Bacteriostatic water calculator

Solve for the water amount

How the math works

The barrel fact is a label fact, not arithmetic: the Humalog prescribing information prints the strength as 100 units/mL (U-100), and the vial panel reads "Use only with a U-100 syringe", so on that barrel each unit is one hundredth of a milliliter. The 20-unit mark sits at 0.2mL and the 50-unit mark at 0.5mL, before any vial enters the picture.

That mapping is not universal, which is why the tool names U-100: the Humulin R U-500 label puts 500 units in the same milliliter and states that each marking on its dedicated syringe represents 5 units. Both labels are in the sources below. Every step after this one is division you can redo by hand; this step is the only one you have to read off a document.

What the dose fixes: for a dose to read as a chosen mark, that many milligrams has to occupy exactly the mark's volume, which pins one concentration: the dose divided by that volume.

What the vial fixes: with the concentration pinned, vial milligrams divided by that concentration gives the volume the finished solution has to occupy.

Finished volume and water added are not the same quantity, and this page used to treat them as one: the powder in the vial occupies space too, so the liquid you end up with is the water you put in plus whatever the cake displaces. For a vial holding a few milligrams of peptide and nothing else, that difference is far below anything a syringe can show. Take the most pessimistic case and assume the powder displaces a volume equal to its own mass in water: 10mg would then displace 0.01mL, one hundredth of a unit on a U-100 barrel. The arithmetic above is the water amount for that kind of vial, to a precision the barrel cannot resolve.

Where it stops being true, with the document that shows it: a manufactured lyophilized product is not a few milligrams of peptide. It carries bulking agents, and the cake is large enough to matter. The EGRIFTA WR label directs that an 11.6mg vial be reconstituted with 1.3mL of bacteriostatic water to give 8mg/mL. Divide 11.6 by 8 and you get 1.45mL, which is the finished volume, not the diluent, and it is 0.15mL more water than the label asks for. Where a manufacturer prints a diluent volume, that number governs and this tool's does not.

The check row: the dose divided by the concentration has to hand back the mark's volume. When it does, the mix is consistent, and any other dose reads off the same barrel by proportion.

Why this one runs backward: the forward form of the same equation starts from a water amount and reports where a dose lands. This tool starts from the mark, because the mark is the number the barrel actually shows.

One proportion to keep: the water amount moves in straight lines with all three inputs: double the vial or the mark and it doubles, double the dose and it halves.

How do I make 2mg read as 20 units?

It scales with the vial: the water amount in mL comes out to one tenth of the vial size in milligrams, so a 10mg vial lands on 1mL, a 5mg vial on 0.5mL, and a 30mg vial on 3mL.

Where the tenth comes from: 20 units is 0.2mL on a U-100 barrel, and holding 2mg inside that volume means a concentration of 10mg per mL; the water amount is always the vial size divided by that concentration.

It scales with the mark too: at a 40-unit target every one of those water amounts doubles, because the mark's volume doubled while the dose stayed put.

How much bacteriostatic water do I add to a 10mg vial?

No single number exists: a 10mg vial takes 1mL to put 2mg on the 20-unit mark and 2.5mL to put the same 2mg on the 50-unit mark, and both are correct arithmetic for the same vial.

Why one-number answers mislead: any page that prints a single water amount for a vial size is quietly assuming a dose and a mark, and the assumptions are doing the work, not the vial.

The honest form of the question: given this vial, this dose to measure, and this mark, what water amount makes the three agree. That is exactly the solve the tool at the top runs.

What if the water amount comes out huge?

What a huge result means: the arithmetic does not break, it outruns the container. A 30mg vial, a 0.5mg dose, and a 50-unit target come out at 30mL of water. Whether that fits is settled by the capacity of the vial actually in hand and by nothing else, and vial capacities vary, so there is no volume above which a result is universally impossible.

Why it blows up: the water amount grows in proportion to the vial and the mark, and shrinks in proportion to the dose, so a small dose against a big vial and a high mark inflates fast.

What moves it back: halving the target mark halves the water amount, and a dose twice the size halves it too. The tool prints every result it computes and adds a note past 10mL. That threshold is a display convention we chose, not a property of any vial, and it is not a claim that 10mL is the point where a mix stops fitting.

Where this sits in the dose-math cluster

The cluster: this tool is one page of a set we built around a single rule: dose math is arithmetic on stated inputs, never advice.

• The peptide dose math index: every tool and explainer in the set, in one place.
• Reconstitution calculator: the forward direction, starting from a water amount and reporting where a dose lands.
• Units to mg converter: converts between the barrel reading and milligrams once the concentration is known.
• Why the water amount changes the units: the explainer behind the proportion this tool uses.
• What a unit is on an insulin syringe: the barrel scale itself, before any vial math enters.

How this page is sourced

Arithmetic, with one exception: every number the tool returns, and every figure in the worked example and the vial-size table, is multiplication or division that can be redone by hand from the inputs printed beside it.

The exception, and it is the one the whole tool rests on: that a U-100 barrel holds 100 units in a milliliter is a device and label fact, not something arithmetic can produce. It is cited below, together with a second label at a different strength that shows the mapping is specific to U-100 rather than general.

Three sources: three approved prescribing informations, all read on DailyMed. Two are the insulin labels that fix the barrel scale. The third is a lyophilized product whose own label directs a diluent volume, which is what shows that the finished volume this tool returns is not always the water to add.

What changed on 30 July 2026: both labels were re-retrieved and both returned HTTP 200. One further sentence was narrowed: this page used to say that most mixing math starts from a water amount, which is a claim about how other people calculate and nothing here measures that. It now names the forward form of the same equation, which is what the linked tool runs.

The standard: the sourcing and citation-verification standard for the site lives on the methodology page, including who checks a page before it ships.

1. 1

   Humalog (insulin lispro) injection, for subcutaneous or intravenous use. Prescribing information, Eli Lilly and Company. Dosage Forms and Strengths, printing the strength as 100 units/mL (U-100); Principal Display Panel for the 10 mL vial, reading "Use only with a U-100 syringe"; and the U-100 Vial Instructions for Use. Retrieved 29 July 2026, HTTP 200. DailyMed SPL, set id c8ecbd7a-0e22-4fc7-a503-faa58c1b6f3f.

2. 2

   Humulin R U-500 (insulin human) injection, for subcutaneous use. Prescribing information, Eli Lilly and Company. Dosage Forms and Strengths, printing 500 units/mL (U-500); and Dosage and Administration, Delivery of Humulin R U-500 Using the Vial Presentation and the U-500 Insulin Syringe, which states that each marking on that syringe represents 5 units. Retrieved 29 July 2026, HTTP 200. DailyMed SPL, set id b60e8dd0-1d48-4dc9-87fd-e14675255e8c.

3. 3

   EGRIFTA WR (tesamorelin) for injection, for subcutaneous use. Prescribing information, Theratechnologies Inc., revised 3/2025. Dosage and Administration, Reconstitution, which directs "Reconstitute 1 vial of EGRIFTA WR lyophilized powder with 1.3 mL of diluent (8 mg per mL)" and states "After reconstitution with 1.3 mL of Bacteriostatic Water for Injection, USP, resultant concentration is 8 mg/mL". Cited here as the formulation-matched case where finished volume and diluent volume come apart: 11.6 divided by 8 is 1.45mL, and the label directs 1.3mL. Retrieved 9 August 2026, HTTP 200. DailyMed SPL, set id 839334d3-8c1d-4c26-9036-2ab524a6ea75.