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MedTestBankDosage Calculation for Nursing

Reconstitution Calculator for Powdered Medications

Calculate the mL to withdraw after reconstituting a powdered medication, using the concentration printed on the label, and how many doses the vial holds.

Enter the values: the answer and every step appear here as you type.

Formula

mL = desired dose ÷ concentration after reconstitution (mg/mL)

Full doses in the vial = total drug ÷ dose ordered (round down)

How to calculate it step by step

  1. Read the label: the type and volume of diluent for the route ordered, and the concentration you get (for example add 3.4 mL to get 250 mg/mL).
  2. Add the diluent, mix as directed, and check that the powder is fully dissolved.
  3. Divide the dose ordered by the concentration after mixing.
  4. Round the volume as for any injection and label the vial if it holds more doses.

Worked examples

Example 1. A vial contains 2 g of an antibiotic powder. The label says: add 4.2 mL of sterile water to obtain 400 mg/mL. The order is 1,000 mg IM. How many mL will you withdraw?

  1. Use the concentration after mixing (400 mg/mL), not the diluent volume.
  2. 1,000 mg ÷ 400 mg/mL = 2.5 mL, rounded to 2.5 mL.

Answer: 2.5 mL

Example 2. A vial contains 1 g of an antibiotic powder. The label says: add 3.4 mL of sterile water to obtain 250 mg/mL. The order is 500 mg IM. How many mL will you withdraw?

  1. Use the concentration after mixing (250 mg/mL), not the diluent volume.
  2. 500 mg ÷ 250 mg/mL = 2 mL, rounded to 2 mL.

Answer: 2 mL

Rounding

Round like any injection: tenths of a mL at 1 mL or more, hundredths under 1 mL.

Common mistakes

  • Using the diluent volume instead of the concentration: the powder adds volume, so 1 g plus 3.4 mL of diluent gives 4 mL at 250 mg/mL.
  • Using the wrong diluent or the IV volume for an IM dose: many labels give different directions for each route.

Nursing safety points

  • Label a multiple-dose vial with the date and time of mixing, the concentration, the expiration and your initials, and store it as the label says.
  • Split an IM dose larger than the site allows (usually 3 mL in adults) between two sites, as your facility directs.

Frequently asked questions

How do you calculate a reconstituted dose?

Divide the dose ordered by the concentration on the label after mixing. For 500 mg with 250 mg/mL: 500 ÷ 250 = 2 mL.

Why not use the volume of diluent?

The powder itself takes up volume, so the final volume is larger than the diluent added. The label gives the final concentration to use.