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Child Dosage Calculator: Find the Right Pediatric Drug Dose by Weight
TL;DR: A 20 kg child prescribed 200 mg/kg/day of amoxicillin at 3 doses per day needs 1,333 mg per dose and 4,000 mg total daily. If the liquid concentration is 50 mg/mL, that works out to 26.7 mL per dose. The calculator above converts weight, prescribed dose, frequency, and concentration into exact milligram and millilitre values so you never have to guess.
Table of Contents
- Why Getting a Pediatric Drug Dose Right Matters More Than You Think
- Six Scenarios Where a Child Dosage Calculator Saves You Time
- The Formulas Behind Pediatric Dose Calculation
- How to Calculate a Child's Medication Dose Step by Step
- Two Real-World Examples: See How the Numbers Play Out
- Seven Errors That Throw Off Your Child's Dose
- FAQ
- Assumptions and Notes
- Your Next Step
- Further Reading
Why Getting a Pediatric Drug Dose Right Matters More Than You Think
A toddler who weighs 12 kg and an adolescent who weighs 50 kg cannot take the same amount of medicine. That statement sounds obvious, yet medication errors in children are three times more common than in adults according to a 2018 analysis in Pediatrics. The reason is simple: pediatric medication dosing almost always depends on body weight, and converting a "mg/kg" prescription into the actual number of millilitres a parent should draw from a syringe involves multiple arithmetic steps that are easy to get wrong at 2 a.m.
A child dosage calculator takes a child's weight in kilograms, the prescribed dose in mg/kg (per day or per dose), the dosing frequency, and the liquid medicine concentration in mg/mL. It returns the single dose in milligrams, total daily dose, volume per dose in millilitres, and daily volume. The source formulas align with the British National Formulary for Children (BNFC), the standard reference for pediatric prescribing in clinical practice. Children metabolise drugs differently from adults because of higher liver-to-body-weight ratios and different renal clearance rates, which is why flat adult doses scaled down by fraction are unreliable for kids.
Plug in your numbers above and skip the mental arithmetic.
Six Scenarios Where a Child Dosage Calculator Saves You Time
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Your paediatrician prescribes a weight-based antibiotic. Amoxicillin at 80 mg/kg/day split into 3 doses for a 15 kg child means 400 mg per dose, but a parent staring at a 250 mg/5 mL bottle still needs to figure out that equals 8 mL per syringe. The calculator bridges that gap in seconds.
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You are switching between liquid and tablet forms. A 30 kg child moving from liquid ibuprofen (100 mg/5 mL) to junior tablets (200 mg each) needs the exact milligram dose confirmed first. Getting the conversion wrong by even 50% can mean under-treating a fever or overshooting the 40 mg/kg/day ceiling for ibuprofen.
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Your baby's weight has changed since the last prescription. Infants can gain 150 to 200 g per week in the first 3 months. A dose calculated for a 4.5 kg baby at the 2-week checkup may be 10% too low by week 6 if the infant now weighs 5 kg. Recalculating with current weight keeps the dose therapeutic.
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You are a pharmacy technician double-checking a prescription. The Institute for Safe Medication Practices reports that 1 in 10 pediatric prescriptions contains a dosing error that is caught at the dispensing stage. A quick weight-based cross-check against the calculator flags discrepancies before they reach the patient.
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You are dosing an over-the-counter medicine for your child at home. Paracetamol (acetaminophen) dosing charts on the box group children by age bands, but a 3-year-old can weigh anywhere from 11 to 18 kg. Using the recommended 15 mg/kg/dose with the child's actual weight is more precise than relying on a broad age bracket.
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Your child has a chronic condition requiring daily medication. Kids with epilepsy, asthma, or ADHD often take weight-based doses that need recalculation every 3 to 6 months as they grow. A 25 kg child on valproate at 30 mg/kg/day takes 750 mg daily; at 28 kg six months later that becomes 840 mg, a 12% increase the prescriber should review.
The Formulas Behind Pediatric Dose Calculation
Every pediatric dose calculation reduces to a small set of multiplication and division steps that depend on which value the prescriber specifies.
