Calculate weight-based medication doses for common OTC medications. Get dose per kg calculations for acetaminophen, ibuprofen, and other non-prescription drugs.
The Medication Dose by Weight Calculator computes weight-based doses for common over-the-counter (OTC) medications. Many medications, especially for children, are dosed by body weight (mg/kg) rather than by age. Weight-based dosing is more accurate because children and adults of the same age can vary significantly in size.
This calculator covers common OTC medications including acetaminophen (Tylenol), ibuprofen (Advil/Motrin), diphenhydramine (Benadryl), cetirizine (Zyrtec), and amoxicillin (commonly prescribed). For each medication, it calculates the single dose, shows the minimum dosing interval, and highlights the maximum daily dose to prevent toxicity.
Weight-based dosing is especially critical for pediatric patients, where overdosing is a leading cause of medication-related emergency room visits. This tool provides quick, accurate dose calculations to support safe medication administration. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation. By automating the calculation, you save time and reduce the risk of costly errors in your planning and decision-making process.
Medication dosing errors are among the most common medical errors, particularly in children. A 2018 study found that one-third of parents make errors when dosing liquid medications for their children. Weight-based dosing eliminates the guesswork of age-based charts that don't account for size variation. This calculator provides instant, accurate doses with clear maximum limits to prevent accidental overdose.
Single Dose (mg) = Dose per kg (mg/kg) × Body Weight (kg) Maximum Daily Dose = max(single dose × doses per day, daily max limit) Common OTC Dosing: • Acetaminophen: 10–15 mg/kg every 4–6 hours (max 75 mg/kg/day or 4,000 mg/day) • Ibuprofen: 5–10 mg/kg every 6–8 hours (max 40 mg/kg/day or 1,200 mg/day) • Diphenhydramine: 1–1.25 mg/kg every 6–8 hours (max 5 mg/kg/day or 300 mg/day) • Cetirizine: 0.25 mg/kg once daily (max 10 mg/day) • Amoxicillin: 25–45 mg/kg/day divided in 2–3 doses
Result: Single dose: 375 mg | Max daily: 1,875 mg (5 doses)
A 25 kg child receiving acetaminophen at 15 mg/kg gets a single dose of 375 mg. With dosing every 4–6 hours (up to 5 doses/day), the maximum daily dose is 1,875 mg, which is well under the absolute maximum of 4,000 mg/day. This corresponds to about 12 mL of Children's Tylenol (160 mg/5 mL concentration).
Children's medications come in specific concentrations. Infant acetaminophen drops are 160 mg/5 mL (same as children's liquid since 2011 standardization). Children's ibuprofen is typically 100 mg/5 mL. After calculating the mg dose, divide by the concentration to get the volume in mL. A dosing syringe is far more accurate than a dosing cup for volumes under 10 mL.
The most frequent errors include: confusing mg with mL (giving 5 mL instead of 5 mg), using household teaspoons (which range from 2.5 to 7.5 mL), not accounting for acetaminophen in combination products, and continuing infant drop concentrations when switching to children's liquid. Electronic medical records and pharmacy systems help catch errors in clinical settings, but home dosing relies on caregiver accuracy.
Obese patients may need adjusted-body-weight dosing for some medications because drug distribution differs in adipose tissue. Patients with liver disease (especially for acetaminophen) or kidney disease (especially for ibuprofen) may need reduced doses. Elderly patients often need lower doses due to decreased metabolism. Always consult a healthcare provider for patients with chronic conditions.
Children of the same age can differ dramatically in weight. A 2-year-old might weigh anywhere from 22 to 35 pounds. Age-based dosing uses averages, which could mean a small child gets too much or a large child gets too little. Weight-based dosing provides individualized, accurate dosing that accounts for actual body size.
Package labels use simplified age-based charts for convenience. The weight-based calculation is generally more accurate. However, NEVER exceed the maximum dose listed on the package without consulting a healthcare provider. If the calculated dose seems significantly different from the label, verify the weight, concentration, and medication before administering.
The mg/kg formulas apply to adults as well, but adults typically use standard fixed doses (e.g., 500–1000 mg acetaminophen) because most adults fall within a weight range where the standard dose is appropriate. For very small or very large adults, weight-based dosing may be more accurate. This calculator caps doses at adult maximums.
Acetaminophen (Tylenol) is the leading cause of acute liver failure in the United States, often from accidental overdose. The margin between therapeutic and toxic doses is relatively narrow: therapeutic maximum is 4,000 mg/day, while 7,500 mg/day can cause severe liver damage. Always check ALL medications (cough syrups, sleep aids, pain relievers) for acetaminophen content to avoid doubling up.
Yes, alternating is a common practice for managing fever or pain. Typically: give acetaminophen, then ibuprofen 3 hours later, then acetaminophen 3 hours after that. This provides more consistent relief while keeping each medication within its safe frequency. However, this approach increases the risk of dosing errors. Keep a written log of what was given and when.
Aspirin is contraindicated in children and teenagers (under 18) for fever or viral illness due to the risk of Reye's syndrome, a rare but potentially fatal condition affecting the brain and liver. Low-dose aspirin may be prescribed by a doctor for specific conditions (e.g., Kawasaki disease), but over-the-counter aspirin should never be given to children for routine fever or pain.