Peptide Dosage & Reconstitution Calculator
Turn the dose on your prescription into the number on your syringe. Enter what's in your vial and how much bacteriostatic water you add (or the mg/mL on a pre-mixed vial), and see exactly how many units to draw on a U-100 insulin syringe.
Used only to count doses per vial.
= 0.1 mL of a 2.5 mg/mL solution, delivering 0.25 mg (250 mcg).
The three-step formula
Every peptide calculation, whatever the medication, comes down to the same three steps:
- Concentration = mg in the vial ÷ mL of water added
- Volume to inject = your dose in mg ÷ concentration
- Syringe units = volume in mL × 100 (on a U-100 insulin syringe)
The calculator above runs these for you, but it's worth being able to check the math yourself. Here is how one 5 mg vial behaves with different amounts of water, drawing the same 250 mcg dose:
| Water added | Concentration | 250 mcg dose | Doses per vial |
|---|---|---|---|
| 1 mL | 5 mg/mL | 5 units (0.05 mL) | 20 |
| 2 mL | 2.5 mg/mL | 10 units (0.10 mL) | 20 |
| 3 mL | 1.67 mg/mL | 15 units (0.15 mL) | 20 |
| 5 mL | 1 mg/mL | 25 units (0.25 mL) | 20 |
The number of doses stays the same. Only the size of each draw changes, which is why your pharmacy's mixing instructions and your dose in units have to match each other.
Units are volume, not medication
An insulin syringe is marked in units because it was designed for U-100 insulin, where 100 units of insulin fill 1 mL. For anything else, a unit is simply 0.01 mL. The mark tells you how much liquid you drew, not how much drug. Two vials of the same peptide at different concentrations need different unit counts for the same mg dose.
This confusion causes real overdoses. In 2024 the FDA warned about dosing errors with compounded semaglutide: patients told to draw 5 units drew 50, and some prescribers converted mg to units incorrectly, leading to doses five to twenty times higher than intended. If the units your calculation gives don't match what your pharmacy told you, stop and call them.
Pick the right syringe
Insulin syringes usually come in 0.3 mL (30 units), 0.5 mL (50 units) and 1 mL (100 units) sizes. Smaller barrels have wider spacing between marks, so small doses are easier to read accurately. A 0.3 mL syringe often has half-unit marks; a 1 mL syringe usually marks every 2 units. Choose the smallest syringe that holds your full dose, and select it in the calculator so the diagram matches what you see in your hand.
Before you mix: checklist
- Confirm the vial strength on the label matches your prescription.
- Use bacteriostatic water unless your pharmacy says otherwise. Plain sterile water has no preservative for multi-dose use.
- Add the water slowly down the side of the vial and swirl gently; don't shake.
- Write the mixing date and resulting concentration on the vial.
- Run your dose through the calculator and compare it to the pharmacy's instructions before the first injection.
Frequently Asked Questions
How do I calculate a peptide dose in units?
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Divide the milligrams in the vial by the millilitres of bacteriostatic water you add to get the concentration (mg/mL). Divide your dose in mg by that concentration to get millilitres, then multiply by 100 to get units on a U-100 insulin syringe. Example: 5 mg + 2 mL water = 2.5 mg/mL; a 250 mcg (0.25 mg) dose is 0.1 mL, or 10 units.
How much bacteriostatic water should I add to a peptide vial?
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Use the amount your prescriber or pharmacy specifies. The water volume does not change how much medication is in the vial, only how concentrated it is, so it changes how many units each dose takes. More water means a larger, easier-to-measure draw for small doses, but the volume per injection grows too. If your instructions don't say, ask the pharmacy before mixing.
Is 1 unit on an insulin syringe the same as 1 mg?
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No, and confusing the two is the most common dosing error. A unit on a U-100 syringe is a volume, 0.01 mL. How many milligrams that volume contains depends entirely on the vial's concentration. At 2.5 mg/mL, 1 unit holds 0.025 mg; at 10 mg/mL it holds 0.1 mg.
What is the difference between mg and mcg?
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1 milligram (mg) equals 1,000 micrograms (mcg). Many peptide doses are written in mcg (for example 250 mcg = 0.25 mg). The calculator lets you enter the dose in either unit and converts for you.
How long does a reconstituted peptide last?
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Storage and beyond-use dates depend on the specific product and are set by the dispensing pharmacy. Most reconstituted peptides are kept refrigerated and protected from light. Check the label or pharmacy instructions rather than relying on a general rule, and discard anything past its beyond-use date.
Can I use this for peptides I bought online without a prescription?
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The arithmetic works for any vial, but we don't recommend it. 'Research use only' peptides are not made under pharmacy standards, so the actual contents, purity and sterility of the vial are unknown, and an accurate calculation can't fix an inaccurate label. Several popular peptides also have no approved or legally settled compounding pathway in the US.
Medical disclaimer: This calculator does arithmetic on the numbers you enter. It does not recommend a dose, and it can't check your vial label. Always use the dose, concentration and reconstitution volume from your prescriber and dispensing pharmacy, and confirm with your pharmacist if anything doesn't match. Not medical advice.