Set 6 of 10 · Reconstitution
Reconstitution Dosage Calculation Practice
Some drugs come as a powder that must be mixed with a liquid, the diluent, before use. Reconstitution means adding the exact type and volume of diluent the label states, then calculating the dose from the concentration of the mixed solution. The practice questions on this page are free, with no sign-up.
The key point: the powder takes up space, so the final volume is usually more than the diluent added. Always work from the final volume or the concentration the label gives, not from the diluent volume.
Reconstitution dosage calculation formula
- Concentration after mixing
- mg/mL = drug in vial (mg) ÷ final volume (mL)
- Volume to give
- (D ÷ H) × Q, using the new concentration
- Dividing by the diluent volume is the classic error. Here it gives 476.19 mg/mL, and the volume drawn up would be too small.
- Some labels give more than one dilution, for example for different routes. Use the one that matches the route prescribed, with the diluent named.
- If the vial will be used again, label it with the concentration, the date and time it was mixed and your initials, and check how long it stays stable.
Worked example: reconstitution
Drug R 750 mg IM is prescribed. The vial holds 2 g of powder, and the label says to add 4.2 mL of sterile water for a final volume of 5 mL. How many mL will the nurse give? Round to the nearest tenth.
- Convert the vial's contents: 2 g × 1,000 = 2,000 mg.
- Concentration after mixing: 2,000 mg ÷ 5 mL = 400 mg/mL. Use the 5 mL final volume, not the 4.2 mL of diluent.
- Volume: (750 mg ÷ 400 mg) × 1 mL = 1.875 mL.
- Round: a volume of 1 mL or more goes to the nearest tenth, so 1.9 mL.
Answer: 1.9 mL
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Answer key
Reconstitution practice questions: answers and rationales
Try the questions first. Open one to read its answer and rationale.
Question 1A 38-year-old client on a medical unit who has no IV access is prescribed cefazolin 250 mg IM every 8 hours. The vial holds 1 g of cefazolin powder. The label says to add 2.5 mL of sterile water for injection, which gives a concentration of about 330 mg/mL. How many mL will the nurse draw up for one dose? Round to the nearest hundredth. Record your answer using 2 decimal places.
The label gives the concentration after mixing, 330 mg/mL, so that is the dose on hand. Desired over have: (250 mg ÷ 330 mg) × 1 mL = 0.7576 mL. A volume under 1 mL is rounded to the nearest hundredth and written with a leading zero: 0.76 mL. Common error: dividing the 1,000 mg in the vial by the 2.5 mL of diluent gives 400 mg/mL and a dose of 0.63 mL, which is too little drug. The powder takes up space, so the mixed solution is about 3 mL, not 2.5 mL. Sense check: 250 mg is a quarter of the vial, and 0.76 mL is about a quarter of 3 mL. Takeaway: after reconstitution, calculate from the concentration printed on the label, never from the volume of diluent added.
Sources: Open RN Nursing Skills (ACC adaptation), 5.10 Reconstituted Medication; Open RN Nursing Skills (ACC adaptation), 5.5 Rounding; StatPearls: Dose Calculation Desired Over Have Formula Method; ISMP List of Error-Prone Abbreviations, Symbols, and Dose Designations; DailyMed: Cefazolin for Injection label (1 g with 2.5 mL gives about 330 mg/mL; adult dosage)
Question 2A 76-year-old resident of a long-term care facility is prescribed ampicillin 500 mg IM every 6 hours. Only 2 g vials of ampicillin powder are in stock. The label says to add 6.8 mL of sterile water for injection, which gives 8 mL of solution. What is the concentration of the reconstituted solution in mg/mL? Round to the nearest whole number. Record your answer as a whole number.
Concentration after mixing = drug in the vial ÷ final volume. Convert the vial's contents first: 2 g × 1,000 = 2,000 mg. Then divide by the 8 mL of solution, not the 6.8 mL of diluent: 2,000 mg ÷ 8 mL = 250 mg/mL. Common errors: dividing by the diluent volume (2,000 ÷ 6.8 = 294 mg/mL), or leaving the vial in grams (2 ÷ 8 = 0.25). The question asks for the concentration, not the volume of one dose. Each 500 mg dose would then be (500 mg ÷ 250 mg) × 1 mL = 2 mL. Takeaway: the powder adds volume, so divide the mg in the vial by the final volume after mixing.
Sources: Open RN Nursing Skills (ACC adaptation), 5.10 Reconstituted Medication; Open RN Nursing Skills (ACC adaptation), 5.4 Equivalencies; DailyMed: Ampicillin for Injection label (2 g with 6.8 mL gives 8 mL at 250 mg/mL; adult dosage)
Question 3A 41-year-old client on a medical unit is prescribed penicillin G potassium 3,000,000 units IV every 4 hours. The vial holds 5,000,000 units of powder. The label lists three dilutions: add 18.2 mL of diluent for 250,000 units/mL, 8.2 mL for 500,000 units/mL, or 3.2 mL for 1,000,000 units/mL. The nurse adds 8.2 mL of sterile water for injection. How many mL will the nurse withdraw to add to the IV bag? Round to the nearest tenth. Record your answer using 1 decimal place.
