Displacement values: how to reconstitute a powder injection
A displacement value is the space the powder takes up. Forget it and your solution comes out weaker than you meant. Here is the method, three worked examples and the mistakes to watch for.
Not affiliated with the General Pharmaceutical Council. This guide is for revision only. It is not clinical or legal advice, and its worked examples use invented numbers. Check a current source before you rely on any point.
What a displacement value is
A vial of powder is not empty space. When you add liquid, the powder dissolves but still takes up room, so the final volume is more than the liquid you added. The extra volume is the displacement value.
Product information gives it in one of two ways. Some products state a displacement value. Others give a reconstitution table with the volume of diluent to add and the approximate volume you end up with. The difference between the two columns is the displacement.
A real example: the SmPC for a ceftriaxone 1 g powder lists a row for intravenous injection. Add 10 mL of water for injections and the approximate available volume is 10.8 mL, with an approximate displacement volume of 0.8 mL. That is 10 + 0.8 = 10.8. The same SmPC lists a different row for an intramuscular injection of 1 g made up with 1% lidocaine: 3.5 mL of diluent, 4.1 mL available, 0.6 mL displacement. The figures change with the diluent and the route, so read the row that matches what you are making. The SmPC says these values are approximate, and its footnote notes that the 2 g intravenous figures assume water for injections.
The method
- Work out the final volume. Final volume = amount of drug in the vial ÷ the concentration you want.
- Work out the diluent to add. Diluent = final volume − displacement value.
- Work out the volume for the dose. Volume = dose ÷ concentration.
If the question tells you how much diluent was added instead, run it the other way. Final volume = diluent added + displacement value, then concentration = amount of drug ÷ final volume.
Worked example 1: the basic case
A vial holds 500 mg of Drug A as powder. The powder displaces 0.3 mL. You want 100 mg/mL. How much water do you add, and what volume gives a 350 mg dose?
Final volume = 500 ÷ 100 = 5 mL. Water to add = 5 − 0.3 = 4.7 mL. Dose volume = 350 ÷ 100 = 3.5 mL.
Worked example 2: working backwards from the water added
A vial holds 1 g of Drug B. The product information says the powder displaces 0.6 mL. You add 4 mL of water. What is the concentration, and what volume gives a 600 mg dose?
Final volume = 4 + 0.6 = 4.6 mL. Concentration = 1,000 mg ÷ 4.6 mL = 217.39 mg/mL. Dose volume = 600 ÷ 217.39 = 2.76 mL, which is 2.8 mL to one decimal place.
Now see what ignoring the displacement does. Treat the final volume as 4 mL and you get 250 mg/mL and a dose volume of 2.4 mL. That is about 13% less than the 2.76 mL you need, so the patient would get roughly 520 mg instead of 600 mg. This is the standard trap in these questions.
Worked example 3: more than one gram of powder
Drug C powder displaces 0.4 mL for every 500 mg. You need a 2 g dose in a final volume of 10 mL. How much water do you add?
2 g is four lots of 500 mg, so the powder displaces 4 × 0.4 = 1.6 mL. Water to add = 10 − 1.6 = 8.4 mL.
Read what the displacement value refers to. It may be per vial, per gram or per 500 mg, and it scales with the amount of powder you use.
Mistakes to watch for
- Adding the whole final volume as water. The water you add is always a little less than the final volume.
- Using the wrong row or unit. Check the route, the diluent and whether the value is per vial or per gram.
- Rounding too early. Keep the full figure for the concentration and round the dose volume at the end, as the question asks.
- Not asking if the answer is practical. A dose volume that no syringe could measure is a sign something went wrong.
Our pharmacy calculations guide has one example of each calculation type. The infusion rates guide picks up where reconstitution ends.
Sources
- Ceftriaxone 1g Powder for Solution for Injection or Infusion, Summary of Product Characteristics (emc), see the reconstitution table. https://www.medicines.org.uk/emc/product/1361/smpc. Accessed 22 October 2026.
Not affiliated with the General Pharmaceutical Council. Revision only, not clinical or legal advice. Worked examples use invented numbers and products. Every source above was read on 22 October 2026; check anything that matters against a current source. See our terms of use and privacy notice.