Short answer: to find how many units to draw from a reconstituted vial, divide the amount in the vial by the water you added to get the strength (mg per mL). Divide the amount you want by that strength to get mL. On a U-100 insulin syringe, multiply mL by 100 to get units. For example, 5 mg mixed with 2 mL of bacteriostatic water is 2.5 mg/mL, so 250 mcg is 0.1 mL, which is 10 units.
What is the formula for peptide reconstitution?
Three steps, always in this order:
- Strength (mg/mL) = mg in the vial ÷ mL of water added
- Volume (mL) = amount you want (mg) ÷ strength (mg/mL)
- Units (U-100 syringe) = volume (mL) × 100
Convert micrograms to milligrams first: 1,000 mcg = 1 mg, so 250 mcg = 0.25 mg.
Worked examples
Vial | Water added | Strength | Amount | Volume | Units (U-100) |
|---|---|---|---|---|---|
5 mg | 2 mL | 2.5 mg/mL | 250 mcg | 0.10 mL | 10 |
5 mg | 1 mL | 5 mg/mL | 250 mcg | 0.05 mL | 5 |
10 mg | 2 mL | 5 mg/mL | 500 mcg | 0.10 mL | 10 |
10 mg | 3 mL | 3.33 mg/mL | 1 mg | 0.30 mL | 30 |
More water does not change how much is in the vial. It only makes each unit weaker, so you draw more units for the same amount.
How many units are in 1 mL?
On a U-100 insulin syringe, 1 mL is 100 units, so each unit is 0.01 mL. A 0.3 mL syringe holds 30 units, a 0.5 mL syringe 50 units and a 1 mL syringe 100 units.
On a U-40 syringe, 1 mL is 40 units. Using U-100 math on a U-40 syringe gives the wrong volume, so check the label on the syringe before you count.
A full worked example, step by step
Say the vial label reads 10 mg and you add 2 mL of bacteriostatic water.
- Step 1, strength: 10 mg ÷ 2 mL = 5 mg/mL. Every 1 mL in that vial now holds 5 mg.
- Step 2, volume: the amount you want is 500 mcg. Convert first: 500 mcg = 0.5 mg. Then 0.5 mg ÷ 5 mg/mL = 0.1 mL.
- Step 3, units: 0.1 mL × 100 = 10 units on a U-100 syringe.
Check it backwards: 10 units is 0.1 mL, and 0.1 mL × 5 mg/mL = 0.5 mg. If the reverse check does not land on the amount you started with, one of the steps used the wrong unit.
How long will a vial last?
Divide what is in the vial by the amount you take each time.
Doses per vial = mg in the vial ÷ mg per dose
A 10 mg vial at 0.5 mg per dose holds 20 doses. Taken once a day, that is 20 days. Taken five days a week, it is four weeks. The water does not change this number. It only changes how many units each dose takes.
In practice you get slightly fewer doses than the math says, because a little liquid stays in the vial and the needle. Planning your next order a few doses early avoids running out.
Vial | Amount per dose | Doses per vial | Daily use | 5 days a week |
|---|---|---|---|---|
5 mg | 250 mcg | 20 | 20 days | 4 weeks |
10 mg | 500 mcg | 20 | 20 days | 4 weeks |
10 mg | 250 mcg | 40 | 40 days | 8 weeks |
5 mg | 1 mg | 5 | 5 days | 1 week |
What about blends?
A blend vial holds two or more compounds mixed together, for example 10 mg of one and 5 mg of another in the same vial. You only add water once, so each compound gets its own strength:
- Compound A: 10 mg ÷ 2 mL = 5 mg/mL
- Compound B: 5 mg ÷ 2 mL = 2.5 mg/mL
Every unit you draw contains both, in the same ratio. You cannot draw only one of them. Pick the units based on the compound you are measuring by, then work out how much of the other comes with it. If 10 units (0.1 mL) gives 0.5 mg of A, the same 10 units also gives 0.25 mg of B.
A quick checklist before you draw
- Read the vial label: total mg, not the mg per dose.
- Write down how much water you added, and when.
- Convert mcg to mg before dividing.
- Check the syringe: U-100 or U-40, and its total volume.
- Do the reverse check from units back to mg.
- Note the dose so the next one uses the same numbers.
Common mistakes
- Mixing up mg and mcg. 250 mg is 1,000 times more than 250 mcg.
- Keeping the old number of units after mixing a new vial with a different amount of water.
- Counting U-100 units on a U-40 syringe.
- Running several compounds and using one vial's strength for another.
How to stop doing this by hand
Dose Track is an iPhone and Android app for tracking TRT, GLP-1 medications and peptides in one place. Its reconstitution calculator works out the water to add and the units to draw, and saves them with each vial, so every dose you log afterwards uses that vial's strength. It also counts down what is left in each vial and warns you before it runs out, reminds you when each dose is due, suggests the next injection site, and estimates how much of each compound is still active from your logged doses. A blend calculator gives the units for each compound in a mixed vial.
For a quick answer without the app, use the free peptide reconstitution calculator or the U-40 vs U-100 syringe guide.
FAQ
Does adding more water change the dose? No. The amount in the vial stays the same. More water lowers the strength, so the same amount takes more units.
How many units is 250 mcg? It depends on the strength. At 2.5 mg/mL it is 10 units on a U-100 syringe; at 5 mg/mL it is 5 units.
What is 0.1 mL in units? 10 units on a U-100 syringe, 4 units on a U-40 syringe.
Which water is used for reconstitution? Bacteriostatic water is commonly used for multi-dose vials because it contains a preservative. Follow the product's instructions.
This article shows the math only. It is not dosing advice. Check any dose with your provider.