How much bacteriostatic water do I add to a peptide vial?

Any amount the vial holds. The water does not change your dose, only the volume it arrives in. Pick the amount that puts your usual dose between roughly 10 and 50 units on the barrel.

Does more bacteriostatic water mean a weaker dose?

No. The vial contains a fixed amount of peptide. More water spreads it through more liquid, so you draw a larger volume for the same dose. The amount of peptide you inject is unchanged.

Can I add more water later?

Adding water to an already reconstituted vial changes the concentration of everything left in it, so every number you worked out before becomes wrong. If you do it, recalculate from the new total volume.

What if I added the wrong amount?

Nothing is ruined. Work out the real concentration from what you actually added and use that. The peptide in the vial has not changed.

Bacteriostatic water calculator

Most calculators ask how much water you added. This one answers the question the other way round: how much should you add, so that your dose lands somewhere you can actually read.

Pick the draw you want, read off the water

Vial	Your dose	for 10 units	for 20 units	for 25 units	for 40 units	for 50 units
10 mg	2 mg	0.5 mL	1 mL	1.25 mL	2 mL	2.5 mL
10 mg	0.5 mg	2 mL	4 mL	5 mL	8 mL	10 mL
5 mg	250 mcg	2 mL	4 mL	5 mL	8 mL	10 mL
5 mg	500 mcg	1 mL	2 mL	2.5 mL	4 mL	5 mL
30 mg	2.5 mg	1.2 mL	2.4 mL	3 mL	4.8 mL	6 mL

Read across to the draw size you want, and that cell is how much bacteriostatic water to add.

IndependentThis site does not sell peptides and is not owned by a vendor. Every other calculator on page one of Google belongs to a shop.

The maths is openThe formula is written out on this page and the code behind it is a single file with an automated test suite .

How it earnsAffiliate links on one clearly marked page, and nowhere else. Never inside the calculator.

ReviewedDose ranges and half-life figures last checked 3 September 2026.

The rule behind the table

water (mL) = target units x vial (mg) / (100 x dose in mg)

The reason to care is readability. A dose that lands at 3 units sits between two marks you cannot reliably distinguish, and being one mark out is a 33 percent error. The same dose at 30 units is easy, and one mark out is 3 percent. Nothing about the drug changes, only your ability to measure it.

Check what your vial physically holds before committing. Most peptide vials take about 2 to 3 mL. Some take more, but a 5 mL answer in a 3 mL vial is not a plan.

Or work it from your own numbers

How much bacteriostatic water should I add?

There is no single correct amount. The water does not change the dose, only how far along the barrel that dose lands. More water means a bigger, easier-to-read draw of a weaker solution. Less water means a small, concentrated draw that is harder to measure precisely.

The practical rule is to pick a volume that puts your usual dose somewhere between roughly 10 and 50 units, because that part of the barrel is easy to read and small errors matter least. If the calculator tells you to draw 3 units, add more water. If it tells you to draw 90 units, add less.

Whatever you choose, write it on the vial. The number is meaningless a week later if you cannot remember how much water went in, and that is the single most common reason people end up re-dosing wrong.

Common questions

How much bacteriostatic water do I add to a peptide vial?: Any amount the vial holds. The water does not change your dose, only the volume it arrives in. Pick the amount that puts your usual dose between roughly 10 and 50 units on the barrel.
Does more bacteriostatic water mean a weaker dose?: No. The vial contains a fixed amount of peptide. More water spreads it through more liquid, so you draw a larger volume for the same dose. The amount of peptide you inject is unchanged.
Can I add more water later?: Adding water to an already reconstituted vial changes the concentration of everything left in it, so every number you worked out before becomes wrong. If you do it, recalculate from the new total volume.
What if I added the wrong amount?: Nothing is ruined. Work out the real concentration from what you actually added and use that. The peptide in the vial has not changed.

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Tools

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mg to units calculator Both directions, any concentration
Bacteriostatic water calculator (this page)
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The Protocol Pack Free 25-page printable reference
Where to buy Vendor notes and what to check first
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Read this before you use the numbers. This is a unit-conversion tool. It converts a vial strength and a volume of bacteriostatic water into a concentration, a draw volume and a mark on an insulin syringe. It does not tell you what to take, whether to take it, or whether a compound is safe for you. Most of the compounds listed here are sold for laboratory research use and are not approved for human use. Nothing on this site is medical advice. Dosing decisions belong with a licensed clinician who knows your history. Always confirm the strength printed on your own vial rather than trusting a preset.