrim)

Summary

This calculator checks the arithmetic of a weight-based antibiotic prescription written by a licensed prescriber. Daily dose (mg/day) = weight (kg) × mg/kg/day; dose per administration = daily dose ÷ doses per day; volume (mL) = mg per dose × bottle mL ÷ bottle mg. It applies an optional daily maximum, rounds the volume to your syringe markings and shows the milligrams actually delivered, and can compare the result with published reference regimens (FDA product labels, AAP and IDSA guidelines) for amoxicillin, amoxicillin-clavulanate, azithromycin, cephalexin, cefdinir, clindamycin and trimethoprim-sulfamethoxazole. It does not choose a drug or a dose and is not medical advice.

How it works

Enter the weight in kg or lb (1 kg = 2.2046226218 lb). Pounds are converted without intermediate rounding.
Choose the prescription format: mg/kg/day divided into doses, or mg/kg/dose given a set number of times a day. Daily mg = kg × mg/kg/day; per dose = daily mg ÷ doses per day (or kg × mg/kg/dose).
Optionally enter the daily maximum written by the prescriber (mg/day). In mg/kg/day mode the total is capped; in mg/kg/dose mode a conflict is flagged instead of changing the order.
Enter the bottle strength (for example 250 mg in 5 mL = 50 mg/mL) or tap a common strength. mL per dose = mg per dose ÷ mg per mL. Pick the syringe increment to see the rounded volume and the mg it delivers.
Optionally select a reference regimen. The tool compares the prescribed mg/kg/day, per-dose and daily totals with the label or guideline range and its maximums, and flags weights of 40 kg or more where labels switch to adult dosing.
Measure liquids with the oral syringe or dosing cup from the pharmacy, never a kitchen spoon.

Use cases

Double-check a pediatric amoxicillin prescription such as 90 mg/kg/day divided twice daily.
Convert a prescribed mg dose into mL for the exact bottle strength dispensed.
See whether a weight-based total reaches the daily maximum set by the prescriber or the label.
Round a dose to 0.1, 0.2 or 0.5 mL syringe markings and see the mg actually delivered.
Practice weight-based dosage calculations for nursing and pharmacy exams.

Frequently Asked Questions

Last updated: 2026-10-10 · Reviewed by Nham Vu

Antibiotic Dose by Weight Calculator

Prescription Arithmetic Inputs

Liquid Suspension Concentration (Optional)

Active Dose Arithmetic (mg)

Liquid Oral Suspension Volume

Reference check

Arithmetic Standards & Unit Factors

Published Pediatric Oral Antibiotic Regimens

Summary

How it works

Use cases

Frequently Asked Questions

Antibiotic Dose by Weight Calculator

Use the Antibiotic Dose by Weight Calculator

Prescription Arithmetic Inputs

Liquid Suspension Concentration (Optional)

Active Dose Arithmetic (mg)

Liquid Oral Suspension Volume

Reference check

Arithmetic Standards & Unit Factors

Published Pediatric Oral Antibiotic Regimens

Summary

How it works

Use cases

Frequently Asked Questions