Drug dosing in children: how to correctly calculate the dose per kilogram

In children, a drug's dose is not “guessed” from age, and you do not simply halve the adult dose. A child's body processes substances differently, and the correct reference is body weight in kilograms: almost all paediatric medicines are dosed in milligrams per kilogram of body weight (mg/kg). A calculation error or a confusion between millilitres and milligrams can mean either an ineffective dose or a dangerous overdose. This guide shows you how to calculate correctly, with real examples for paracetamol and ibuprofen, which mistakes are the most common, and what should never be given to a child without a doctor's approval.

Why children are not “small adults”

A 15 kg child is not “a quarter of an adult”. The differences are physiological, not just a matter of size:

• The liver and kidneys are immature, especially in infants. These organs eliminate medicines more slowly or, in other cases, faster than in an adult, which changes both the dose and the interval between doses.
• The proportion of water in the body is higher, and the distribution of the substance throughout the body differs.
• The safety margin is narrower: in young children, the difference between the useful dose and the toxic dose is reached more easily.

That is why EMA and the WHO (World Health Organization) treat paediatric medication as a distinct field, with its own formulations and doses, rather than an “eyeballed” adaptation of adult treatment. In Romania, the patient information leaflets of products approved for paediatric use by ANMDMR (the Romanian National Agency for Medicines and Medical Devices) contain dosing sections based on weight and age.

The golden rule: the dose per kilogram of body weight

For most paediatric medicines, the leaflet indicates a dose expressed in mg/kg. The basic formula is simple:

Dose (mg) = child's weight (kg) × recommended dose (mg/kg)

Then, to know how many millilitres of syrup or suspension to pour, one more step is needed, because the bottle contains a certain concentration (mg in a given volume). More on that below.

How to calculate, step by step

1. Weigh the child (the real, recent weight — not the one “from a few months ago”). The dose is calculated by weight, not by age.
2. Read the dose per kg/dose in the leaflet and the minimum interval between doses.
3. Multiply: weight × mg/kg = milligrams per dose.
4. Convert to millilitres using the bottle's concentration.
5. Check the maximum dose over 24 hours and do not exceed it, even if the fever returns.

If any step seems unclear, ask the pharmacist before giving the first dose — your calculation can be checked in a few seconds at the counter.

Paracetamol in children: 15 mg/kg per dose

Paracetamol is the first-line antipyretic and analgesic in children. The usual recommended dose in paediatrics is approximately 15 mg/kg per dose, every 4–6 hours, generally without exceeding 60 mg/kg over 24 hours (and usually a maximum of 4 doses per day), according to the BNF for Children (NICE) and the leaflets approved by ANMDMR. You can find more about the mechanism and precautions in our guide on paracetamol.

Example: a 12 kg child. The dose per intake = 12 × 15 = 180 mg. If you use a syrup with 120 mg per 5 ml (24 mg/ml), the volume = 180 ÷ 24 = 7.5 ml. Note: other products have other concentrations, so the volume in ml differs from bottle to bottle.

Paediatric ibuprofen: alternative and differences

Ibuprofen, a non-steroidal anti-inflammatory drug (NSAID), is the second common option for fever and pain. The usual paediatric dose is approximately 5–10 mg/kg per dose, every 6–8 hours, without exceeding the maximum dose in the leaflet. Unlike paracetamol, ibuprofen:

• is given after a meal, to protect the stomach;
• is generally not recommended for infants under 3 months or below a certain weight (see the leaflet);
• requires caution in a dehydrated child, a child with chickenpox, or with kidney problems — ask the doctor.

One essential detail: ibuprofen suspensions for children come in different concentrations (for example 100 mg/5 ml and the “forte” variant 200 mg/5 ml). The same quantity in ml means double the dose if you take the wrong bottle. Always read the concentration on the label.

Reference table: two common antipyretics

The values are indicative and may differ from product to product. The exact dose, interval and duration are established with the doctor or pharmacist, based on the child's real weight and the leaflet of the product used.

