Free tool · Solve the water before you mix · Arithmetic only
Bacteriostatic Water Calculator
The water volume is the one decision you make before a vial is mixed, and it is the only one you cannot take back. It does not change potency and it does not change how many draws you get — it decides how many syringe marks a given amount occupies, and therefore how much a misread mark costs.
This solves that volume two ways: from a strength you want to end up with, or backwards from the mark you want to be able to read. Below the tool is what bacteriostatic water actually is, and why the 28-day figure keeps coming up.
The math, before you use the tool
- From a target strength
- water mL = vial (mg) ÷ target (mg/mL)
- From a target syringe mark
- water mL = (target units × vial mg) ÷ (amount mg × 100)
- Check the result
- mg/mL = vial (mg) ÷ water (mL)
- Per unit mark
- mcg per unit = (mg/mL × 1000) ÷ 100
The second line is just the units formula rearranged. Because units = (amount ÷ concentration) × 100 and concentration = vial ÷ water, fixing the units you want to read fixes the water — there is exactly one volume that puts a given amount on a given mark.
Read this before you use the calculator
This tool performs unit arithmetic on numbers you enter, and nothing else. It is not medical advice, it does not recommend a dose, and it is not a suggestion that you use any compound. Most of the peptides these calculators get used with are not FDA-approved for the purposes they are marketed for, are sold through a grey market where purity and labelling are not guaranteed, and many are prohibited in tested sport — check yours with the WADA prohibited-status checker. Any decision to use a compound, and any amount, must come from a licensed clinician who knows your history. Bacteriostatic Water for Injection is itself a USP pharmaceutical product; most of the compounds people dissolve in it are not approved for that use, and this page takes no position on whether any of them should be used at all.
Enter the vial size and the strength you want to end up with.
This is arithmetic, not advice. Every figure above is derived only from the numbers you typed. Nothing here is a recommended dose, a claim that any compound is approved, safe, or effective, or a suggestion that you should use one. Confirm the vial label, the water volume, and the amount with a licensed clinician before you draw anything.
Worked examples
Real numbers, every step shown, so you can check the tool against a pen and paper. These illustrate the arithmetic — the amounts in them are inputs chosen to make the maths legible, not amounts anyone should use.
A 10 mg vial, targeting 5 mg/mL
- 1water mL = vial mg ÷ target mg/mL
- 2water mL = 10 ÷ 5 = 2 mL
- 3Check: 10 mg ÷ 2 mL = 5 mg/mL
- 4Per unit mark: 5000 mcg/mL ÷ 100 = 50 mcg
2 mL of bacteriostatic water · 50 mcg per unit mark
A 5 mg vial, wanting 250 mcg to land exactly on 20 units
- 1water mL = (target units × vial mg) ÷ (amount mg × 100)
- 2Convert first: 250 mcg = 0.25 mg
- 3water mL = (20 × 5) ÷ (0.25 × 100) = 100 ÷ 25 = 4 mL
- 4Check: 5 mg ÷ 4 mL = 1.25 mg/mL, and 0.25 ÷ 1.25 = 0.2 mL = 20 units
- 5But 4 mL will not fit a 3 mL vial — pick a lower mark or accept a different one
4 mL in theory · in practice, check the vial's capacity first
A 30 mg vial, wanting 2.5 mg to land on 25 units
- 1water mL = (25 × 30) ÷ (2.5 × 100)
- 2water mL = 750 ÷ 250 = 3 mL
- 3Check: 30 mg ÷ 3 mL = 10 mg/mL, and 2.5 ÷ 10 = 0.25 mL = 25 units
3 mL of bacteriostatic water · 10 mg/mL
What bacteriostatic water actually is
Bacteriostatic Water for Injection, USP is sterile water containing 0.9% benzyl alcohol as a preservative. The benzyl alcohol is bacteriostatic — it inhibits bacterial growth rather than killing what is already there — and it is the only reason a vial can be entered more than once. Sterile Water for Injection is the same water without the preservative, and is intended for a single use.
