Reconstitution calculations

Pup, the Pharm Pup mascot, thinking

A powder vial becomes a liquid you can draw up once you add diluent. The trick is to use the concentration the label gives after mixing, not the amount of diluent.

Read the label in this order

  1. Diluent: which one (sterile water, normal saline, 1% lidocaine for some IM antibiotics) and how much.
  2. Route: IM and IV directions often use different volumes.
  3. Final concentration: the mg per mL after mixing. This is the number you calculate with.
  4. Storage: how long the mixed drug keeps, and at what temperature. Label the vial with date, time, concentration and your initials.

Why not just divide by the diluent?

The powder itself takes up space (displacement). If you add 2.5 mL to a 1 g vial, the total volume is a little more than 2.5 mL, so the concentration is lower than 1,000 ÷ 2.5. Labels state the true concentration; use it.

Worked examples

Practice problem: cefazolin 500 mg IM; the 1 g vial label says add 2.5 mL sterile water for about 330 mg/mL
  1. 500 mg ÷ 330 mg/mL = 1.52 mL

= 1.5 mL

Practice problem: ceftriaxone 500 mg IM; the 1 g vial label says add 2.1 mL 1% lidocaine for about 350 mg/mL
  1. 500 mg ÷ 350 mg/mL = 1.43 mL

= 1.4 mL

Practice problem: vancomycin 750 mg IV; the 1 g vial label says add 20 mL sterile water for 50 mg/mL
  1. 750 mg ÷ 50 mg/mL

= 15 mL

Label concentrations differ between manufacturers. The numbers above are for practice; in clinical work use the label in your hand and your facility's references.

Common mistakes

  • Using 1,000 mg ÷ 2.5 mL = 400 mg/mL for cefazolin, which ignores displacement.
  • Choosing the IV dilution for an IM dose, which gives a volume too large to inject.
  • Forgetting that 1 g = 1,000 mg.

Practise: reconstitution practice problems.

Questions, answered.

What is reconstitution in nursing?

Adding a liquid (the diluent) to a powdered drug so it can be given as a liquid. The label tells you which diluent, how much, and the final concentration.

Why is the final volume more than the diluent?

The powder takes up space when it dissolves. This is called displacement, and it is why you use the concentration stated on the label.

What should I write on a reconstituted vial?

Typically the date and time mixed, the final concentration, the expiry and your initials. Follow your facility policy.

Practise it in the app

Pharm Pup app screen: The replay: lb, kg and mg knocked out until mL stands
The replay: lb, kg and mg knocked out until mL stands.
Pharm Pup app screen: An IV drip round: 31 gtt/min by gravity, 125 mL/h on a pump
An IV drip round: 31 gtt/min by gravity, 125 mL/h on a pump.
Pharm Pup app screen: A timed mock round in your school format
A timed mock round in your school format.

Sources

Practice problems for study only, not clinical advice. Follow your school's rounding policy, your facility's policy and the prescriber's order. Reviewed 2026-10-30.

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