Syrups and suspensions: concentration matters enormously

In children, liquid medicines are the most used. The trap is that the same substance comes in different concentrations. One syrup may have 120 mg/5 ml, another 250 mg/5 ml; an ibuprofen suspension may be 100 mg/5 ml or 200 mg/5 ml. If you switch the bottle without checking, you can double the dose without realising it.

The practical rule: do not memorise “so many ml”, but recalculate every time you open a new product. Write on the box the dose in ml suitable for your child, written by the pharmacist, so you don't get confused at 3 in the morning.

The dosing syringe vs. the kitchen spoon

Use only the device from the box — the graduated oral syringe, the pipette or the dosing cup. An ordinary kitchen spoon is not a medical instrument: it can hold between 2 and 7 ml, depending on the model, so it leads to large errors. The dosing syringe allows precise measurement, in ml, and administration into the corner of the mouth, without the risk of choking. If you have lost the device, ask for another one at the pharmacy — do not improvise. For the general administration technique, see our guide on the correct administration of medicines.

Common mistakes that lead to overdose

• Confusing mg ↔ ml. “5 ml” does not mean “5 mg”. The leaflet gives you the dose in mg or in mg/kg; you convert it into ml according to the bottle's concentration.
• Dosing by age, not by weight. Two children of the same age can have very different weights. Weight wins.
• Two products with the same INN. A “fever” syrup and a “cold” syrup may both contain paracetamol — given together, they add up the dose and can exceed it. Always check the active substance, not just the commercial name. Be especially careful with combinations with cough syrups that also contain paracetamol.
• Repeating the dose too early. If the fever returns before the minimum interval, you do not give another dose — you use physical methods (fluids, light clothing) and call the doctor.
• “If one dose is good, two is better.” False and dangerous. A paracetamol overdose can seriously damage the liver.
• Switching the bottle without recalculating. New concentration = new dose in ml.

If you suspect you have exceeded the dose or the child has accidentally swallowed medicine, call 112 or a doctor immediately — do not wait for symptoms to appear. And if you have simply forgotten an intake, first read what to do when you have forgotten a dose before doubling the next one.

What NOT to give children: aspirin and Reye's syndrome

Acetylsalicylic acid (aspirin) is not given to children and adolescents for fever or viral infections (cold, flu, chickenpox). The reason is Reye's syndrome, a rare but very serious condition that causes damage to the liver and brain. For this reason, the BNF for Children / NICE and the regulatory authorities recommend avoiding aspirin under the age of 16, except for strict medical indications established by a specialist. For fever in a child, the first-line options remain paracetamol and ibuprofen, in the correct doses.

Other classes require special caution in children and are not given without a prescription: antibiotics (only on the doctor's recommendation, with a dose per kg), codeine-containing cough suppressants (contraindicated for cough and cold in children under 12 according to the EMA restriction, and not recommended between 12 and 18 years with respiratory problems) and some decongestants. When in doubt, ask the pharmacist.

When to call the doctor or go to the emergency room

• a child under 3 months with a fever — always a medical evaluation;
• a fever that does not respond to the correctly dosed antipyretic or that lasts more than 2–3 days;
• signs of dehydration, unusual drowsiness, breathing difficulties, rashes;
• suspicion of overdose or accidental ingestion — call 112;
• a child with chronic illnesses or already taking other medicines — check the interactions with the doctor.

Key points

Drug dosing in children is done by kilogram of body weight, not by age and not by halving the adult dose. Weigh the child, read the dose in mg/kg from the leaflet, convert it into ml according to the bottle's exact concentration, use the dosing syringe from the box and respect the maximum dose over 24 hours. Avoid aspirin in children because of the risk of Reye's syndrome and do not combine two products with the same INN. Before the first dose of a new product, learn how to read the patient information leaflet and, for any uncertainty, ask the paediatrician or pharmacist — a correct calculation is safer than an estimate.