The formula everyone uses is:

C1 x V1 = C2 x V2
C1 = starting concentration (mg/mL)
V1 = starting volume (mL)
C2 = final concentration (mg/mL)
V2 = final volume (mL)

For peptides, it's usually simpler:

You start with a powder amount, not a liquid concentration.

Concentration after mixing = Total mg in vial / Total mL you add

Example: 10 mg powder + 2 mL bacteriostatic water
10 mg / 2 mL = 5 mg/mL

Want to know what volume gives you a specific dose?
Dose volume = Desired dose / Concentration

If you want 0.5 mg from that 5 mg/mL vial:
0.5 / 5 = 0.1 mL. On a U-100 syringe, that's 10 units.

If you want to dilute further — say you have 5 mg/mL and you want 2.5 mg/mL — you double the total volume. Using C1V1 = C2V2: 5 x 1 = 2.5 x V2, so V2 = 2 mL. You'd need to add 1 mL more diluent to your 1 mL.

More diluent = lower mg/mL = larger, more forgiving dose volume. Always label final mg/mL and date, and confirm your math and storage with your pharmacist or prescriber.