That preservative is also where the 28-day in-use limit comes from: the USP labelling directs that the vial be discarded within 28 days of the first needle entry. It is a statement about the water, not about whatever was dissolved into it — how long a given compound stays intact in solution is a separate question the water label does not answer, and for unapproved compounds nobody has answered it properly.
Why “which bacteriostatic water is best” is the wrong question
It is a monographed pharmaceutical with a fixed composition, so between compliant products there is nothing to compare — one manufacturer's compliant vial is chemically the same as another's. The question worth asking is whether what arrived is genuinely that product: correctly labelled, properly sealed, in date, and not sterile water relabelled or repackaged by an intermediary. That is a sourcing question, not a product-comparison one, and it is the same question that applies to everything else in this supply chain.
Frequently asked questions
- How much bacteriostatic water should I add to a peptide vial?
- There is no lookup answer, because water does not change potency — it changes readability. Divide the vial's mg by the strength you want to end up with: a 10 mg vial at 5 mg/mL needs 2 mL. Or solve from the syringe instead: water mL = (target units × vial mg) ÷ (amount mg × 100). Two hard constraints apply — the volume must physically fit the vial, and the amount you draw should land on a mark you can read.
- What is bacteriostatic water and how is it different from sterile water?
- Bacteriostatic Water for Injection, USP is sterile water containing 0.9% benzyl alcohol as a preservative, which inhibits the growth of bacteria and is what makes a multiple-dose vial possible. Sterile Water for Injection contains no preservative at all and is intended for single use — once a needle enters it, there is nothing stopping microbial growth. That preservative is the entire practical difference.
- How long is bacteriostatic water good for after opening?
- The USP labelling for Bacteriostatic Water for Injection directs that it be discarded within 28 days of first entry. That is a property of the preservative, not of whatever you mixed into it, and it is why the 28-day figure appears everywhere in this context. Storage conditions and the stability of the compound in solution are separate questions the water label says nothing about.
- What counts as 'good' bacteriostatic water?
- The honest answer is that there is nothing to optimise between compliant products. Bacteriostatic Water for Injection is a USP-monographed pharmaceutical with a fixed composition — sterile water plus 0.9% benzyl alcohol — so a compliant vial from one manufacturer is the same as a compliant vial from another. What varies is whether what you received is genuinely that: whether it is a real pharmaceutical-grade product, correctly labelled, sealed and in date, rather than sterile water relabelled or something repackaged.
- Does adding more water make a peptide weaker?
- It makes the solution less concentrated, but it does not change how much peptide the vial contains or how many draws it yields. A 5 mg vial holds 5 mg whether you add 1 mL or 3 mL. The mass per draw is set by the amount you pull; the water only decides how many millilitres — and therefore how many syringe marks — that mass occupies.
- Can I use tap water, saline, or sterile water instead?
- Tap water is not sterile and has no place near an injectable at all. Sterile Water for Injection is sterile but unpreserved and is intended for single use. Sodium chloride 0.9% is a different diluent with its own labelling and compatibility considerations. Which diluent is appropriate for a given product is a question for the labelling and a licensed clinician, not for a calculator — this page does the arithmetic once that decision has been made by someone qualified to make it.
The part a calculator cannot answer
A number on a syringe is the easy half. Whether a compound does anything in humans, whether it is legal to buy, and whether it will end a season are the questions worth your time:
Medical disclaimer: This content is for general educational purposes only and is not medical advice, diagnosis, or treatment. Always consult a licensed healthcare professional before starting, stopping, or changing any treatment.
This calculator converts units. It does not account for your health, the specific product in your hand, its actual purity, injection technique, or whether using it is appropriate at all. Nothing on this page states or implies that any compound is FDA-approved, safe, or effective. Talk to a licensed clinician before acting on any figure here.