How the calculation works
Reconstitution is a dilution problem with three steps. Work in micrograms (mcg) throughout: 1 mg is 1,000 mcg, and mixing the two up is a 1,000-fold error.
concentration (mcg/mL) = vial amount (mcg) ÷ BAC water (mL) volume to draw (mL) = amount per draw (mcg) ÷ concentration units on a U-100 syringe = volume (mL) × 100Worked example. A 5 mg vial is 5,000 mcg. Add 2 mL of BAC water and the concentration is 5,000 ÷ 2 = 2,500 mcg/mL. For 250 mcg you need 250 ÷ 2,500 = 0.10 mL, which is 10 units on a U-100 insulin syringe. That vial holds 5,000 ÷ 250 = 20 draws of that size.
Quick reference: units for a 250 mcg draw
| Vial | BAC water | Concentration | Per unit | Units for 250 mcg |
|---|---|---|---|---|
| 2 mg | 1 mL | 2,000 mcg/mL | 20 mcg | 12.5 |
| 5 mg | 1 mL | 5,000 mcg/mL | 50 mcg | 5 |
| 5 mg | 2 mL | 2,500 mcg/mL | 25 mcg | 10 |
| 5 mg | 3 mL | 1,667 mcg/mL | 16.7 mcg | 15 |
| 10 mg | 2 mL | 5,000 mcg/mL | 50 mcg | 5 |
| 10 mg | 3 mL | 3,333 mcg/mL | 33.3 mcg | 7.5 |
Choosing how much water to add
The water does not change how much peptide is in the vial. It only changes how much each unit contains. That gives you a trade-off:
- More water means each unit holds less, so your draw is a bigger number of units. Bigger draws are easier to measure accurately on the syringe markings.
- Less water means a stronger solution and a smaller draw. Very small draws (a couple of units) are hard to measure precisely, and a small misread is a large percentage error.
A common aim is a draw that lands on a clear marking, somewhere between roughly 5 and 50 units, and that fits the syringe you use. The calculator warns you when the draw is under 2 units or larger than the syringe.
Reading a U-100 insulin syringe
U-100 means 100 units per mL, so 1 unit is 0.01 mL. The syringe size only changes how much it holds and how finely it is marked: a 0.3 mL syringe holds 30 units and often has half-unit marks, a 0.5 mL holds 50 and a 1 mL holds 100, usually marked every 2 units. The unit value is the same on all of them.
Frequently asked questions
How many units is 250 mcg from a 5 mg vial with 2 mL of BAC water?
10 units on a U-100 insulin syringe. 5 mg in 2 mL is 2.5 mg/mL (2,500 mcg/mL). 250 mcg ÷ 2,500 mcg/mL = 0.10 mL, and on a U-100 syringe 0.10 mL is 10 units.
What is the formula for peptide reconstitution?
Concentration (mcg/mL) = vial amount in mcg ÷ water in mL. Volume to draw (mL) = target amount in mcg ÷ concentration. Units on a U-100 syringe = volume in mL × 100.
Does adding more BAC water change the amount of peptide?
No. The vial holds the same amount of peptide whatever you add. More water makes each unit contain less, so you draw more units for the same amount; less water makes each unit stronger.
How many units are in 1 mL on an insulin syringe?
A U-100 insulin syringe holds 100 units per mL, so 1 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.
What is the difference between mg and mcg?
1 milligram (mg) equals 1,000 micrograms (mcg). A 5 mg vial contains 5,000 mcg. Mixing the two up is an easy mistake to make, and it is a 1,000-fold error. The calculator converts both to mcg first.
Does the calculator work for semaglutide, tirzepatide or other compounds?
Yes. The math is the same for any powder you reconstitute: it depends only on the amount in the vial, the water added and the amount per draw.
Do this math once, not every time
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