Mode: Per kg Per Day (dose is prescribed as mg/kg/day)
Daily Dose = dose (mg/kg/day) × weight (kg)
Single Dose = Daily Dose ÷ frequency (times/day)
Mode: Per kg Per Dose (dose is prescribed as mg/kg/dose)
Single Dose = dose (mg/kg/dose) × weight (kg)
Daily Dose = Single Dose × frequency (times/day)
Mode: Per Day (flat daily dose given directly)
Daily Dose = dose (mg/day)
Single Dose = Daily Dose ÷ frequency (times/day)
Mode: Per Dose (flat single dose given directly)
Single Dose = dose (mg/dose)
Daily Dose = Single Dose × frequency (times/day)
Volume conversion (all modes):
Volume Per Dose = Single Dose (mg) ÷ concentration (mg/mL)
Daily Volume = Daily Dose (mg) ÷ concentration (mg/mL)
Source: British National Formulary for Children (BNFC), dosing principles section.
| Formula Mode | Prescriber Gives | Calculator Derives |
|---|---|---|
| Per kg Per Day | mg/kg/day + weight + frequency | Single dose (mg), daily dose (mg), volumes (mL) |
| Per kg Per Dose | mg/kg/dose + weight + frequency | Single dose (mg), daily dose (mg), volumes (mL) |
| Per Day | mg/day + frequency | Single dose (mg), volumes (mL) |
| Per Dose | mg/dose + frequency | Daily dose (mg), volumes (mL) |
| Common Pediatric Drug | Typical Dose | Usual Frequency | Max Daily Dose |
|---|---|---|---|
| Amoxicillin (standard) | 40–90 mg/kg/day | 3 times/day | 3,000 mg |
| Ibuprofen | 5–10 mg/kg/dose | 3–4 times/day | 40 mg/kg or 1,200 mg |
| Paracetamol (oral) | 15 mg/kg/dose | 4 times/day | 75 mg/kg or 4,000 mg |
| Cefalexin | 25–50 mg/kg/day | 2–3 times/day | 4,000 mg |
| Prednisolone | 1–2 mg/kg/day | 1–2 times/day | 60 mg |
Genetic polymorphisms in cytochrome P450 enzymes (particularly CYP2D6 and CYP2C19) can shift how fast a child metabolises certain drugs by 2 to 10 fold. The calculator assumes standard metabolism. Prescribers adjust doses for known poor or ultra-rapid metabolisers.
Limitations: The formulas assume the prescriber has already selected the correct mg/kg dose for the drug and indication. The calculator performs arithmetic only; it does not validate whether a dose is appropriate for a given medication. Always confirm the prescribed mg/kg value against a formulary or pharmacist.
How to Calculate a Child's Medication Dose Step by Step
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Weigh the child in kilograms. Use a recent weight, ideally within the last 2 weeks for infants under 6 months. If you only have pounds, divide by 2.205 to convert. A 33 lb toddler weighs approximately 15 kg.
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Identify the prescribed dose and its unit. Read the prescription carefully. "40 mg/kg/day" and "40 mg/kg/dose" produce very different numbers. For a 20 kg child at 3 times daily, the first gives 267 mg per dose while the second gives 800 mg per dose.
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Select the correct formula mode. Match the prescription unit to one of the four modes: Per kg Per Day, Per kg Per Dose, Per Day, or Per Dose. This is the step most people skip, and it is the single largest source of calculation errors.
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Enter the dosing frequency. Count how many times per day the medicine is given. "Every 8 hours" means 3 times per day. "Every 6 hours" means 4 times per day. "Twice daily" means 2.
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Enter the liquid concentration from the bottle label. Common concentrations include 125 mg/5 mL (which equals 25 mg/mL) and 250 mg/5 mL (which equals 50 mg/mL). Always convert to mg/mL before entering if the label states mg per 5 mL.
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Read all four outputs. The calculator returns single dose in mg, daily dose in mg, volume per dose in mL, and daily volume in mL. Cross-check: single dose multiplied by frequency should equal daily dose.
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Verify against the drug's maximum dose. Many pediatric drugs have an absolute ceiling regardless of weight. Ibuprofen caps at 400 mg per dose even if 10 mg/kg would suggest more for a heavy adolescent. The reference table above lists common ceilings.
Two Real-World Examples: See How the Numbers Play Out
Example 1: 8-Month-Old Infant With an Ear Infection
An 8.5 kg baby is prescribed amoxicillin at 90 mg/kg/day, divided into 3 doses. The pharmacy dispenses a suspension at 50 mg/mL.
Mode: Per kg Per Day
Daily Dose = 90 × 8.5 = 765 mg
Single Dose = 765 ÷ 3 = 255 mg
Volume Per Dose = 255 ÷ 50 = 5.1 mL
Daily Volume = 765 ÷ 50 = 15.3 mL
| Output | Value | Unit |
|---|---|---|
| Single Dose | 255 | mg |
| Daily Dose | 765 | mg |
| Volume Per Dose | 5.1 | mL |
| Daily Volume | 15.3 | mL |
What to do with this number: Draw 5.1 mL into an oral syringe three times a day. At 15.3 mL per day, a standard 100 mL bottle lasts about 6.5 days, which covers most 7-day antibiotic courses with a small margin. Ask the pharmacist for a 5 mL oral syringe marked in 0.1 mL increments for accurate measurement.
