Dosage calculation

Reconstitution dosage calculation

Powdered drugs must be reconstituted with a diluent before use. The vial label states the concentration you get after adding the diluent (e.g. add 5 mL to yield 250 mg/mL). Divide the ordered dose by that concentration to find the volume to draw up.

The formula

Volume (mL) = Ordered dose (mg) ÷ Concentration after mixing (mg/mL)

How to solve it

  1. Read the post-reconstitution concentration from the label (mg/mL).
  2. Set up dimensional analysis: ordered mg × (1 mL ÷ concentration mg).
  3. Cancel mg and solve for milliliters.

Common mistakes

  • Using the total vial strength instead of the per-mL concentration after mixing.
  • Ignoring the diluent volume's effect on the final concentration.
Safety check: The concentration used in the math is the value after reconstitution, not the total amount of powder in the vial.

Worked examples

See it solved

The prescriber orders 2250 mg of piperacillin-tazobactam IV. The vial directs: add 20 mL of diluent to yield 200 mg/mL.

Ordered dose2250 mg
After reconstitution200 mg/mL
Diluent20 mL

How many milliliters should the nurse draw up?

Answer: 11.25 mL

Step-by-step solution

1. Read the concentration after reconstitution
Add 20 mL diluent → 200 mg per 1 mL
2. Set up dimensional analysis so mg cancels
2250 mg × ( 1 mL ÷ 200 mg )
3. Cancel mg and solve
= 2250 ÷ 200 mL
= 11.25 mL

The prescriber orders 80 mg of methylprednisolone IV. The vial directs: add 1 mL of diluent to yield 40 mg/mL.

Ordered dose80 mg
After reconstitution40 mg/mL
Diluent1 mL

How many milliliters should the nurse draw up?

Answer: 2 mL

Step-by-step solution

1. Read the concentration after reconstitution
Add 1 mL diluent → 40 mg per 1 mL
2. Set up dimensional analysis so mg cancels
80 mg × ( 1 mL ÷ 40 mg )
3. Cancel mg and solve
= 80 ÷ 40 mL
= 2 mL

The prescriber orders 2000 mg of ceftriaxone IV. The vial directs: add 10 mL of diluent to yield 100 mg/mL.

Ordered dose2000 mg
After reconstitution100 mg/mL
Diluent10 mL

How many milliliters should the nurse draw up?

Answer: 20 mL

Step-by-step solution

1. Read the concentration after reconstitution
Add 10 mL diluent → 100 mg per 1 mL
2. Set up dimensional analysis so mg cancels
2000 mg × ( 1 mL ÷ 100 mg )
3. Cancel mg and solve
= 2000 ÷ 100 mL
= 20 mL