Why are paediatric dose calculations so error-prone?
Most adult doses are fixed. Most children's doses are calculated for each child, usually from body weight and sometimes from age or body surface area. Every calculation is a chance for an arithmetic slip, and liquid medicines add a second step, turning milligrams into millilitres.
The volumes are small, so a misplaced decimal point can make a dose ten times too high or too low. A tenfold overdose in an adult often looks obviously wrong, such as ten tablets instead of one. In a child it may be 5 mL instead of 0.5 mL, which looks perfectly normal in a syringe. Young children also have little reserve to cope with an overdose.
What is the formula for paediatric dose calculation?
There are two formulas you will use again and again. The first gives the dose for the child. The second turns that dose into a volume of liquid.
| Purpose | Formula | Watch out for |
|---|---|---|
| Dose for this child | Dose per kg × weight in kg | Is the reference dose per dose or per day? |
| Each dose from a daily total | Daily dose ÷ number of doses per day | Giving the daily total at every dose |
| Volume to give | (Dose required ÷ stock strength) × stock volume | Dose and stock must be in the same unit |
| Infusion rate in mL per hour | Dose per hour ÷ concentration per mL | Using the wrong concentration of bag or syringe |
- Confirm the weight: Use a recent measured weight in kilograms from the chart or record. If a parent gives pounds, convert carefully (1 kg is about 2.2 lb) and weigh the child as soon as you can.
- Find the dose: Use the current BNF for Children (BNFC) or your local paediatric formulary. Write down the dose per kg, whether it is per dose or per day, and the frequency.
- Multiply: Multiply the dose per kg by the weight. If it is a daily dose, divide by the number of doses.
- Check the maximum: Compare with the maximum single and daily dose. If your answer is higher, the maximum applies. In general, a child's dose should not exceed the usual adult dose.
- Convert to a volume: Make sure units match, then use the volume formula.
- Sense check: Is this a sensible volume for a child of this age? If not, start again from the beginning.
Worked examples: from simple to tricky
The medicines below are hypothetical, and the doses are for practice only. Always take real doses from the BNFC or your local guideline.
Example 1: a single weight-based dose
A child weighs 16 kg. The prescription is 15 mg/kg per dose. The liquid is 250 mg in 5 mL. Dose: 15 × 16 = 240 mg. Assume a maximum single dose of 500 mg, so 240 mg is within range. Volume: (240 ÷ 250) × 5 = 0.96 × 5 = 4.8 mL. Sense check: just under 5 mL for a preschool child is plausible.
Example 2: per day, in divided doses
A child weighs 20 kg. The dose is 30 mg/kg per day in 3 divided doses. Daily dose: 30 × 20 = 600 mg. Each dose: 600 ÷ 3 = 200 mg every 8 hours. The classic error is to give 600 mg three times a day, which is three times the intended daily amount.
Example 3: micrograms and milligrams
A baby weighs 3.8 kg. The dose is 10 micrograms/kg. The injection is labelled 0.1 mg in 1 mL. Dose: 10 × 3.8 = 38 micrograms. Convert the stock first: 0.1 mg = 100 micrograms, so the stock is 100 micrograms in 1 mL. Volume: (38 ÷ 100) × 1 = 0.38 mL. If you had divided 38 by 0.1 without converting, you would get 380 mL, a thousandfold error.
Example 4: when the maximum dose applies
A 14-year-old weighs 68 kg. The dose is 10 mg/kg, maximum 500 mg per dose. Calculated: 10 × 68 = 680 mg. This is above the maximum, so the correct dose is 500 mg. Maximums are most often missed in larger adolescents and in children living with obesity. For some medicines the BNFC advises using ideal body weight in obese children, so read the monograph.
Example 5: an infusion rate
An infusion contains 100 mg in 50 mL, which is 2 mg per mL. The prescription is 12 mg per hour. Rate: 12 ÷ 2 = 6 mL per hour. Many children's units use standard concentrations and smart pump drug libraries so that rates can be checked against a table. Use them where your organisation provides them.
| Unit | Equals | Error if confused |
|---|---|---|
| 1 g | 1,000 mg | Thousandfold |
| 1 mg | 1,000 micrograms | Thousandfold |
| 1 microgram | 1,000 nanograms | Thousandfold |
| 1 litre | 1,000 mL | Thousandfold |
| 0.5 mg written as 5 mg | Missing decimal point | Tenfold |
What causes tenfold errors, and how do you prevent them?
