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Please wait while the page loadsMedication Safety · Paediatric Calculations
The formulas students keep coming back to for oral meds, IV rates, and paediatric maths, explained without the extra noise.
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Student note
This page is for the maths part only. Set the question up calmly, write each step out, and sense-check the answer at the end. If the final number looks wildly too big or too small, that is your cue to stop and rework it before anything is given.
Come here for
Oral formulas, IV drip rates, pump rates, and paediatric dose maths in one place.
If you were looking for
The old paediatric dose calculator or IV drip-rate page, it all lives here now so you are not jumping between near-duplicate pages.
Pair it with
9 Rights of Medication Administration for the safety checks and Medication Abbreviations Guide for chart shorthand.
The one you will use most — oral liquids and tablets
Need
What the chart says the patient should have.
Have
The strength you actually have in your hand, per tablet or per ml.
Stock
The amount that strength comes in, such as 5 ml or 1 tablet.
Worked example — oral liquid
Child needs 150mg ibuprofen. You have ibuprofen 100mg/5ml.
Worked example — tablets
Patient needs 7.5mg bisoprolol. You have 5mg tablets.
Drops per minute — for gravity sets without a pump
Important
Drop factor is not something you calculate from memory. It is printed on the IV giving set packaging, so check the pack first. Common examples are 20 drops/ml for a standard set, 15 drops/ml for blood, and 60 drops/ml for a microdrip or paediatric set.
Post-op fluids
1L 0.9% saline over 8 hours. Standard set (20 drops/ml).
Paediatric IV
Child needs 250ml over 6 hours. Microdrop set (60 drops/ml).
ml per hour — for electronic infusion pumps
Maintenance fluids
3L over 24 hours via pump.
Antibiotic infusion
500mg vancomycin in 100ml over 2 hours.
The standard paediatric maintenance-fluid estimate
| Weight range | Daily allowance |
|---|---|
| First 10 kg | 100 ml/kg/day |
| Next 10 kg (11–20 kg) | 50 ml/kg/day |
| Each kg above 20 kg | 20 ml/kg/day |
Plain English
Holliday-Segar is just a standard way to estimate how much maintenance fluid a child needs over 24 hours. Work through the weight in blocks: first 10 kg, next 10 kg, then anything above 20 kg.
8 kg
→ 33 ml/hr
18 kg
→ 58 ml/hr
25 kg
→ 71 ml/hr
When the dose depends on the patient's weight in kilograms
Gentamicin (adult)
Patient weighs 80kg. 5mg/kg once daily.
Paracetamol (child)
Child weighs 25kg. Paracetamol 15mg/kg PRN.
Divided doses — step by step
Step 1: Daily dose = mg/kg/day × weight (kg). Step 2: Single dose = Daily dose ÷ Number of doses.
Example
Child weighs 20 kg. Amoxicillin is prescribed at 25 mg/kg/day in 3 divided doses (TDS). Step 1: 25 × 20 = 500 mg/day. Step 2: 500 ÷ 3 = 166.6 mg per dose.
Always check the safe range in the BNF or BNFc
Worked example
Doctor prescribes 200mg ibuprofen TDS for an 18kg child. The BNFc (the children's British National Formulary) says the maximum is 30mg/kg/day. Is it safe?
Moving down the staircase (to a smaller unit) — multiply by 1,000. Moving up the staircase (to a larger unit) — divide by 1,000.
How to use the staircase
Convert 0.25 mg to mcg: you are moving one step down from mg to mcg, so multiply by 1,000. Answer: 250 mcg. Convert 500 ml to L: you are moving one step up from ml to L, so divide by 1,000. Answer: 0.5 L.
Calculation safety reminders
Also practice with