Three numbers, one division
A lyophilised peptide vial is a powder. Mixing it with bacteriostatic water gives a solution whose strength depends only on two things: the amount of peptide printed on the vial and the volume of water you add. A U-100 insulin syringe is marked in units, where 100 units is 1 mL, so once the strength is known, a dose in micrograms converts to a mark on the barrel.
concentration (mcg/mL) = peptide in vial (mcg) ÷ water added (mL)
units to draw = dose (mcg) ÷ concentration (mcg/mL) × 100
Equivalently, each unit holds the concentration divided by 100. The syringe size changes nothing in the arithmetic; it only decides whether one draw fits.
The calculator takes the dose as an input and never fills one in. Where a dose comes from is between you and whoever prescribed it; the tool only does the conversion.
U-100 syringes and their sizes
"U-100" means the barrel is graduated for a liquid with 100 units per millilitre. The three common sizes hold different amounts but share the same scale:
| Syringe | Capacity | Typical markings |
|---|---|---|
| 0.3 mL | 30 units | Every unit, sometimes every half unit |
| 0.5 mL | 50 units | Every unit |
| 1 mL | 100 units | Every 2 units |
A smaller syringe spreads the same units over more barrel, so small draws are easier to read on a 0.3 mL syringe than on a 1 mL one. The calculator's syringe setting checks the draw against the capacity and says when it will not fit.
Check the arithmetic
Example: a 5 mg vial, 2 mL of water, a 250 mcg dose.
5,000 mcg ÷ 2 mL = 2500 mcg/mL, so each unit holds 25 mcg
250 ÷ 2500 × 100 = 10 units (0.1 mL)
The vial holds 20 such doses. Add 3 mL of water instead and the same dose becomes 15 units, because the solution is weaker. These are arithmetic examples with round numbers, not doses.
The reverse direction works the same way. With 10 mg in 1 mL, a draw to 8 units on a 0.5 mL syringe contains 800 mcg (0.8 mg), because 8 units is 0.08 mL of a 10 mg/mL solution.
How much water to add
More water makes a weaker solution and a larger draw for the same dose; less water makes a stronger solution and a smaller draw. Neither changes the dose. Two practical rules follow:
- Aim for a draw you can read. Under 2 units is hard to measure on any barrel. If the calculator shows a tiny draw, add more water to the next vial.
- Stay inside the syringe. If the draw is larger than the syringe holds, add less water or use a larger syringe.
Vial size limits how much water fits; most small vials take 1 to 3 mL comfortably. Whatever volume you choose, write it on the vial and use the same volume every time, so a mark on the syringe keeps meaning the same dose. Bacteriostatic water is a multiple-dose diluent with 0.9% benzyl alcohol as a preservative; its label says it is not for use in neonates.
Blended vials
A blend lists more than one peptide in one vial, say 10 mg of one and 10 mg of another, 20 mg in total. Enter the total milligrams as the vial amount and the dose of the total you intend to draw; the components come along in the vial's fixed proportion. To see one component's share of a draw, multiply the draw by that component's fraction of the total: in a 10 + 10 vial, half of any draw is each peptide.
Keep the schedule beside the math
The units are the easy part; what people lose track of is the rest: which days, which site, what was skipped, how it felt. Brolic's Protocols keep user-entered peptide, GLP-1, TRT and supplement routines on one timeline with reminders, as-needed logs, injection-site rotation and symptom notes, beside the day's food and training. It is tracking only. Brolic never recommends a dose, a site, a schedule or a compound.
One more thing worth knowing before anything goes in a vial: the FDA lists many of the peptides sold online, including BPC-157, TB-500, CJC-1295, ipamorelin and GHK-Cu, among bulk substances that may present significant safety risks in compounding, and products labelled "for research use only" are not approved medicines. The sources below link the list.
Common questions
Why does the calculator not fill in a typical dose?
Because it is not a dosing tool. Any number it suggested would be a recommendation for a compound and a person it knows nothing about. Enter the dose you were given and it converts that, nothing more.
Does the syringe size change the units I draw?
No. Units are the same on every U-100 syringe: 100 units is always 1 mL. The size only decides whether the draw fits and how easy the marks are to read.
My vial is labelled in milligrams but my dose is in micrograms. Which do I enter?
Enter each in the unit it comes in and set the unit toggles to match; the calculator converts. One milligram is 1,000 micrograms.
What if the dose is larger than the syringe holds?
The result says so. Mix the next vial with less water so the same dose is a smaller draw, or use a larger syringe. Do not change the dose to make it fit.
Can I use this for GLP-1 vials or insulin?
The arithmetic is the same for any powder reconstituted with a known volume and drawn with a U-100 syringe. Branded pens and pre-filled products have their own dosing instructions; follow those, not a calculator.
Sources & method
Written by the Brolic team. These references explain the evidence and assumptions; this page does not replace individualized professional advice.
- Bacteriostatic Water for Injection, USP — prescribing information (Hospira, a Pfizer company) — 0.9% benzyl alcohol as preservative; a multiple-dose container for diluting or dissolving drugs for injection; not for use in neonates.
- Syringe, U-100 insulin, 1 mL — Médecins Sans Frontières medical catalogue — Specification of a U-100 insulin syringe: 1 mL graduated in 100 units.
- Certain bulk drug substances for use in compounding that may present significant safety risks — U.S. FDA — Names BPC-157, TB-500, CJC-1295, ipamorelin, GHK-Cu, MOTS-c and others; last updated April 22, 2026.