Tenfold errors in children are well recognised and can be fatal. They usually come from the way a number is written or read, from mixing up units, or from using a different strength of the same medicine than the one the calculation assumed.
| Cause | Example | Safe practice |
|---|---|---|
| Trailing zero | 5.0 mg read as 50 mg | Never write a trailing zero. Write 5 mg |
| No leading zero | .5 mg read as 5 mg | Always write 0.5 mg |
| Abbreviated units | mcg or µg read as mg; U read as 0 | Write micrograms, nanograms and units in full |
| Two strengths of one liquid | Dose worked out for 120 mg in 5 mL but measured from 250 mg in 5 mL | State the strength on the prescription and check the bottle label |
| Wrong weight | Pounds recorded as kilograms, or 3.2 kg entered as 32 kg | Record kg, check the weight fits the child's age and size |
| Adult stock on a children's ward | Adult vial strength used for a paediatric calculation | Check the vial label against the strength used in the working |
Paracetamol oral suspension, for example, is sold as both 120 mg in 5 mL and 250 mg in 5 mL. A dose measured from the wrong bottle is a large error even when the arithmetic was perfect. Morphine oral solution also comes in very different concentrations.
Electronic prescribing helps with legibility and can calculate doses, but it creates new risks, such as picking the wrong weight or the wrong drop-down option, or overriding an alert without reading it. A computer-calculated dose still needs a human sense check.
What does an independent check really mean?
An independent check means the second person works the dose out from the original sources, before looking at the first person's answer. Reading someone else's working and agreeing with it often repeats their mistake.
Nurse A: "I've drawn up 0.4 mL for bed 6, can you sign?" Nurse B glances at the syringe and the chart, says it looks fine, and signs.
Nurse B: "Let me work it out before I look at yours." She checks the wristband against the chart, confirms the weight, reads the prescribed dose of 16 mg and the strength on the paediatric vial (10 mg in 1 mL), and gets 1.6 mL. Nurse A then realises she used the adult strength. They discard the syringe and redraw together.
- Prescribers: write the mg/kg dose and the calculated dose where local policy asks for both, state the strength of liquids, and ask a senior or paediatric pharmacist before prescribing if unsure.
- Pharmacists: recalculate from the weight, check frequency, maximum dose and formulation, and in community pharmacy contact the prescriber and ask for a recent weight if a dose looks unusual for the child's age.
- Nurses and midwives: follow local double-checking policy, often for IV medicines, controlled drugs and neonatal medicines, and involve parents, who often know the child's usual dose.
- Everyone: if you are told to just give it and you are not satisfied, stop and get the dose checked.
Why do competent professionals make paediatric dosing errors?
Most errors are made by capable people in busy, complex settings. Recognising the human and system factors is part of insight and the basis of any improvement plan.
- Interruptions that break a calculation part way through.
- Unfamiliarity, for example locums, rotating doctors or agency staff used to adult doses.
- An old, estimated or wrongly entered weight.
- Adult and paediatric strengths stored side by side.
- Mental arithmetic at night or in an emergency.
- A culture where juniors or parents do not feel able to question a dose.
In emergencies, many departments use pre-calculated weight-based dose charts so no one has to calculate under pressure. Find out where yours are before you need them.
What should you do if a paediatric dosing error happens?
- Make the child safe: Assess the child, including observations and conscious level. Stop any infusion. Call for help through your emergency process if the child is unwell.
- Escalate: Tell the nurse in charge and the medical team straight away. Seek pharmacy advice, and poisons information advice through your clinical team where needed.
- Be open: Tell the parents or carers, and the child where appropriate, what happened, apologise and explain what is being done.
- Document and report: Write a factual account with times, the dose given and intended, and who was told. Never alter earlier entries. Complete an incident report.
