Paediatric Safe Dose
Check an ordered dose against the safe mg/kg/day range for a child’s weight.
Within the safe range of 360–720 mg/day for 18 kg.
Per dose that is 120 – 240 mg across 3 doses.
Multiply the child’s weight by the low and high ends of the reference range to get a safe daily window, then compare the ordered total. For an 18 kg child with a 20–40 mg/kg/day range that window is 360–720 mg/day; 150 mg three times a day is 450 mg, so the order is safe.
Checking the order, not just calculating it
Paediatric med-math has an extra step that adult dosing usually does not: before giving anything, you verify that what was ordered actually falls inside the range the drug reference allows for that child’s weight. This is the step examinations test hardest, because the correct answer is sometimes to withhold the dose and query the order.
The check has three parts. Work out the safe window from the weight, work out the total the order delivers over 24 hours, and compare. An order can look reasonable per dose and still be unsafe once you count the frequency — 500 mg four times a day is 2000 mg, not 500.
Compare like with like
The reference range is almost always written per day, so the ordered amount has to be converted to a daily total before comparing: the single dose multiplied by the number of doses in 24 hours. Comparing a single dose against a daily range is the most common way this check goes wrong, and it makes a safe order look dangerously low.
If the ordered daily total falls inside the safe range, the order is within reference. Outside it — high or low — the order should be clarified before administration.
Worked example: 18 kg child, 20–40 mg/kg/day, 150 mg every 8 hours
Build the window first, then total the order:
- 1 Weigh the child in kilograms. The child weighs 18 kg. Paediatric weights should be current, not from a previous admission.
- 2 Look up the safe range. The drug reference gives 20 to 40 mg/kg/day for this indication.
- 3 Calculate the low end. 20 mg/kg/day × 18 kg = 360 mg per day.
- 4 Calculate the high end. 40 mg/kg/day × 18 kg = 720 mg per day.
- 5 Total what has been ordered. 150 mg every 8 hours is three doses: 150 × 3 = 450 mg per day.
- 6 Compare and act. 450 mg sits between 360 and 720 mg, so the order is within range. Outside the window, hold and clarify with the prescriber.
Safe daily windows by weight
Weight multiplied by the low and high ends of the reference range. Ranges are examples for practising the method — always use the range in your own drug reference.
| Weight | 20–40 mg/kg/day | 25–50 mg/kg/day | 10–20 mg/kg/day |
|---|---|---|---|
| 6 kg | 120–240 mg | 150–300 mg | 60–120 mg |
| 10 kg | 200–400 mg | 250–500 mg | 100–200 mg |
| 18 kg | 360–720 mg | 450–900 mg | 180–360 mg |
| 25 kg | 500–1000 mg | 625–1250 mg | 250–500 mg |
| 32 kg | 640–1280 mg | 800–1600 mg | 320–640 mg |
| 45 kg | 900–1800 mg | 1125–2250 mg | 450–900 mg |
When the order falls outside the range
An order above the safe range is not given. It is held, and the prescriber is contacted — the calculation is your evidence, so record the weight, the range you used, and the daily total you arrived at. An order below the range is queried too: an underdose of an antibiotic is a treatment failure and can drive resistance, so it is a clinical problem rather than a safe error.
Two further checks are worth building into the habit. Compare the total against any absolute maximum daily dose, which applies regardless of weight and can be reached by an older child. And confirm the weight itself is current and measured, since a stale or estimated weight silently shifts the entire window.
This tool is a study aid for practising calculations. Safe ranges must come from your own drug reference, and paediatric doses must be independently verified by a qualified professional before administration.