Pediatric Weight-Based Dose Calculator (mg/kg to mL)
Turn a children’s mg/kg/day or mg/kg/dose order into milligrams and millilitres per dose, capped at the maximum single and daily dose, rounded down to what your oral syringe can measure, with the dosing interval, tablets per dose, a prescribed-dose check and the bottle size for the course.
How it works
Per dose (mg) = mg/kg/day × weight (kg) ÷ doses per day, or mg/kg/dose × weight · capped at the maximum single dose and at the maximum daily dose ÷ doses per day · exact volume (mL) = mg ÷ (strength mg ÷ mL) · measured volume = exact volume rounded DOWN to the syringe marks · dose given = measured volume × mg/mL
Children are dosed by weight because their size varies twenty-fold from newborn to teenager. Most references quote milligrams per kilogram per day, divided into a number of doses; some quote milligrams per kilogram per dose. Confusing the two is the commonest paediatric dosing error, so choose the form your order uses at the top. Weight can be entered in kilograms or pounds. The calculator then applies two caps: the maximum single dose (a child should never get more than an adult) and, if you enter it, the maximum daily dose, which reduces each dose when the total would go over. The liquid’s strength, always written as milligrams in a set volume such as 400 mg/5 mL, turns milligrams into millilitres. The exact volume is then rounded down — never up — to what your oral syringe can measure, and the calculator shows the milligrams that volume actually delivers, the dose per kilogram that results and a table of the same dose measured with finer or coarser syringe marks. Under More options you can count tablets per dose (rounded down to a half tablet) and compare a prescribed amount with the weight-based dose. Every result is arithmetic only: confirm the drug, dose and limits with the prescriber, pharmacist or product label.
Worked example
Amoxicillin 45 mg/kg/day in two doses for an 18 kg child: 45 × 18 = 810 mg a day, so 405 mg per dose (below the 875 mg cap). With a 400 mg/5 mL suspension (80 mg/mL) the exact volume is 405 ÷ 80 = 5.063 mL. With 0.1 mL syringe marks that rounds down to 5.0 mL, which delivers 400 mg (98.8% of the calculated dose, 44.4 mg/kg/day), every 12 hours. Over 7 days that is 5.0 × 2 × 7 = 70 mL, so a 100 mL bottle.
Sources: NIH/NLM DailyMed — drug labels (maximum doses and strengths) · FDA — Use the right dosing device for liquid medicines · ISMP — List of error-prone abbreviations, symbols and dose designations
Frequently asked questions
What if I only know the weight in pounds?
Choose lb next to the weight field and the calculator converts it (1 kg = 2.2046 lb). A 40 lb child is about 18.1 kg. Typing pounds into a kilogram field more than doubles the dose, so check the unit.
Why does the calculator round down instead of to the nearest mark?
Rounding down keeps the dose at or below the calculated amount, which is the safer direction when giving medicine at home. The result shows what percentage of the calculated dose the rounded volume delivers, and the table shows how a syringe with finer marks gets closer. Never “top up” a dose to a convenient number.
What is the difference between mg/kg/day and mg/kg/dose?
mg/kg/day is the total for 24 hours and is split across the doses: 45 mg/kg/day in 2 doses is 22.5 mg/kg each time. mg/kg/dose is given in full every time. Picking the wrong one multiplies or divides the dose by the number of doses a day.
What do the maximum dose fields do?
The maximum per dose stops a heavy child getting more than an adult single dose. The maximum per day, if you enter it, lowers each dose when the daily total would pass the label limit. Take both from the product label or a paediatric formulary; the defaults are only an example.
How do I read a strength like 250 mg/5 mL?
Enter 250 in the mg field and 5 in the mL field. That is 50 mg in each mL. Strengths are per volume, not per bottle — a 100 mL bottle of 250 mg/5 mL contains 5,000 mg in total.
Related calculators
Educational tool for clinicians and carers — not a prescription. Always confirm doses against the product label, local formulary and the child’s actual weight, and consult a doctor or pharmacist before giving medicine to a child.