Peptide units to mg explained: syringe units, mL, mg, mcg and IU

A "unit" on an insulin syringe is a volume, not an amount of peptide: on a standard U-100 syringe, 1 unit = 0.01 mL and 100 units = 1 mL. To convert units to milligrams you need the solution's concentration: mg = units x 0.01 x concentration (mg/mL). For example, a 10 mg vial reconstituted with 2 mL of bacteriostatic water is 5 mg/mL, so each unit carries 50 mcg and 20 units carry 1 mg. The same 20 units from a vial at 10 mg/mL carry 2 mg. That is why a unit count on its own means nothing, and why the concentration must be written on every reconstituted vial.

The four quantities, defined

Syringe hardware (which barrel sizes exist, how fine the graduations are) is covered in insulin syringes for peptide research. This article is only about the arithmetic.

Peptide units to mg: the formula

Three relationships do everything:

1. Concentration: C (mg/mL) = vial mass (mg) / solvent volume (mL)
2. Units to mg: mg = units x 0.01 x C
3. mg to units: units = (mg / C) x 100

For mcg, multiply mg by 1,000. A useful shortcut on U-100 syringes: mcg per unit = C x 10. At 2.5 mg/mL each unit is 25 mcg; at 5 mg/mL it is 50 mcg; at 10 mg/mL it is 100 mcg.

Worked example: retatrutide units to mg

Retatrutide is supplied as 5 mg, 10 mg and 20 mg vials. Here is how the same unit reading converts to mass in each common preparation (calculated, not estimated):

Twenty units is anywhere from 0.5 mg to 2 mg depending on how the vial was prepared, a four-fold spread from the same syringe mark. Full charts for each vial are in how to reconstitute retatrutide and at /reconstitution/retatrutide-10mg/.

Worked example: BPC-157 in mcg

BPC-157 work is usually expressed in micrograms. For 5 mg and 10 mg vials:

Notice the half-unit results. A 12.5-unit reading is hard to measure accurately on a syringe marked in whole or double units, which is a good reason to choose a solvent volume that produces round numbers. Per-vial charts: BPC-157 5 mg and BPC-157 10 mg.

IU is not a syringe unit: the HCG case

International units (IU) measure biological activity against a reference standard, defined separately for each substance. HCG is labelled in IU rather than mg, so the conversion uses IU per mL instead of mg per mL, but the syringe arithmetic is identical:

The trap is the shared word. "10 units" of an HCG solution at 1,000 IU/mL is 0.1 mL and 100 IU, not 10 IU. Always state which unit you mean.

Common unit-conversion mistakes

• Treating units as mcg. "10 units" is not 10 mcg. At 5 mg/mL it is 500 mcg, fifty times more.
• Slipping a factor of 1,000. 250 mcg is 0.25 mg, not 2.5 mg. Write the unit next to every number.
• Using the wrong concentration. Mixing up a 10 mg and a 20 mg vial, or 2 mL and 3 mL of solvent, silently changes every result.
• U-40 vs U-100 syringes. A U-40 syringe holds 40 units per mL, so 20 units on a U-40 barrel is 0.5 mL, while 20 units on a U-100 barrel is 0.2 mL: a 2.5-fold difference.
• Misreading graduations. Many 1 mL syringes mark every 2 units. Count the lines before drawing.
• Forgetting dead volume. Some liquid stays in the needle hub, so the last draw from a vial is usually short.

Peptide units calculator

For any combination not shown here, the peptide reconstitution calculator takes vial size, solvent volume and target amount and returns concentration and syringe units. The reconstitution hub has ready-made charts for each vial size we stock. For the reconstitution step itself, see the reconstitution reference and bac water vs sterile water; our bacteriostatic water comes in 3 mL vials.

Frequently asked questions