Doses filled in when you pick a peptide are starting examples so the maths has something to work with, not a recommendation. Research use only, and not medical advice.
How the calculation works
Three steps, and the calculator does all of them for you:
- Concentration = peptide in the vial ÷ water added. A 10 mg vial with 2 mL of water is 5 mg per mL.
- Volume to draw = your dose ÷ concentration. A 2 mg dose from that vial is 2 ÷ 5 = 0.4 mL.
- Units = volume × 100, because every U-100 insulin syringe has 100 units per mL. 0.4 mL is 40 units.
Choosing how much water to add
The water does not change how much peptide you get, only how strong each unit is. So the useful question is which volume makes your dose easy to measure. Two rules of thumb:
- Aim for a draw between about 5 units and 80% of your syringe. Below that, a small slip is a large share of the dose; above it, the plunger is near the end of the barrel.
- A dose that lands on a numbered mark (5, 10, 15) is easier to read than one at 13.3 units. When your draw falls between marks, the calculator suggests a volume that fixes it.
Use bacteriostatic water, not plain sterile water, for any vial you will draw from more than once: the benzyl alcohol in it slows bacterial growth. Our bacteriostatic water guide covers why, and how to mix without damaging the peptide.
Once the vial is mixed
Keep it in the fridge at 2 to 8°C, away from light, and never freeze it. For mixed GLP-1 vials the FDA advises not to keep using one after 28 days, even if a label says longer. Write the mixing date on the vial so you know when that is. For compound-specific numbers, see the retatrutide, semaglutide and tirzepatide unit charts.
Frequently asked questions
How many units is 250 mcg?
It depends on how much water is in the vial. In a 5 mg vial mixed with 2 mL of bacteriostatic water, each unit holds 25 mcg, so 250 mcg is 10 units. Mix the same vial with 1 mL and 250 mcg becomes 5 units. Enter your own vial and water above to get your number.
How much bacteriostatic water should I add?
There is no single correct amount. The water only sets the concentration; the peptide in the vial stays the same. Pick a volume that puts your dose on an easy-to-read mark, ideally between about 5 units and 80% of your syringe. The calculator suggests one when your current draw lands between marks.
Is a syringe unit the same as an IU?
No. A unit on an insulin syringe is a volume: on a U-100 syringe, 1 unit is 0.01 mL. An IU (international unit) measures biological activity and is used for products such as HGH and HCG. For a peptide measured in mg or mcg, syringe units are just a way to read the volume.
Which syringe size should I use?
Use the smallest barrel that holds your whole dose. All three common sizes are U-100, so the units mean the same thing, but a 0.3 mL syringe spreads 30 units over the same length that a 1 mL syringe spreads 100, which makes small draws far easier to read.
Does adding more water change the dose?
No. More water makes each unit weaker, so you draw more units for the same dose. The amount of peptide you inject stays the same. What changes is how readable the draw is and how quickly the vial runs out of volume, not how much peptide is in it.