Water changes concentration, not the total amount
Every vial contains a fixed amount of peptide, printed on the label in mg or mcg. Adding BAC water does not add or remove any of that peptide; it dissolves the powder into a liquid so you can measure out small portions of it with a syringe. Add more water and the same total amount is spread through more liquid, so it's more dilute. Add less water and that same amount is concentrated into less liquid, so it's more potent per mL. The vial's total content never changes, only how much of it sits in each unit you draw.
The trade-off: more water vs. less water
- More water lowers the concentration, so drawing a given amount takes more units. Larger numbers of units are easier to line up against the markings on a syringe, so the draw is easier to measure accurately.
- Less water raises the concentration, so the same amount takes fewer units. That makes the vial last through more, smaller draws, but small draws are harder to measure precisely. As a rule of thumb, readings under about 2 units are difficult to read reliably on a standard insulin syringe, so a small misjudgment of the plunger position is a large percentage error at that size.
There's no universally "correct" volume; it's a balance between an easy-to-read draw and how many draws you want out of one vial. The peptide reconstitution calculator shows you the resulting units as you adjust the water, so you can see the trade-off for your own numbers before you draw anything.
Concentration and per-unit amount by vial size
The table below shows the concentration and the amount in a single unit (0.01 mL on a U-100 syringe) for three common vial sizes, each reconstituted with 1, 2 or 3 mL of BAC water. Figures are rounded to one decimal place.
| Vial | BAC water | Concentration | Per unit (0.01 mL) |
|---|---|---|---|
| 2 mg | 1 mL | 2,000 mcg/mL | 20 mcg |
| 2 mg | 2 mL | 1,000 mcg/mL | 10 mcg |
| 2 mg | 3 mL | 666.7 mcg/mL | 6.7 mcg |
| 5 mg | 1 mL | 5,000 mcg/mL | 50 mcg |
| 5 mg | 2 mL | 2,500 mcg/mL | 25 mcg |
| 5 mg | 3 mL | 1,666.7 mcg/mL | 16.7 mcg |
| 10 mg | 1 mL | 10,000 mcg/mL | 100 mcg |
| 10 mg | 2 mL | 5,000 mcg/mL | 50 mcg |
| 10 mg | 3 mL | 3,333.3 mcg/mL | 33.3 mcg |
Concentration is the vial's amount in mcg divided by the water in mL; the per-unit amount is that concentration divided by 100, since a U-100 syringe puts 100 units in each mL.
concentration (mcg/mL) = vial amount (mcg) ÷ BAC water (mL) per unit (mcg) = concentration (mcg/mL) ÷ 100Choosing water so a draw lands on a round number
If you already know roughly what amount you'll be drawing each time, you can work backward from a target number of units to the water volume that produces it:
water (mL) = vial amount (mcg) × target units ÷ (amount per draw (mcg) × 100)Worked example 1. A 5 mg (5,000 mcg) vial, with an example draw amount of 250 mcg, aimed at landing on 25 units: water = 5,000 × 25 ÷ (250 × 100) = 125,000 ÷ 25,000 = 5 mL. Checking it the other way: 5,000 mcg in 5 mL is 1,000 mcg/mL, and 250 ÷ 1,000 = 0.25 mL, which is 25 units. It matches.
Worked example 2. A 10 mg (10,000 mcg) vial, with an example draw amount of 500 mcg, aimed at landing on 20 units: water = 10,000 × 20 ÷ (500 × 100) = 200,000 ÷ 50,000 = 4 mL. Checking it the other way: 10,000 mcg in 4 mL is 2,500 mcg/mL, and 500 ÷ 2,500 = 0.20 mL, which is 20 units. It matches again.
You can run either direction of this calculation, plus the resulting draws-per-vial, on the reconstitution calculator instead of doing it by hand.
Syringe capacity
Whatever water volume you land on, the draw still has to fit the syringe you're using. On a U-100 insulin syringe:
- A 0.3 mL syringe holds 30 units
- A 0.5 mL syringe holds 50 units
- A 1 mL syringe holds 100 units
If your calculated draw is close to or larger than the syringe's capacity, either add more water to lower the concentration or switch to a larger syringe.
Frequently asked questions
Does more BAC water make a peptide weaker?
No. The vial still holds the same total amount of peptide. More water only spreads that amount across more liquid, so each unit on the syringe contains less and you draw more units to get the same amount.
What happens if I use too little BAC water?
The solution becomes more concentrated, so the draw for a given amount becomes a smaller number of units. Very small draws, under roughly 2 units, are hard to measure precisely on a syringe, and a small misread becomes a large percentage error.
How do I pick a water volume that gives round units?
Use water (mL) = vial amount (mcg) × target units ÷ (amount per draw (mcg) × 100). Pick the units number you want to land on, plug in your vial size and typical draw amount, and the formula gives you the water volume to add.
How many units does a syringe hold?
On a U-100 insulin syringe, a 0.3 mL syringe holds 30 units, a 0.5 mL syringe holds 50 units, and a 1 mL syringe holds 100 units.
Is there a best amount of BAC water to add?
There's no single right answer; it's a trade-off between a bigger, easier-to-read draw and a smaller draw volume. Many people pick a volume that puts their usual draw somewhere in the middle of the syringe's markings rather than at either extreme.
Skip the manual math
PeptidesAI works out the concentration and the units for every draw as soon as you enter the vial and the water, and it keeps tracking your remaining doses from there. Free to download on iPhone.
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