Reconstitution calculator (powder volume and concentration)
Enter your values. The answer and the full worked solution update as you type.
Result
Use the diluent and volume stated on the product label: different routes (for example IM and IV) often need different volumes. For exam practice only. This is a study aid, never a clinical decision: always follow the prescription, local protocols and the product information.
Work out what happens when a powdered drug is reconstituted. Enter the amount of drug in the vial, the diluent you add and the powder volume from the label, and the calculator gives the final volume, the concentration and the volume to draw up for the ordered dose. The second mode works backwards: how much diluent gives a target concentration. Drugs measured in units work the same way as mg.
Step-by-step solution
Final volume in the vial = diluent + powder volume
Concentration after reconstitution
Volume to give = dose ÷ concentration
Key formulas
- Reconstitution
- final volume = diluent + powder volume
- Concentration
- mg/mL = drug in vial / final volume
How to calculate a reconstitution
- Read the label: the amount of drug in the vial, the diluent to use and, if stated, the powder volume (displacement value).
- Final volume = diluent added + powder volume. The powder takes up space once it dissolves, so the final volume is larger than the diluent volume.
- Concentration = amount of drug ÷ final volume, for example 500 mg ÷ 5 mL = 100 mg/mL.
- Volume to give = ordered dose ÷ concentration: 350 mg ÷ 100 mg/mL = 3.5 mL.
- For a target concentration, the final volume is the drug amount ÷ target concentration; subtract the powder volume to get the diluent to add.
Questions and answers
What is the powder volume or displacement value?
The volume the dry powder adds once it is dissolved. A vial labelled “add 4.6 mL to give 5 mL” has a powder volume of 0.4 mL. Ignoring it gives a concentration that is too high and a wrong volume to give.
What if the label does not give a powder volume?
Leave the field empty (0). Many labels state the final concentration directly, for example “add 2 mL to give 250 mg/mL”; the final volume then follows from the drug amount ÷ concentration.
Why are there different volumes for IM and IV use?
Intramuscular injections need a small volume, so less diluent is used and the solution is more concentrated. For IV use more diluent is added. Always use the volume and diluent for the route that is ordered.
Can the calculator handle drugs in units?
Yes. Choose units as the drug unit; the concentration is then in units/mL, as for many penicillin products.
What if the dose is more than the vial contains?
The calculator stops with a message, because one vial is not enough. Check the order; more than one vial would be needed.
Practise it yourself
Practise Pharmacy calculations
A calculator gives the answer. In the exam you need the method: practise it with new problems, hints and worked solutions.