Divide the mg in the vial by the mL of water: that is its strength. Divide your dose by the strength to get mL, then multiply by 100 for the units on a U-100 insulin syringe.
Two formulas and one conversion
Everything you will ever need to calculate a peptide dose fits in three lines.
Concentration (mg/mL) = peptide amount (mg) ÷ water volume (mL)
Dose volume (mL) = desired dose (mg) ÷ concentration (mg/mL)
On a U-100 insulin syringe, 1 unit = 0.01 mL.
That last one is the conversion that trips people up, so anchor it: 10 units is 0.1 mL. 50 units is 0.5 mL. 100 units is the full 1 mL barrel.
Worked example: BPC-157
You have a 5 mg vial and you want a 250 mcg dose.
Add 2 mL of bacteriostatic water. Concentration = 5 ÷ 2 = 2.5 mg/mL.
250 mcg is 0.25 mg. Dose volume = 0.25 ÷ 2.5 = 0.1 mL.
0.1 mL × 100 = 10 units on a U-100 syringe.
Worked example: tesamorelin
You have a 2 mg vial and you want a 2 mg dose.
Add 0.5 mL. Concentration = 2 ÷ 0.5 = 4 mg/mL.
Dose volume = 2 ÷ 4 = 0.5 mL = 50 units.
Worked example: retatrutide at 12 mg
You have a 10 mg vial and you want 12 mg.
Add 1 mL. Concentration = 10 ÷ 1 = 10 mg/mL.
Dose volume = 12 ÷ 10 = 1.2 mL = 120 units.
A U-100 barrel holds 100 units. This does not fit. The Playbook's answer is to draw 60 units, inject, then draw and inject 60 more from the same vial. A 3 mL syringe with an appropriate needle gauge is the alternative.
Notice that the arithmetic surfaced a practical problem before you were standing over the vial with a syringe in your hand. That is the point of doing it in advance.
The reference table
| Vial | Water | Concentration | 250 mcg = | 500 mcg = |
|---|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL | 5 units | 10 units |
| 5 mg | 2 mL | 2.5 mg/mL | 10 units | 20 units |
| 10 mg | 2 mL | 5 mg/mL | 5 units | 10 units |
And the broader version from our reconstitution guide:
| Vial | Water | Concentration | 10 units = | 50 units = |
|---|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL | 500 mcg | 2.5 mg |
| 5 mg | 2 mL | 2.5 mg/mL | 250 mcg | 1.25 mg |
| 10 mg | 1 mL | 10 mg/mL | 1 mg | 5 mg |
| 10 mg | 2 mL | 5 mg/mL | 500 mcg | 2.5 mg |
| 3 mg | 3 mL | 1 mg/mL | 100 mcg | 500 mcg |
Choosing your dilution
The volume you add does not change how much peptide you own. It changes the concentration, and therefore how many syringe ticks a dose occupies.
More water means each tick represents less peptide, so small doses are easier to measure accurately, but injection volumes are larger.
Less water means smaller injections but demands more precision, and sub-5-unit measurements on a U-100 barrel get unreliable fast.
1 to 2 mL is the sweet spot for most peptides. Pick the dilution that puts your intended dose at a round, comfortably readable number of units. If your dose lands at 7.5 units, use more water and make it 15.
The mistake to avoid
Confusing micrograms and milligrams. 1,000 mcg = 1 mg. Most peptide doses are measured in micrograms; most vial sizes are labelled in milligrams. Every calculation above crosses that boundary, and a factor-of-1,000 error is not a rounding problem.
Double-check the units before every draw, not just the number.