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Please wait while the page loadsMedication Safety · Free Resource
Essential formulas and step-by-step methods for common drug calculations — from oral liquids to IV rates and weight-based paediatric dosing.
Need
What the prescription says — the dose prescribed
Have
Strength of medication you have (per tablet or ml)
Stock
Volume or quantity it comes in (ml or number of tablets)
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.
Important
Drop factor is NOT calculated — it's printed on the IV giving set packaging. Just look at the box. Standard: 20 drops/ml. Blood: 15 drops/ml. Micro/Paeds: 60 drops/ml.
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).
Maintenance fluids
3L over 24 hours via pump.
Antibiotic infusion
500mg vancomycin in 100ml over 2 hours.
| 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 |
8 kg
→ 33 ml/hr
18 kg
→ 58 ml/hr
25 kg
→ 71 ml/hr
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
Step 2
Example
Child weighs 20kg. Amoxicillin 25mg/kg/day in 3 divided doses (TDS).
Step 1: 25 × 20 = 500mg/day. Step 2: 500 ÷ 3 = 166.6mg per dose.
Always verify against BNF/BNFc
Doctor prescribes 200mg ibuprofen TDS for an 18kg child. BNFc says max 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 0.25mg to mcg: moving one step down from mg to mcg, so multiply by 1,000. Answer: 250 mcg.
Convert 500ml to L: moving one step up from ml to L, so divide by 1,000. Answer: 0.5 L.
Calculation safety reminders