How Many Units of Bacteriostatic Water to Add to Peptides

One of the most common questions in peptide reconstitution is deceptively simple: how much bacteriostatic water do I add?

The answer depends on three things — your vial size, your target dose, and what makes the math easy. But before diving into calculations, there’s an important distinction that trips up nearly every beginner: “units” on a syringe measure volume, not dosage.

Understanding this one concept eliminates most of the confusion around peptide reconstitution.

Quick Summary

• “Units” on an insulin syringe = volume (100 units = 1 mL)
• The amount of BAC water you add determines your concentration
• More water = more dilute solution = easier to measure small doses
• Common starting point: 1–2 mL of BAC water per vial
• Use our Peptide Reconstitution Calculator to skip the math entirely

Understanding Syringe Units vs Peptide Dosage

This is where most confusion starts, so let’s clear it up immediately.

On a standard U-100 insulin syringe, the markings show units of volume:

• 100 units = 1 mL
• 50 units = 0.5 mL
• 10 units = 0.1 mL
• 1 unit = 0.01 mL

These are units of volume, not units of peptide. When someone says “inject 10 units,” they’re really saying “inject 0.1 mL of whatever solution is in the syringe.” How much actual peptide is in that 0.1 mL depends entirely on how concentrated you made the solution.

For a full breakdown of syringe markings, read our insulin syringe units guide.

The Basic Formula

The math for peptide reconstitution is simple division:

Concentration = Peptide Amount ÷ Water Volume

Or, rearranged to find your dose volume:

Volume to Inject = Desired Dose ÷ Concentration

That’s it. Everything else is just plugging in numbers.

Worked Example 1: A 5 mg Peptide Vial

Let’s say you have a vial containing 5 mg of peptide and you want to dose 250 mcg per injection.

Adding 1 mL (100 units) of BAC Water

• Concentration: 5 mg ÷ 1 mL = 5 mg/mL (or 5,000 mcg/mL)
• Volume for 250 mcg dose: 250 ÷ 5,000 = 0.05 mL = 5 units

That works, but 5 units is a very small amount to measure accurately on most syringes. You’d need a 0.3 mL syringe with fine markings to get this right.

Adding 2 mL (200 units) of BAC Water

• Concentration: 5 mg ÷ 2 mL = 2.5 mg/mL (or 2,500 mcg/mL)
• Volume for 250 mcg dose: 250 ÷ 2,500 = 0.1 mL = 10 units

Ten units is much easier to measure. You can see it clearly on any insulin syringe. This is why 2 mL is a popular choice for 5 mg vials.

Adding 2.5 mL (250 units) of BAC Water

• Concentration: 5 mg ÷ 2.5 mL = 2 mg/mL (or 2,000 mcg/mL)
• Volume for 250 mcg dose: 250 ÷ 2,000 = 0.125 mL = 12.5 units

Also workable, and creates very round numbers at common dose levels.

Worked Example 2: A 10 mg Peptide Vial

Now with a 10 mg vial, dosing 500 mcg per injection:

Adding 1 mL (100 units) of BAC Water

• Concentration: 10 mg ÷ 1 mL = 10 mg/mL (or 10,000 mcg/mL)
• Volume for 500 mcg dose: 500 ÷ 10,000 = 0.05 mL = 5 units

Again, 5 units is hard to measure precisely. Works if you have a fine syringe, but there’s a better option.

Adding 2 mL (200 units) of BAC Water

• Concentration: 10 mg ÷ 2 mL = 5 mg/mL (or 5,000 mcg/mL)
• Volume for 500 mcg dose: 500 ÷ 5,000 = 0.1 mL = 10 units

Clean and easy. Ten units on the syringe, every time.

Adding 3 mL (300 units) of BAC Water

• Concentration: 10 mg ÷ 3 mL = 3.33 mg/mL (or 3,333 mcg/mL)
• Volume for 500 mcg dose: 500 ÷ 3,333 = 0.15 mL = 15 units

Also fine, but the math isn’t as clean. There’s nothing wrong with this — just less convenient.

Quick Reference Table

Here are common vial sizes with popular water volumes:

How to Decide How Much Water to Add

There’s no single “correct” amount. The amount of water simply changes the concentration — the total peptide in the vial stays the same. Here’s how to choose:

Aim for Doses That Land on Easy Syringe Lines

The whole point is making your dose easy to measure. Look at your syringe:

• 1 mL syringe (100 units): Each small line = 1 unit (0.01 mL). Reasonable accuracy down to about 2–3 units.
• 0.5 mL syringe (50 units): Each small line = 1 unit (0.01 mL). Same precision, but easier to read for small volumes.
• 0.3 mL syringe (30 units): Each small line = 0.5 units (0.005 mL). Best precision for very small doses.

Pick a water volume that puts your dose at 5, 10, 15, or 20 units. These are easy to see and measure consistently.

Don’t Go Below 0.5 mL of Water

Very small water volumes (0.25 mL, for example) create highly concentrated solutions. While this works in theory, measuring tiny doses (1–2 units) is difficult and introduces dosing errors. Stick to at least 0.5 mL, preferably 1–2 mL.

Don’t Go Above 3 mL

Most peptide vials are small — typically 2 mL or 3 mL total capacity. Adding 5 mL of water to a 3 mL vial simply won’t fit. Even if it did, overly dilute solutions mean injecting large volumes, which isn’t ideal for subcutaneous injection.

Consider How Many Doses You Need

If you’ll use the entire vial within a few days, a smaller water volume (1 mL) is fine — you’ll draw it up quickly. If the vial will last 3–4 weeks, a slightly larger volume gives you more liquid to work with and reduces the percentage lost to syringe dead space.

Common Mistakes with BAC Water Volume

Confusing “Units” with Dose

If a protocol says “take 250 mcg,” that does not mean “draw 250 units.” Units on the syringe are volume, not peptide mass. You must calculate the volume based on your specific concentration.

Not Accounting for Syringe Dead Space

Every syringe retains a tiny amount of liquid in the needle hub — typically 0.05–0.07 mL. Over many draws from a vial, this adds up. It’s usually not enough to worry about, but be aware that the last draw from a vial may come up slightly short.

Mixing Up mL and Units

A quick reminder:

• 1 mL = 100 units
• 0.5 mL = 50 units
• 0.1 mL = 10 units

When someone says “add 2 mL,” that’s the same as “add 200 units” on a U-100 syringe. When they say “inject 10 units,” that means 0.1 mL.

Adding Water in Multiple Sessions

Add all your water at once. Don’t add 1 mL today and another 1 mL tomorrow. Each needle puncture increases contamination risk, and it’s harder to track your final concentration if you’re adding water in stages.

Why BAC Water and Not Something Else?

Bacteriostatic water is the standard reconstitution solvent for peptides used across multiple doses. The 0.9% benzyl alcohol acts as a preservative, keeping the solution safe for repeated needle punctures over its shelf life (typically 28 days once reconstituted).

Plain sterile water is appropriate only for single-use vials that will be consumed entirely in one session. For anything else, BAC water is the safer choice.

Read our complete comparison: bacteriostatic water vs sterile water for peptides.

Skip the Math: Use a Calculator

If formulas aren’t your thing, our reconstitution calculator does all of this for you. Enter your peptide amount, the volume of water you’re adding (or want to add), and your target dose — it tells you exactly how many units to draw.

Try our free Peptide Reconstitution Calculator to calculate your exact dosing.

This content is for educational and research purposes only.