A careful mg/kg medicine dose calculator for liquid medicines
This medicine dose calculator, for clinicians, nurses, pharmacists and students, turns a prescription in mg/kg into millilitres for an oral syringe — the core of any pediatric dose calculator. You type the dose the prescriber gave you, the patient’s weight in kilograms or pounds and the strength printed on the bottle (for example 120 mg per 5 mL), and it shows the dose in mg, the exact and rounded volume, and the daily totals.
It deliberately contains no drug database and no default doses: the correct dose depends on the indication, age, kidney and liver function, other medicines and the exact product. Use it to double-check a prescribed dose against your local protocol or paediatric formulary — never for self-treatment. A study panel shows the historical Clark’s, Young’s and Fried’s rules for exam practice. Everything runs in your browser; nothing is stored or sent.
How to use the Medicine Dose Calculator (mg/kg)
- Enter the Prescribed dose as written, mg/kg per dose or per day (divided), and the Doses per day.
- Enter a recent weight in kg or lb.
- Copy the Liquid strength from the bottle: the milligrams and the millilitres they are in.
- Optionally enter a maximum single dose and a maximum per 24 hours, and how finely your syringe is marked.
- Read the volume per dose, the notes and the working, and check it against your protocol.
Medicine Dose Calculator (mg/kg) features
No built-in doses
Every number comes from your prescription and your bottle, so the tool cannot suggest an inappropriate dose.
mg/kg per dose or per day
Handles both ways prescriptions are written; per-day doses are split evenly across the doses.
Maximum caps you control
A single-dose and a 24-hour maximum are applied before rounding, and rounding is never allowed to go above them.
Measurable volumes
Rounds to 1, 3, 5 or 10 mL syringe markings or half a 5 mL spoon, warning when rounding changes the dose by more than 10%.
Working shown
Weight conversion, dose, strength in mg/mL and volume are printed for a quick second check.
Sanity warnings
Flags unusual mg/kg values, very large volumes and unmeasurably small doses.
Historical rules for study
Clark’s, Young’s and Fried’s rules are worked side by side, clearly labelled as not for prescribing.
How weight-based dosing is calculated
Three steps: dose (mg) = mg/kg × weight (kg), divided by the number of doses if the order is per day; strength (mg/mL) = label mg ÷ label mL; volume (mL) = dose ÷ strength.
Worked example with made-up numbers: 15 mg/kg for a 12 kg child is 15 × 12 = 180 mg. A liquid of 120 mg per 5 mL has 24 mg per mL, so the volume is 180 ÷ 24 = 7.5 mL; four doses a day are 720 mg. With a 150 mg single-dose maximum the dose is capped at 150 mg = 6.25 mL, and with 0.5 mL steps the tool rounds down to 6.0 mL rather than up to 6.5 mL (156 mg — over the cap).
Clark’s, Young’s and Fried’s rules — for study only
Older texts scaled an adult dose. Clark’s rule: weight in lb ÷ 150 × adult dose (150 lb ≈ 68 kg). Young’s rule for ages 1–12: age in years ÷ (age + 12) × adult dose. Fried’s rule for infants: age in months ÷ 150 × adult dose. With a made-up adult dose of 500 mg, a 12 kg (26.5 lb) four-year-old gets 88 mg by Clark, 125 mg by Young and 160 mg by Fried — three different answers, which is why they are taught as history, not for prescribing (StatPearls, “Clark’s Rule”, NBK541104, updated 30 July 2023; checked 24 October 2026). Real paediatric doses come from mg/kg or body-surface-area figures in a formulary.
For tablet and D/H × Q practice questions use the dosage calculator; for volumes from mg/mL, % or 1 : 1,000 strengths, the mg to mL calculator.
Common mistakes this helps you catch
- mg versus mL: the order is in milligrams, the syringe in millilitres — the same number only at 1 mg per mL.
- Different strengths: many medicines come in two strengths (for example 120 mg/5 mL and 250 mg/5 mL); switching bottles without recalculating doubles or halves the dose.
- Per dose versus per day: “30 mg/kg/day in 3 divided doses” means 10 mg/kg each time, not 30.
- Kitchen spoons: household teaspoons hold about 2.5 to 7 mL; use an oral syringe.
- Out-of-date weight: recalculate after re-weighing, but change a prescribed dose only with the prescriber.
Volume conversions on the label
| Label says | Strength in mg/mL | Volume for 100 mg |
|---|---|---|
| 120 mg / 5 mL | 24 | 4.17 mL |
| 125 mg / 5 mL | 25 | 4 mL |
| 250 mg / 5 mL | 50 | 2 mL |
| 100 mg / mL (drops) | 100 | 1 mL |
| 40 mg / mL | 40 | 2.5 mL |
Medicine Dose Calculator (mg/kg) FAQ
How do I calculate a medicine dose in mg/kg?
Multiply the prescribed mg/kg by the weight in kilograms, divide by the number of doses if it is per day, then divide by the strength in mg per mL to get millilitres.
Why doesn’t the calculator suggest a dose for a medicine?
Because the right dose depends on far more than weight — the indication, age, other conditions and the product. It only does the arithmetic on a dose already prescribed; check it against your formulary.
How do I convert mg to mL for a syrup?
Divide the strength by its volume — 250 mg in 5 mL is 50 mg per mL — then divide the dose by it: 200 mg is 4 mL.
Should I round the dose up or down?
To the nearest syringe marking, but never above a maximum dose: the calculator rounds down when rounding up would exceed a cap, and warns when rounding changes the dose noticeably.
Can I use this for adult medicines or tablets?
The arithmetic works for any weight, but most adult medicines are not dosed by weight and tablets cannot be measured like liquids — use the dosage calculator for tablets.
Should I use Clark’s or Young’s rule to work out a child’s dose?
No. They are historical approximations that disagree with each other and with modern data; they appear here only for study. Use the mg/kg or body-surface-area dose in a paediatric formulary.
What if my result is different from what the pharmacist said?
Trust the pharmacist and the label and discuss the difference — often a different strength, a cap or a typing error. The medication schedule maker prints a dose tick chart.