- Get support: Talk to your supervisor. If you are contacted by your regulator or are under investigation, get advice from your defence organisation, trade union or legal representative before responding.
On his first night on a children's ward, Amir prescribes for a 12 kg toddler. The BNFC dose is 5 mg/kg per day in 2 divided doses. He calculates 60 mg and prescribes 60 mg twice daily, so the child receives double the intended dose. The ward pharmacist spots it the next morning. The child is reviewed and stays well. Amir tells his registrar, corrects the prescription, apologises to the parents and completes an incident report. Afterwards he completes calculation training and an assessment marked by the pharmacist, writes the mg/kg dose and his working on every paediatric prescription, and asks himself 'per dose or per day?' before signing. His supervisor audits his prescriptions over the next month.
Regulators such as the GMC, NMC and GPhC look at whether a professional has understood what went wrong and why, and whether they have made changes that others can verify over time. A single error handled honestly is very different from one that is hidden.
Practising and evidencing safe dose calculation
The best protection is a habit: the same steps, written down, every time, followed by a genuine independent check. Practise with calculation questions from your trust or university, and ask a paediatric pharmacist to review your working.
If you want a structured refresher, or need to show learning after an error, our course Paediatric Prescribing and Dose Calculation covers weight-based calculations, safe prescribing conventions, independent checking and a case study with weak and strong reflections. Use it alongside, not instead of, the BNFC and your local policy.
Frequently asked questions
How do you calculate a paediatric dose by weight?
Multiply the dose per kilogram from the BNFC or local guideline by the child's weight in kilograms. If the reference dose is per day, divide by the number of doses. Check the result against the maximum dose, then convert to a volume using (dose required ÷ stock strength) × stock volume.
How do I know if a dose is per dose or per day?
Read the BNFC monograph or local guideline carefully. It will say, for example, mg/kg per dose or mg/kg per day in divided doses. Write the full wording down before you calculate. Confusing the two is one of the most common causes of paediatric overdose.
What is a tenfold medication error?
A tenfold error is a dose ten times higher or lower than intended. In children it usually comes from a misplaced or missing decimal point, a trailing zero, a unit mix-up or using the wrong strength of a medicine. Tenfold errors can be fatal in small children.
Should I use actual or ideal body weight for an overweight child?
It depends on the medicine. For some medicines the BNFC advises calculating the dose using ideal body weight in children who are obese, and for others actual weight is used. Check the specific monograph and local guidance, and ask a paediatric pharmacist if unsure.
Can a child's dose ever be higher than the adult dose?
In general, a weight-based dose for a child should not exceed the usual adult dose, and most monographs state a maximum. If your calculation comes out higher than the maximum, the maximum applies. There are some specific exceptions, so always check the BNFC entry.
What is the formula for working out the volume of liquid to give?
Volume = (dose required ÷ stock strength) × stock volume. For example, 240 mg from a 250 mg in 5 mL liquid is (240 ÷ 250) × 5 = 4.8 mL. Make sure the dose and stock strength are in the same unit before dividing.
What should I do if I give a child the wrong dose?
Assess the child and stop any infusion, call for help if needed, and tell the nurse in charge and the medical team immediately. Seek pharmacy advice, tell the parents honestly, document the facts and complete an incident report. If a regulator contacts you, get advice from your union or defence organisation.
Related courses
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Sources and further reading
- BNF for Children, guidance on prescribing in children. BMJ Group, Royal Pharmaceutical Society and RCPCH Publications.
- Good practice in prescribing and managing medicines and devices. General Medical Council.
- The Code: Professional standards of practice and behaviour for nurses, midwives and nursing associates. Nursing and Midwifery Council.
- Standards for pharmacy professionals. General Pharmaceutical Council.
- A Competency Framework for all Prescribers. Royal Pharmaceutical Society.
- Openness and honesty when things go wrong: the professional duty of candour. General Medical Council and Nursing and Midwifery Council.
Practitioner Insight is an independent education provider. It is not affiliated with or endorsed by the GMC, MPTS, NMC, HCPC, GPhC or any NHS organisation, and this information is not legal advice. If you have been referred to a regulator, speak to your defence organisation, trade union or legal representative.
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