The concentration depends on how much diluent was added. The nurse added 8.2 mL, so the matching concentration on the label is 500,000 units/mL. Desired over have: (3,000,000 units ÷ 500,000 units) × 1 mL = 6 mL, written with no trailing zero. Using another row of the label gives a wrong volume: 3 mL at 1,000,000 units/mL or 12 mL at 250,000 units/mL. Dividing the 5,000,000 units by the 8.2 mL of diluent (609,756 units/mL) is also wrong, because the powder adds volume. Sense check: 3,000,000 units is three fifths of the vial, and 6 mL is three fifths of the 10 mL of solution. Takeaway: when a label lists several dilutions, use the concentration that matches the diluent volume actually added.
Sources: Open RN Nursing Skills (ACC adaptation), 5.10 Reconstituted Medication; StatPearls: Dose Calculation Desired Over Have Formula Method; DailyMed: Penicillin G Potassium for Injection label (5,000,000-unit vial dilution table; adult dosage)
Question 4A 29-year-old client on an inpatient mental health unit is prescribed olanzapine 7.5 mg IM once for acute agitation. The nurse dissolves a 10 mg vial of olanzapine powder with 2.1 mL of sterile water for injection, as the label directs, to give about 5 mg/mL. A second nurse checks the syringe and sees that it holds 1.2 mL. How many mg of olanzapine are in the syringe? Round to the nearest tenth. Record your answer using 1 decimal place.
To find the dose in a measured volume, multiply the volume by the concentration after mixing: 1.2 mL × 5 mg/mL = 6 mg. The syringe holds 6 mg, which is less than the 7.5 mg prescribed. Working forward, the prescribed dose needs (7.5 mg ÷ 5 mg) × 1 mL = 1.5 mL, so the syringe is 0.3 mL short and must be corrected before the injection is given. Common error: using 10 mg ÷ 2.1 mL = 4.76 mg/mL, which gives 5.7 mg. The label states the concentration after mixing, and that is the figure to use. Takeaway: dose in the syringe = volume drawn up × concentration after mixing. Checking backward catches an error before it reaches the client.
Sources: Open RN Nursing Skills (ACC adaptation), 5.10 Reconstituted Medication; Open RN Nursing Skills (ACC adaptation), 5.5 Rounding; DailyMed: Olanzapine for Injection label (10 mg with 2.1 mL gives about 5 mg/mL; IM dosage)
Question 5A 4-year-old child who weighs 38 lb is prescribed ceftriaxone 75 mg/kg/day IV in 2 equally divided doses. The 1 g vial label reads: for IM use, add 2.1 mL of diluent to give 350 mg/mL; for IV use, add 9.6 mL of diluent to give 100 mg/mL. How many mL of reconstituted ceftriaxone will the nurse withdraw for each dose? Round the weight to the nearest tenth of a kilogram and the answer to the nearest tenth. Record your answer using 1 decimal place.
Weight: 38 lb ÷ 2.2 = 17.27 kg, rounded to 17.3 kg before use. Daily dose: 75 mg/kg/day × 17.3 kg = 1,297.5 mg/day. Each dose: 1,297.5 mg ÷ 2 = 648.75 mg. The route is IV, so the vial is mixed with 9.6 mL to give 100 mg/mL. Volume: (648.75 mg ÷ 100 mg) × 1 mL = 6.4875 mL, which rounds to 6.5 mL. Common errors: using the IM concentration of 350 mg/mL (1.9 mL), giving the whole daily dose at once (13 mL), or dividing 1,000 mg by the 9.6 mL of diluent. Takeaway: find the dose for one administration first, then use the label dilution that matches the route prescribed.
Sources: Open RN Nursing Skills (ACC adaptation), 5.10 Reconstituted Medication; Open RN Nursing Skills (ACC adaptation), 5.11 Weight-Based Calculations; Open RN Nursing Skills (ACC adaptation), 5.4 Equivalencies; Open RN Nursing Skills (ACC adaptation), 5.5 Rounding; DailyMed: Ceftriaxone for Injection label (IM and IV reconstitution tables; adult and pediatric dosage; infuse over 30 minutes)
FAQ
Reconstitution calculations: FAQ
How to calculate reconstituted medications?
How many mL do you reconstitute?
Sources: Open RN Nursing Skills (ACC adaptation), 5.10 Reconstituted Medication; DailyMed: Ampicillin for Injection label; StatPearls: Dose Calculation Desired Over Have Formula Method