The short answer is a formula
There is no single correct amount of bacteriostatic water for a vial, because the water amount is not a property of the peptide — it is a choice about measurement. The amount you add sets the concentration, the concentration sets how many units a dose takes, and the units are what you read on the syringe. So the question “how much water?” is really the question “how many units do I want this dose to take?” asked backwards. The formula that connects them is: water in millilitres equals target units times vial milligrams times 10, divided by dose micrograms. For the example on this page — a 5 mg vial and a 500 mcg dose — wanting 20 units gives 20 × 5 × 10 ÷ 500 = 2 mL. Wanting 10 units gives 1 mL. Wanting 50 units gives 5 mL. Same vial, same dose, three different answers, all correct arithmetic for three different targets.
Worked table: 5 mg vial, 500 mcg dose
The table below fixes the vial and the dose and varies only the water. Read it as a menu of measurements, not as advice: each row is what the formulas produce for that water amount.
| Water (mL) | Concentration | Volume per dose | Units (U-100) |
|---|---|---|---|
| 0.5 mL | 10,000 mcg/mL | 0.05 mL | 5 units |
| 1 mL | 5,000 mcg/mL | 0.10 mL | 10 units |
| 1.5 mL | 3,333.333 mcg/mL | 0.15 mL | 15 units |
| 2 mL | 2,500 mcg/mL | 0.20 mL | 20 units |
| 2.5 mL | 2,000 mcg/mL | 0.25 mL | 25 units |
| 3 mL | 1,666.667 mcg/mL | 0.30 mL | 30 units |
| 4 mL | 1,250 mcg/mL | 0.40 mL | 40 units |
| 5 mL | 1,000 mcg/mL | 0.50 mL | 50 units |
Arithmetic examples only, for a 5 mg vial and a 500 mcg dose. Your numbers will differ — use the calculator above.
What BAC water is, and why it is the liquid in the math
BAC water is bacteriostatic water: sterile water containing a small amount of benzyl alcohol, which keeps it sterile after the vial is opened. In the math on this site it plays exactly one role — it is the liquid the powder dissolves in, and its volume is the denominator of the concentration formula. Nothing else about it enters the calculation. That is worth saying plainly because the question “how much BAC water” sometimes arrives with the assumption that the water itself does something to the dose. It does not. The dose is an amount of peptide; the water is the carrier. Change the water and you change the measurement, not the amount.
More water versus less water: the real trade-off
Look at the table again and the trade-off is visible in the units column. At 0.5 mL the example dose is 5 units — a small draw on a syringe, where each unit marking is a large share of the dose and a small reading error is a large percentage error. At 5 mL the same dose is 50 units — a large draw, where each marking is a small share and the reading is finer, but the volume is ten times larger. Between those extremes, 2 mL gives 20 units, a round number that is easy to read and easy to double-check. None of these is right or wrong in itself; they are different measurement resolutions for the same amount. The auto-suggest button on the calculator aims at 20 units for exactly that reason — round numbers are easier to verify — and it says so when you press it.
Why this question gets searched so much
In our keyword research, “how much bac water for 5mg retatrutide calculator” runs to about 600 US searches a month, with two related questions at about 300 and 150. It is the most-asked reconstitution question in the niche, and the reason is understandable: the vial and the dose feel like facts, while the water feels like a free choice, and people want to know whether there is a right answer. There is not — there is a formula, a table, and a calculator, and this page gives you all three so the choice is yours and it is informed. The same question appears for other compounds too — “peptide bac water calculator” at about 800 a month and “tirzepatide bac water calculator” at about 1,100 — and the answer is the same formula with different numbers.
Checking your own numbers
Pick any row of the table and verify it. At 2.5 mL: concentration is 5,000 ÷ 2.5 = 2,000 mcg/mL; volume is 500 ÷ 2,000 = 0.25 mL; units are 0.25 × 100 = 25. Or work backwards from a target: if you want 25 units from a 5 mg vial at a 500 mcg dose, the water is 25 × 5 × 10 ÷ 500 = 2.5 mL. Either direction, the same formula. If your numbers are not in the table, the calculator above does the same steps for any vial, water, and dose you enter.
The boundary
This page explains the arithmetic of water amounts. It does not recommend any amount, any dose, or any schedule. Pure math only — not medical advice.