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Reconstitution calculator

For pharmacy-dispensed lyophilised vials with reconstitution instructions: enter the powder mass and diluent volume to get the working concentration, then the volume and syringe units for a prescribed dose and the number of doses in the vial.

By FormBlends editorial teamAssumptions checked September 4, 2026Runs in your browser; nothing is sent to a server

Inputs

mg

The mass printed on the lyophilised vial, as dispensed by your pharmacy.

mL

Bacteriostatic water or the diluent your pharmacy specified. Use the volume from the pharmacy instructions.

mg

Result

Enter the powder mass and the diluent volume from your pharmacy instructions.

Formulashown in full, nothing hidden
concentration (mg/mL) = powder (mg) / diluent (mL)
volume per dose (mL)  = dose (mg) / concentration (mg/mL)
syringe units (U-100) = volume (mL) x 100
doses per vial        = powder (mg) / dose (mg)

Adding more diluent lowers the concentration and raises the volume per dose. The amount of drug in the vial does not change. Some people prefer a larger volume so the syringe reading is easier to see; that is a conversation for your pharmacist.

Assumptions, checked September 4, 2026

  • The powder's own volume is treated as zero, the usual convention for the small masses involved. For large masses the true concentration is slightly lower than shown.
  • Diluent volume is what you actually inject into the vial, not the volume printed on the diluent bottle.
  • This tool is framed for medication dispensed by a licensed pharmacy with reconstitution instructions. It is not for research chemicals and does not pre-fill any value.
  • Storage time after reconstitution follows the pharmacy's beyond-use date, not a calculation.

This calculator is an arithmetic aid for medication a licensed prescriber has already prescribed. It does not recommend a dose. Confirm any result with your prescriber or pharmacist before use.

Sources

  1. FDA: Human Drug Compounding (overview of 503A and 503B, and the statement that compounded drugs are not FDA approved)
  2. ISMP Guidelines for Optimizing Safe Subcutaneous Insulin Use in Adults (U-100 syringe graduations and unit/volume confusion)

Read before you use it

Guides for this calculator

Reconstitution arithmetic for pharmacy-dispensed vials

How powder mass and diluent volume become a working concentration, why adding more water changes the volume but not the dose, and how to check a pharmacy's instructions against your own numbers.

Updated September 4, 2026

Educational, not medical advice. Read the medical disclaimer.