Reconstitution & dilution calculator
Work out the concentration of a reconstituted vial and the exact volume to draw — in millilitres and in units on a U-40 or U-100 insulin syringe.
Your vial
If you change the water volume
Same vial, same amount drawn — only the diluent changes. The row closest to a clean, easy-to-read mark is highlighted.
| BAC water | Concentration | Volume | Units |
|---|
The arithmetic, in full
Nothing here is proprietary — you can check every number the calculator produces with a pen.
Step 1 — concentration
Reconstitution does not change how much peptide is in the vial, only how much liquid it is dissolved in. So the concentration is simply:
concentration = peptide in vial ÷ volume of bacteriostatic water
A 10 mg vial reconstituted with 2 mL of bacteriostatic water gives 10 ÷ 2 = 5 mg/mL, or 5000 mcg/mL.
Step 2 — volume to draw
volume = amount wanted ÷ concentration
To draw 250 mcg from a 5000 mcg/mL solution: 250 ÷ 5000 = 0.05 mL.
Step 3 — converting millilitres to syringe units
This is the step people get wrong, because "units" mean different things on different syringes. An insulin syringe is calibrated for a specific insulin strength:
| Syringe | Meaning | Conversion |
|---|---|---|
| U-100 | 100 units per 1 mL | units = mL × 100 |
| U-40 | 40 units per 1 mL | units = mL × 40 |
So 0.05 mL is 5 units on a U-100 syringe but 2 units on a U-40 syringe. The physical volume is identical; only the printed scale differs. Using the wrong one is a two-and-a-half-fold error in either direction.
Bacteriostatic water vs sterile water
Bacteriostatic water contains 0.9% benzyl alcohol, which inhibits bacterial growth and is what makes a multi-draw vial viable for laboratory work over a period of days. Sterile water has no preservative and is single-use. Some peptides with poor solubility at neutral pH are instead taken up in dilute acetic acid. Both bacteriostatic water and acetic acid are stocked in our lab supplies section.
A note on IU
Growth hormone is conventionally labelled in international units rather than milligrams, because it is standardised by biological activity. If you select IU, the calculator keeps everything in IU — the arithmetic is identical, and no mg conversion is applied, because the mg-per-IU figure differs by preparation and should be taken from the certificate of analysis for the batch in front of you.
