Not medical advice. This calculator does arithmetic on numbers you enter, for general education. It does not diagnose, treat, or recommend a dose or treatment for any condition, and it does not replace an evaluation by a licensed physician. It never suggests an amount to inject: the prescribed amount comes from your prescriber, and mixing follows your pharmacy's instructions. Talk to your own clinician before acting on any result.
A peptide calculator turns three numbers from your vial label and prescription into the volume to draw: the peptide in the vial, the water added to it, and the amount your prescriber set. The concentration after mixing decides how many units on a U-100 insulin syringe hold that amount, and the calculator below shows each step of the arithmetic.
We ship prescribed peptides from our pharmacy, and every plan behind them is set by a physician who has read your labs. The calculator is for checking the arithmetic on figures you already have.
Peptide Reconstitution Calculator
Enter the amount of peptide printed on the vial, the volume of bacteriostatic water added, and the amount your prescriber set for each injection. Every field starts empty because every figure comes from your own label and prescription.
Concentration
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Peptide per syringe unit
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Volume to draw
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U-100 syringe units
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Full injections in the vial
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Enter the three figures from your vial label and prescription to see the working.
This calculator does arithmetic only. It does not check whether an amount is right for you, and it accepts mg or mcg labels only. Confirm every figure against your prescription and the pharmacy's mixing instructions, and ask your pharmacist or prescriber if any result looks different from what you were told.
How the Calculator Works Out Your Draw
Three divisions sit behind every result, and the calculator prints each one so you can check it by hand.
- Concentration. The peptide in the vial divided by the water added. A vial labelled 10 mg mixed with 2 mL of water holds 5 mg in every mL, which is 5,000 mcg per mL.
- Peptide per unit. A U-100 insulin syringe is marked in units, and 100 units fill 1 mL.1 At 5,000 mcg per mL, each unit on the barrel holds 50 mcg.
- Volume to draw. The prescribed amount divided by the concentration. The number of units is that volume multiplied by 100, or the prescribed amount in mcg divided by the mcg in one unit.
The vial count at the end is the vial's total peptide divided by the prescribed amount, rounded down to whole injections. It tells you how many full draws the arithmetic allows, and your prescriber and pharmacy label set how long a mixed vial is used.
What a Unit on an Insulin Syringe Measures
A unit on a U-100 syringe is a volume of 0.01 mL.1 The syringe was designed for U-100 insulin, where 1 unit of volume also holds 1 unit of insulin. With a peptide, the unit marking still measures volume only, so the amount of peptide in 10 units depends entirely on how the vial was mixed.
Confusing volume with amount leads to measuring mistakes. The U.S. Food and Drug Administration (FDA) has reported patients drawing several times their intended amount of compounded semaglutide after confusing units, milliliters, and milligrams.2 Convert the prescribed amount through the concentration every time, and the unit count you draw will match what was prescribed. For semaglutide or tirzepatide drawn from a vial, the GLP-1 units to milligrams calculator runs the same conversion in both directions.
How the Water Volume Changes the Draw
Adding more water does not change how much peptide is in the vial. It spreads the same amount through more liquid, so each unit holds less and the same prescribed amount takes more units to draw. The table uses a vial labelled 5 mg.
| Water added | Concentration | Peptide in 1 unit | Peptide in 10 units |
|---|---|---|---|
| 1 mL | 5 mg/mL | 50 mcg | 500 mcg |
| 2 mL | 2.5 mg/mL | 25 mcg | 250 mcg |
| 2.5 mL | 2 mg/mL | 20 mcg | 200 mcg |
| 3 mL | 1.67 mg/mL | 16.7 mcg | 167 mcg |
The same 10 units drawn from those four vials hold four different amounts. Use the water volume written on your pharmacy's mixing instructions, enter that exact figure, and the units the calculator returns will match the vial in your hand.
Bacteriostatic Water and Sterile Water
Bacteriostatic water for injection is sterile water with 0.9% benzyl alcohol added as a preservative, which slows bacterial growth in a vial that is punctured more than once.3 Sterile water for injection carries no preservative and is meant for a single use. The choice between them, the volume to add, and how long a mixed vial stays in use all come from the pharmacy that compounded your peptide. The calculator takes the volume as given and does the arithmetic on it.
