Reconstitution Calculator: Concentration, Volume, and Dose Draw
Enter any two of the three core variables (solute mass, diluent volume, and target concentration) and this calculator solves for the third. It also tells you exactly how many millilitres to draw for a desired single dose, and shows each step of the working so you can verify the arithmetic before use.
Formula
Worked example
A 5 mg peptide vial with 2 mL of bacteriostatic water: C = 5 mg / 2 mL = 2.5 mg/mL. For a 250 mcg dose: draw = 0.25 mg / 2.5 mg/mL = 0.1 mL = 10 units on a U-100 syringe. The vial holds 5000 mcg total, so 5000 / 250 = 20 doses.
What is reconstitution?
Reconstitution is the process of dissolving a dry solid, usually a freeze-dried (lyophilised) powder, in a specific volume of liquid (the diluent) to create an injectable or liquid solution at a known concentration. The term is used in pharmacy, clinical laboratories, compounding, and research settings. Unlike dilution, which adds solvent to an already-dissolved solution, reconstitution always starts with a solid and ends with a solution. The resulting concentration depends on exactly two things: the mass of solute and the volume of diluent, linked by the equation C = m / V.
How to use this calculator
Use the "Solve for" selector to pick which of the three core variables you want to find. If you know the solute mass and the diluent volume (the most common scenario), choose "Concentration" and enter both values. If you need to work backward from a target concentration, choose "Diluent volume" and the calculator finds how much liquid to add. For a desired dose, fill in the "Desired dose" field and the calculator shows the draw volume in millilitres, the total number of doses in the vial, and the equivalent mark on a U-100 syringe so you can cross-check the reading. All three mass units (mcg, mg, g) and volume units (uL, mL, L) are supported, and the calculator normalises internally to mg/mL to keep the arithmetic consistent.
Choosing the right diluent
The diluent you choose affects stability and shelf life, not the concentration calculation. Sterile water for injection (SWFI) is the standard choice for single-use reconstitutions: it is used immediately and any leftover solution is discarded. Bacteriostatic water for injection (BWFI) contains 0.9% benzyl alcohol, which inhibits bacterial growth and allows the vial to be stored in the refrigerator for up to 28 days with repeated needle punctures. Normal saline (0.9% sodium chloride) is used where the product monograph specifies it or where benzyl alcohol is contraindicated. Always use the diluent specified by the manufacturer or prescriber, as the wrong diluent can cause precipitation, degradation, or incompatibility.
Syringe marks and dose accuracy
A U-100 insulin syringe has 100 graduated units in 1 mL, where each unit equals 0.01 mL. The calculator converts your draw volume to units on this scale to help you cross-check the reading before drawing. For example, 0.1 mL corresponds to 10 units, and 0.05 mL corresponds to 5 units. Very small doses (below 0.01 mL) fall below the resolution of a standard U-100 syringe and require a higher-precision measuring device. If your draw volume is below 0.05 mL, consider reconstituting with less diluent to raise the concentration and bring the draw volume into a more measurable range.
Common reconstitution scenarios
| Solute mass | Diluent added | Concentration | Common application |
|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL | Peptide research vial (standard) |
| 5 mg | 2 mL | 2.5 mg/mL | Peptide research vial (dilute) |
| 10 mg | 2 mL | 5 mg/mL | Higher-mass peptide vial |
| 500 mg | 2 mL | 250 mg/mL | Ceftobiprole antibiotic |
| 350 mg | 5 mL | 70 mg/mL | Vancomycin antibiotic |
| 1 g | 10 mL | 100 mg/mL | Ampicillin injection |
| 250 mg | 5 mL | 50 mg/mL | Amoxicillin suspension |
Typical mass-volume combinations and the resulting concentration in mg/mL.
Frequently asked questions
What is the formula for reconstitution?
The core formula is: Concentration = Mass / Volume, or C = m / V. Mass is typically in milligrams (mg), volume in millilitres (mL), and concentration in mg/mL. You can rearrange this to find any of the three: V = m / C to get the diluent volume needed, and m = C x V to get the solute mass needed. The draw volume for a specific dose is then: draw volume (mL) = dose (mg) / concentration (mg/mL).
How do I calculate how much diluent to add?
Set 'Solve for' to 'Diluent volume', enter the mass of dry powder and the concentration you want to achieve, and the calculator divides mass by concentration to give you the volume of liquid to add. For example, if you have 10 mg of powder and want a 5 mg/mL solution, add 10 / 5 = 2 mL of diluent.
What does the U-100 syringe reading mean?
A U-100 insulin syringe holds 1 mL and divides it into 100 equal marks, each representing 0.01 mL. The calculator converts your draw volume to this scale automatically. Drawing to the 10-unit mark means drawing 0.10 mL; the 50-unit mark means 0.50 mL. It is a convenient cross-check: if your draw volume is 0.1 mL and the syringe says 10 units, the numbers match and you can proceed with confidence.
What is the difference between reconstitution and dilution?
Reconstitution starts with a dry solid (powder) and adds a liquid to create a solution. Dilution starts with an already-dissolved solution and adds more solvent to reduce the concentration. Both use the same C = m / V relationship, but reconstitution combines a solid and a liquid while dilution combines two liquids. This calculator is designed for reconstitution; for dilution from a stock solution use a dilution calculator (C1 x V1 = C2 x V2).
How many doses are in a reconstituted vial?
Once you know the total volume and concentration, the number of doses is: total mass / dose = (C x V) / dose, which is the same as total volume / draw volume. Enter your desired dose in the 'Desired dose' field and the calculator works this out automatically. Note that some volume is typically lost to dead space in the syringe, vial stopper, and needle, so real-world yield may be slightly less than the theoretical maximum.
Does the diluent volume include the volume of the powder?
Strictly speaking, the dissolved powder adds a small volume to the final solution (called the powder volume or PV). For most practical concentrations this is negligible. High-concentration reconstitutions (above about 100 mg/mL) may require a powder volume correction: the true volume added = desired final volume - powder volume. Product monographs for clinical drugs specify this correction. For research-grade peptides at typical concentrations (1-10 mg/mL), the powder volume is less than 1% of the final volume and can be ignored.