Drug Dilution Calculator (C₁V₁ = C₂V₂)
Calculate dilution volumes for clinical drug preparations. By a Registered Nurse working in Australia.
Leave the unknown field empty (or set to 0). The calculator will solve for it.
The formula
The total amount of drug doesn’t change during dilution — only the concentration and the volume.
Worked example
You have heparin 5,000 units/mL stock. You need a final concentration of 100 units/mL in 50 mL. How much stock do you draw up?
- Known: C₁ = 5000, C₂ = 100, V₂ = 50
- Rearrange: V₁ = (C₂ × V₂) / C₁ = (100 × 50) / 5000
- Calculate: V₁ = 1 mL of heparin stock
- Add diluent (NS) to reach 50 mL total
Frequently asked questions
Why use a dilution calculator?
To prevent dose errors in high-risk drugs that come in concentrated stock. Heparin, insulin, vasopressors, and many paediatric medications require dilution before administration.
Does the diluent matter?
Yes — some drugs are incompatible with certain fluids (e.g. some antibiotics with dextrose, calcium with bicarbonate). Always check the drug monograph for compatible diluents.
How accurate is C₁V₁ = C₂V₂ in practice?
Mathematically exact, but volume measurement errors at very small volumes (< 0.5 mL) introduce real-world inaccuracy. For doses requiring tiny stock volumes, use a smaller-concentration stock if available.