Convert between insulin syringe units (IU) and milliliters (mL). Works for all U-100 syringes.
Standard insulin syringes use the U-100 scale, meaning 100 units = 1 mL. This is a universal standard regardless of syringe size (0.3mL, 0.5mL, or 1mL syringes all use U-100 markings).
The conversion is simple: 1 unit = 0.01 mL. So 10 units = 0.10 mL, 25 units = 0.25 mL, and so on.
0.3 mL (30 units): Best for very small doses. Each tick mark is typically 0.5 units. Great for peptides like BPC-157 and ipamorelin.
0.5 mL (50 units): Good balance between precision and capacity. Each tick mark is typically 1 unit.
1.0 mL (100 units): Most common. Each tick mark is typically 2 units. Best for larger doses like tirzepatide or TB-500.
This is the single most important thing to understand about the U-100 scale, and it is where most confusion starts. A syringe unit is a measure of liquid volume. It says nothing about how much peptide is in that liquid.
Ten units is always 0.1 mL. But whether that 0.1 mL contains 250 mcg or 500 mcg of peptide depends entirely on the concentration, and concentration is set by how much bacteriostatic water you added to the vial. Two people can draw the exact same 10 units and receive completely different amounts of peptide.
A worked pair makes this concrete. A 5 mg vial reconstituted with 1 mL of water yields 5 mg/mL, so 10 units (0.1 mL) contains 500 mcg. The same 5 mg vial reconstituted with 2 mL yields 2.5 mg/mL, so those same 10 units now contain 250 mcg. Identical syringe reading, half the peptide. This is why a unit count copied from someone else's protocol is meaningless without their reconstitution volume.
Syringe choice is a precision decision, not just a capacity one. The finer the graduations, the smaller the error introduced by reading between marks.
On a 1 mL syringe where each mark is 2 units, being one mark off is a 0.02 mL error. If your concentration is 5 mg/mL, that single mark represents 100 mcg. On a 0.3 mL syringe where each mark is 0.5 units, the same misread costs 0.005 mL, or 25 mcg at the same concentration. For small doses, the smaller barrel is meaningfully more forgiving.
The practical rule is to pick the smallest syringe that still holds your full dose. If your draw lands around 8 to 12 units, a 0.3 mL barrel puts that reading in the middle of a well-graduated scale. If your draw is 60 units, you need the 1 mL barrel and should accept the coarser marks.
A second consideration: if a dose consistently lands on an awkward fraction of a mark, the fix is usually not a different syringe but a different reconstitution volume. Adjusting how much water you add can move a dose onto a clean unit line. The reconstitution calculator will show which water volumes produce round numbers.
Assuming all syringes are U-100. Most are, but U-40 syringes exist and are used in some veterinary contexts. On a U-40 scale, 40 units equals 1 mL, so a unit is 0.025 mL rather than 0.01 mL. Drawing a U-100 unit count on a U-40 syringe delivers 2.5 times the intended volume. Check the barrel before assuming.
Reading to the wrong part of the plunger. The dose is read at the leading edge of the plunger's rubber stopper, the edge nearest the needle, not the middle of the stopper and not the trailing edge.
Treating units as an interchangeable dose. Covered above, but worth restating because it is the most consequential error. Always carry the concentration alongside the unit count.
Reusing a syringe to both mix and inject. A needle dulls noticeably after puncturing a rubber vial stopper. Using one syringe to add bacteriostatic water and a fresh one to draw each dose is the standard practice.
Related tools: reconstitution calculator, mcg to mg converter, and the injection site guide.
Need to calculate a full peptide dose? Use our reconstitution calculator →