The only equation you need

Concentration = peptide in the vial ÷ water added. A 5 mg vial with 2 mL of bacteriostatic water is 2.5 mg/mL. A 250 mcg dose is then 0.1 mL — which on a standard 100-unit insulin syringe is 10 units, because those syringes are marked in units where 100 units = 1 mL.

That last step is where most mistakes happen: the syringe is marked in units, your dose is in mcg or mg, and nothing on either label bridges them. The reconstitution calculator does the conversion and draws the syringe with the mark to fill to, which is a lot harder to misread than a number.

Choosing how much water to add

There is no single correct volume — you are choosing a trade-off. Less water means a higher concentration and a tiny, hard-to-measure draw. More water means an easier, more forgiving draw, but the vial holds fewer doses before it is gone and the reconstituted volume has a shelf life.

The comfortable range for most people is a draw somewhere between 10 and 30 units on a 100-unit syringe. Work backwards from that: pick the volume that puts your typical dose in that window.

Actually doing it

Let the vial reach room temperature. Wipe both stoppers with alcohol. Draw the bacteriostatic water and add it slowly, down the inside wall of the vial rather than squirting it onto the powder — peptides are fragile and force is what damages them. Then swirl gently or leave it to dissolve. Do not shake. Give it a few minutes; most peptides go clear on their own.

If it stays cloudy, or has visible particles that do not dissolve, do not use it.

Storage and the clock you just started

Lyophilised powder is stable for a long time in the freezer or fridge. The moment you add water that changes: a reconstituted vial goes in the fridge and is typically used within a few weeks, with the exact window depending on the compound and the preservative in the water. Bacteriostatic water contains benzyl alcohol, which is what buys you that window — sterile water without it does not.

Record the reconstitution date, not just the purchase date. The compound pages carry the usual storage window for each compound.

The mistakes that cost a vial

• Shaking it. Denatures the peptide. Swirl.
• Injecting the water hard onto the powder. Same problem, quieter.
• Confusing mL with units. 0.1 mL is 10 units, not 100. This one is a tenfold error.
• Using plain sterile water for a multi-dose vial. No preservative, much shorter life.
• Not writing down the volume you added. A month later you cannot reconstruct the concentration, and every dose after that is a guess.

Questions

How much bacteriostatic water should I use?

Whatever puts your typical dose at a comfortable mark on the syringe — roughly 10 to 30 units on a 100-unit syringe. The calculator will show you what a given volume produces before you commit.

What is the difference between bacteriostatic and sterile water?

Bacteriostatic water contains a preservative (benzyl alcohol) that lets a vial be entered repeatedly over weeks. Sterile water has none, so it is single-use in practice.

Can I change the volume after I have already mixed it?

You can add more water to dilute further, but you cannot go the other way. Recalculate the concentration afterwards and update it wherever you recorded it.

Is the calculator free?

Yes, in the browser with no download or account, and the same arithmetic runs in the app against your actual vials.