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 400 mg of acyclovir IV. A 500 mg vial directs: add 10 mL of diluent to yield 50 mg/mL.

Ordered dose400 mg
Vial500 mg
Diluent10 mL
After reconstitution50 mg/mL

How many milliliters should the nurse draw up?

Answer: 8 mL

Step-by-step solution

1. Read the concentration after reconstitution
Add 10 mL diluent → 50 mg per 1 mL
2. Set up dimensional analysis so mg cancels
400 mg × ( 1 mL ÷ 50 mg )
3. Cancel mg and solve
= 400 ÷ 50 mL
= 8 mL

The prescriber orders 250 mg of vancomycin IV. A 500 mg vial directs: add 10 mL of diluent to yield 50 mg/mL.

Ordered dose250 mg
Vial500 mg
Diluent10 mL
After reconstitution50 mg/mL

How many milliliters should the nurse draw up?

Answer: 5 mL

Step-by-step solution

1. Read the concentration after reconstitution
Add 10 mL diluent → 50 mg per 1 mL
2. Set up dimensional analysis so mg cancels
250 mg × ( 1 mL ÷ 50 mg )
3. Cancel mg and solve
= 250 ÷ 50 mL
= 5 mL

The prescriber orders 250 mg of ceftriaxone IM. A 250 mg vial directs: add 0.9 mL of diluent to yield 250 mg/mL.

Ordered dose250 mg
Vial250 mg
Diluent0.9 mL
After reconstitution250 mg/mL

How many milliliters should the nurse draw up?

Answer: 1 mL

Step-by-step solution

1. Read the concentration after reconstitution
Add 0.9 mL diluent → 250 mg per 1 mL
2. Set up dimensional analysis so mg cancels
250 mg × ( 1 mL ÷ 250 mg )
3. Cancel mg and solve
= 250 ÷ 250 mL
= 1 mL