Drug Dosage Calculator
Calculate the volume to draw up from a prescribed dose and stock concentration. By a Registered Nurse working in Australia.
The formula
Make sure both dose desired and stock strength are in the same unit before applying the formula.
Worked example
A patient is prescribed 500 mg of paracetamol IV. Your stock vial contains 250 mg in 5 mL. How much do you draw up?
- Units already match (mg = mg) — no conversion needed
- Apply formula: (500 × 5) / 250 = 2500 / 250
- Calculate: 10 mL
Unit conversions
| From | To | Multiply by |
|---|---|---|
| g | mg | × 1000 |
| mg | mcg | × 1000 |
| g | mcg | × 1,000,000 |
| mcg | mg | ÷ 1000 |
Frequently asked questions
What if the units don’t match?
The calculator converts units automatically (mg/g/mcg/units). For safety, always sanity-check your own math: a 1000-fold error usually means a unit mismatch.
Why is double-checking so important?
Medication errors are among the most common preventable adverse events in healthcare. Independent double-checking by two RNs is a standard safety net for high-risk drugs.
Can I use this for weight-based dosing?
This calculator handles fixed-dose drugs. For weight-based dosing (e.g. mg/kg), first multiply patient weight by the per-kg dose to get the prescribed dose, then enter that here.