Medication errors are more common and potentially more harmful in children than adults. Doses are calculated by weight (or sometimes body surface area), many medicines come in adult strengths that need dilution, and children have less physiological reserve. A misplaced decimal point can result in a ten-fold overdose. Accurate calculation and double-checking are essential.
1. Get an accurate weight
- Weigh the child in kilograms on calibrated scales; don’t guess or rely on the parent’s estimate except in emergencies.
- Record weight in kg clearly. Confusing pounds and kilograms causes errors.
- In emergencies, age-based formulas or length-based tapes may be used temporarily.
- For obese children, some medicines are dosed on ideal or adjusted weight; check the formulary.
2. The basic formula
Dose (mg) = weight (kg) × dose per kg (mg/kg)
Then calculate the volume:
Volume (ml) = dose required (mg) ÷ stock strength (mg) × stock volume (ml)
Worked example 1
A child weighs 14 kg. Paracetamol is prescribed at 15 mg/kg. Stock: 120 mg in 5 ml.
- Dose = 14 × 15 = 210 mg.
- Volume = 210 ÷ 120 × 5 = 8.75 ml.
- Check: 210 mg is below the usual single maximum for this age per your formulary.
Worked example 2 (daily dose divided)
A child weighs 20 kg. Amoxicillin prescribed at 90 mg/kg/day divided into two doses. Stock: 250 mg in 5 ml.
- Daily dose = 20 × 90 = 1,800 mg/day.
- Each dose = 1,800 ÷ 2 = 900 mg.
- Volume = 900 ÷ 250 × 5 = 18 ml per dose.
Watch for “per day” vs “per dose”: confusing the two causes many errors.
3. Check against maximum doses
A weight-based dose should never exceed the adult maximum. Always check the calculated dose against a paediatric formulary (such as the BNF for Children, WHO Model Formulary for Children, or your national formulary).
4. Avoid ten-fold errors
- Always use a leading zero: write 0.5 mg, never .5 mg.
- Never use a trailing zero: write 5 mg, never 5.0 mg (which can be read as 50).
- Write micrograms in full; don’t abbreviate as µg or mcg where policy says so.
- Ask: does this answer make sense? A tiny baby rarely needs a large volume of an oral medicine.
5. Use the right equipment
- Use oral/enteral syringes for liquid medicines, not household spoons, and never IV syringes for oral medicines.
- Choose a syringe size close to the volume to improve accuracy.
- Infusions: use pumps and standardised concentrations.
6. Double-check
Many hospitals require an independent second check for paediatric medicines, especially IV and high-alert medicines. The second checker should calculate independently, not just confirm your answer.
Teach parents
Show parents how to measure doses with an oral syringe, warn about giving the same medicine in different brands (e.g. paracetamol in combination products), and explain maximum doses per day.
Related: Drug calculation formulas · Rights of medication administration
Examples are for calculation practice only, not dosing recommendations. Always use your local paediatric formulary and prescription. Based on WHO Medication Without Harm and paediatric medication safety guidance.