Reading the Vial Label Before You Calculate
Peptide vials are labelled in milligrams (mg) or micrograms (mcg), and 1 mg equals 1,000 mcg. A prescription written in mcg against a vial labelled in mg is normal, and the calculator converts both to mcg before it divides. Enter each figure in the unit printed on its source, and the conversion happens inside the tool.
Some products are labelled in international units (IU). An IU is a measure of biological activity, and the mg equivalent differs from one peptide to the next, so the calculator accepts mg and mcg only. For an IU-labelled vial, the mg figure comes from the compounding pharmacy's label or from your prescriber.
Three Common Measuring Errors
- Units against milliliters. A prescription written in mL drawn as units, or the reverse, is a 100-fold error. Confirm which one your instructions use before you draw.2
- Milligrams against micrograms. A swapped mg and mcg entry is a 1,000-fold error. If the calculator says the prescribed amount is larger than the whole vial, that swap is the usual cause.
- Syringe size. A 1 mL syringe used for a small volume makes each marking harder to read. FDA cited oversized syringes as one contributor to measuring errors with compounded semaglutide.2 Ask your pharmacist which syringe size your volume calls for.
Peptides Prescribed and Monitored by a Physician
A compounded peptide is not FDA-approved, so FDA does not review it for safety, effectiveness, or quality before it is dispensed.4 That puts the weight on two things the calculator cannot provide: a pharmacy you can trust and a prescriber who sets the amount from your history and labs. Our guide to where your peptides come from covers the sourcing side. For the options a physician weighs, see how a physician chooses a peptide and what peptide therapy costs.
The Opt Take on Peptide Reconstitution
Checking the arithmetic before you draw guards against the 100-fold and 1,000-fold errors above. The amount itself is set by a physician. At Opt Health, a peptide enters a plan only after a 1:1 consultation with an Opt Health physician. That physician reviews a blood panel of 55+ biomarkers, your existing conditions, and your current medications. Prescribed peptides ship from our pharmacy to your home, and the plan is reassessed with a full blood panel and a physician consultation every 3 to 4 months.
Questions about mixing or drawing go to the care team, which you can reach by chat in the Opt app 7 days a week. Bring your vial label and your calculator result to that conversation, and the answer starts from your actual numbers.
Get started: Begin with Opt Health, where a physician reads your labs, builds your plan, and adjusts it over the loop.
Frequently Asked Questions
Divide the peptide in the vial by the water added to get the concentration, then divide the prescribed amount by that concentration to get the volume to draw. Multiply the volume in mL by 100 for the units on a U-100 syringe. A 10 mg vial mixed with 2 mL holds 5,000 mcg per mL, which is 50 mcg in each unit.
Use the volume written on the mixing instructions from the pharmacy that supplied the vial, or the volume your prescriber specified. The calculator shows how that volume changes the concentration and the units you draw. It does not choose a volume for you.
A U-100 insulin syringe holds 100 units in 1 mL, so each unit is 0.01 mL. A 0.3 mL syringe holds 30 units and a 0.5 mL syringe holds 50 units.
Adding water does not change the total peptide in the vial. It lowers the concentration, so the same prescribed amount takes more units to draw. Enter the exact water volume you used and the calculator adjusts the units to match.
The calculator accepts mg and mcg only, because the mg equivalent of an international unit differs from one peptide to another. The compounding pharmacy's label or your prescriber can give you the mg figure for an IU-labelled vial.
A peptide plan works best when the prescription, the pharmacy, and the follow-up labs come from one place. An Opt Health membership pairs prescribed peptides with a physician who reviews your labs and a care team you can message 7 days a week.
References
- International Organization for Standardization. ISO 8537:2016. Sterile single-use syringes, with or without needle, for insulin. Geneva: ISO; 2016.
- U.S. Food and Drug Administration. FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products. Silver Spring, MD: FDA; July 26, 2024.
- Hospira, Inc. Bacteriostatic Water for Injection, USP [prescribing information]. Lake Forest, IL: Hospira; DailyMed, U.S. National Library of Medicine.
- U.S. Food and Drug Administration. Compounding and the FDA: questions and answers. Silver Spring, MD: FDA.
This content is for informational purposes and does not replace evaluation, diagnosis, or treatment by a qualified medical professional.
This article is part of our guide to peptides. See how peptide therapy is prescribed and monitored at Opt Health.
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