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.
Reconstitution, in plain terms
How much bacteriostatic water do I add to a 5mg vial?
That is your choice of working concentration, not a fixed rule. Adding 2ml to a 5mg vial gives 2.5mg per ml. Adding 1ml gives 5mg per ml. Less water means a more concentrated solution and smaller volumes drawn, more water means the opposite. The calculator works out the concentration for whatever volume you enter.
What is the difference between a U-40 and a U-100 insulin syringe?
Only the printed scale. A U-100 syringe marks 100 units per millilitre, a U-40 marks 40 units per millilitre. The same 0.05ml of liquid reads as 5 units on a U-100 and 2 units on a U-40. Using the wrong scale is a two and a half fold error in either direction, which is why the calculator asks which one you have.
How do I convert millilitres to insulin syringe units?
On a U-100 syringe, units equal millilitres multiplied by 100. On a U-40 syringe, units equal millilitres multiplied by 40. So 0.1ml is 10 units on a U-100 and 4 units on a U-40.
Can I use sterile water instead of bacteriostatic water?
They behave differently. Bacteriostatic water contains 0.9% benzyl alcohol, which inhibits bacterial growth and is what makes a multi-draw vial viable over a period of days. Sterile water has no preservative and is single use. Some compounds with poor solubility at neutral pH are taken up in dilute acetic acid instead.
Why is HGH measured in IU rather than mg?
Growth hormone is standardised by biological activity rather than mass, so it is conventionally labelled in international units. If you select IU the calculator keeps everything in IU and applies no mg conversion, 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.
How should a reconstituted vial be stored?
Follow the certificate of analysis for that batch rather than a general rule, since it varies by compound. Storage conditions are printed on every certificate, and you can look yours up by batch number on our verification page.
