Many medicines, vaccines, peptides, antibiotics and lab reagents are supplied as a dry powder (lyophilised) because they keep far longer that way. Before use, the powder is reconstituted: a measured volume of a liquid diluent – sterile water, saline or a buffer – is added so that the solution has a known concentration, typically in mg/mL. Reconstitution is a single step from solid to solution, unlike dilution, which starts from a liquid that is already concentrated.
The calculation is mass divided by volume, with one subtlety: the powder itself takes up space. Labels often say “add 4.8 mL to give 5 mL” – the 0.2 mL difference is the displacement volume. Ignoring it makes the actual concentration lower than intended, which matters for small vials and concentrated solutions.
This calculator works in all three directions and then tells you how much of the reconstituted solution to draw up for a given dose and how many full doses the vial provides. Always follow the product’s own instructions and local protocols for clinical use.
How to use the Reconstitution Calculator
- Find
- Amount of powder in the vial
- Volume of diluent added
- Target concentration
- Powder displacement volume (0 if unknown)
- Prescribed dose
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose what you want to find, enter the other two values and, if the label gives one, the displacement volume. The final volume is the diluent plus the displacement; the concentration is the amount divided by that volume. The volume to administer is the prescribed dose divided by the concentration.
Worked example
A vial holds 500 mg of powder. The label says to add 4.8 mL of diluent and that the powder displaces 0.2 mL, so the final volume is 5.0 mL and the concentration is 500 ÷ 5.0 = 100 mg/mL (10% w/v). For a 125 mg dose draw up 125 ÷ 100 = 1.25 mL; the vial gives 4 full doses. Had the displacement been ignored, you would expect 500 ÷ 4.8 = 104.2 mg/mL – about 4% too high.
Safety notes
Check the product, strength and expiry date, use the diluent specified by the manufacturer, and label the vial with the concentration, date and time of reconstitution. Reconstituted products often have a short shelf life. This calculator does arithmetic only and does not replace professional or manufacturer guidance.
Tips
- Use the final volume of the solution, not the volume of solvent added, for molarity.
- For dilutions, C₁V₁ = C₂V₂ only holds when both concentrations use the same unit.
- Add acid to water, never water to acid.
- Everything is calculated in your browser.
Common mistakes to avoid
- Mixing w/w, w/v and v/v percentages.
- Forgetting that pH is logarithmic – one unit is a tenfold change in [H⁺].
- Using pKa where Ka is asked for (or the other way round).
Inputs explained
| Field | What to enter |
|---|---|
| Find | Choose one: Concentration after reconstitution, Diluent volume to add, Amount of powder (dose in vial) |
| Amount of powder in the vial | Units: mg, g, µg, kg |
| Volume of diluent added | Units: mL, L, µL, cm³ (cc), fl oz (US) |
| Target concentration | Units: mg/mL, g/L, µg/mL, g/mL, mg/L |
| Powder displacement volume (0 if unknown) | Units: mL, L, µL, cm³ (cc), fl oz (US) |
| Prescribed dose | Units: mg, g, µg, kg |
What each result means
- Concentration = [Amount of powder in the vial]÷([Volume of diluent added] + [Powder displacement volume (0 if unknown)])
- Diluent to add = [Amount of powder in the vial]÷[Target concentration] – [Powder displacement volume (0 if unknown)]
- Amount of powder = [Target concentration]×([Volume of diluent added] + [Powder displacement volume (0 if unknown)])
- Final volume of solution = [Amount (g)]÷[Concentration (g÷L)]
- Volume to draw up for the dose = [Prescribed dose]÷[Concentration (g÷L)]
- Number of full doses in the vial = floor([Amount (g)]÷[Prescribed dose] + 1e-9)
- Strength as a percentage = [Concentration (g÷L)]÷10
Example values
How the main result changes when Amount of powder in the vial changes and everything else stays at the example values above:
| Amount of powder in the vial (mg) | Concentration (mg/mL) |
|---|---|
| 250 | 50 |
| 375 | 75 |
| 500 | 100 |
| 625 | 125 |
| 750 | 150 |
| 1,000 | 200 |
Formula
Concentration = amount of powder ÷ final volumeFinal volume = diluent volume + powder displacement volumeDiluent = amount ÷ target concentration − displacementVolume per dose = dose ÷ concentrationWorked example
Inputs — Find: Concentration after reconstitution; Amount of powder in the vial: 500 mg; Volume of diluent added: 4.8 mL; Powder displacement volume (0 if unknown): 0.2 mL; Prescribed dose: 125 mg.
Results — Concentration = 100 mg/mL; Final volume of solution = 5 mL; Volume to draw up for the dose = 1.25 mL; Number of full doses in the vial = 4; Strength as a percentage = 10 % w/v.
Key terms
- Reconstitution
- Adding a liquid diluent to a dry powder to make a solution of known strength.
- Diluent
- The liquid used to dissolve the powder.
- Displacement volume
- Extra volume contributed by the dissolved powder.
- Lyophilised
- Freeze-dried for storage.
Frequently asked questions
How do I calculate the concentration after reconstitution?
Divide the amount of powder by the final volume. 350 mg in 5 mL gives 70 mg/mL.
How much diluent do I add for 250 mg/mL from a 500 mg vial?
500 ÷ 250 = 2 mL of final volume; subtract any displacement volume given on the label.
What is displacement volume?
The volume the powder occupies once dissolved. If the label says "add 4.8 mL to make 5 mL", the displacement is 0.2 mL.
How much do I draw up for a dose?
Dose ÷ concentration: 125 mg from a 100 mg/mL solution is 1.25 mL.
What is the difference between reconstitution and dilution?
Reconstitution turns a dry powder into a solution; dilution lowers the concentration of a solution that is already liquid.
Which diluent should I use?
The one named in the product information – often sterile water for injection, 0.9% sodium chloride or bacteriostatic water – because some products are incompatible with others.
How do I convert mg/mL to a percentage?
Divide by 10: 100 mg/mL is 10% w/v.
What formula does the Reconstitution Calculator use?
Concentration = amount of powder ÷ final volume.