Example 2: 6-Year-Old With a Fever
A 22 kg child needs ibuprofen for fever. The parent has a 100 mg/5 mL liquid bottle and wants to give 10 mg/kg per dose, 3 times daily.
Mode: Per kg Per Dose
Single Dose = 10 × 22 = 220 mg
Daily Dose = 220 × 3 = 660 mg
Volume Per Dose = 220 ÷ 20 = 11 mL (100 mg/5 mL = 20 mg/mL)
Daily Volume = 660 ÷ 20 = 33 mL
| Output | Value | Unit |
|---|---|---|
| Single Dose | 220 | mg |
| Daily Dose | 660 | mg |
| Volume Per Dose | 11 | mL |
| Daily Volume | 33 | mL |
What to do with this number: 660 mg/day is within the 40 mg/kg/day ceiling (40 × 22 = 880 mg), so this dose is safe. Give 11 mL per dose using the measuring cup or syringe included with the bottle. Space doses at least 6 to 8 hours apart. If the fever persists beyond 3 days, see a doctor rather than increasing the dose.
Seven Errors That Throw Off Your Child's Dose
Confusing mg/kg/day with mg/kg/dose. This is the most dangerous mistake. A prescription for 30 mg/kg/day given 3 times daily means 10 mg/kg per dose. Reading it as 30 mg/kg per dose triples the intended amount. For a 15 kg child, that is the difference between 150 mg per dose and 450 mg per dose. Always confirm which unit the prescriber intended.
Using an outdated weight. A 9-month-old who weighed 7.5 kg at the 6-month visit may now weigh 9 kg. Dosing on the old weight under-delivers by 20%. For infants, re-weigh at every new prescription. For children over 2, a weight from the past 1 to 2 months is generally acceptable.
Ignoring the concentration on the bottle. Amoxicillin suspension comes in 125 mg/5 mL, 250 mg/5 mL, and 400 mg/5 mL strengths. Pouring 5 mL from the 400 mg/5 mL bottle when the dose was calculated for the 250 mg/5 mL strength delivers 60% too much drug. Check the label every time, even if "it looks like the same bottle."
Measuring with a kitchen spoon instead of a syringe. A household teaspoon holds anywhere from 2.5 to 7.5 mL depending on size and how full it is. A 2014 study in the Annals of Internal Medicine found that 84% of parents using kitchen spoons delivered inaccurate doses. Use the oral syringe or measuring cup from the pharmacy.
Forgetting to convert the label concentration. Many bottles list concentration as "250 mg per 5 mL" rather than "50 mg/mL." Entering 250 into the calculator's mg/mL field produces a volume that is 5 times too small. Divide the label number by 5 first (250 ÷ 5 = 50 mg/mL) before entering.
Rounding the volume too aggressively. A calculated dose of 3.7 mL rounded to "about 4 mL" represents an 8% overdose. Over 3 daily doses, the child receives an extra full dose worth of drug every 4 days. Round to the nearest 0.1 mL when using a syringe, or to the nearest 0.5 mL when using a measuring cup.
Splitting adult tablets without verifying the arithmetic. Giving "half a 500 mg tablet" to a child who needs 200 mg delivers 250 mg, which is 25% too much. Liquid formulations exist for paediatric dosing precisely because tablets cannot be split to arbitrary fractions. Ask the pharmacist for the liquid form if the dose does not align with a clean tablet fraction.
Assumptions and Notes
- Margin of error: The calculator performs exact arithmetic on the values entered. Real-world accuracy depends on the precision of the child's weight (aim for plus or minus 0.1 kg for infants) and the measuring device used for liquid volumes. A calibrated oral syringe delivers accuracy within 0.1 mL.
- Professional disclaimer: This calculator is an arithmetic tool. It does not validate whether a specific dose is appropriate for a given drug, condition, or child. Always confirm the prescribed mg/kg value and maximum dose limits with a pharmacist, paediatrician, or current formulary (BNFC, BNF, AAP guidelines). Do not self-prescribe pediatric medications based on calculator output alone.
Your Next Step
The hardest part of pediatric dosing is not the formula. It is reading the prescription unit correctly and measuring the liquid accurately. Run your child's numbers through the calculator above, then grab an oral syringe from the pharmacy and measure to the nearest 0.1 mL. If the result seems unusually high or low compared to what you expected, call your pharmacist before giving the dose.