What a "unit" actually is
A U-100 insulin syringe is marked so that 100 units equal 1 mL. That makes each unit a fixed volume: 1 unit = 0.01 mL, whatever is in the syringe. The "100" in U-100 refers to that scale, not to any peptide amount.
1 unit = 0.01 mL 100 units = 1 mLSyringe sizes hold the same unit differently
U-100 syringes come in a few common barrel sizes, and the size only changes how much liquid the syringe holds and how finely the markings are spaced, not the value of a unit. A 0.3 mL syringe holds 30 units and is often marked in half-unit or single-unit steps, a 0.5 mL syringe holds 50 units, and a 1 mL syringe holds 100 units, typically marked every 2 units. Whichever barrel you use, 1 unit is still 0.01 mL; a smaller barrel just makes each unit occupy more visible space on the scale, which can make small draws easier to read.
Why units aren't a fixed amount of peptide
Because a unit is a volume, the amount of peptide inside it depends entirely on how concentrated the solution is. The same 10 units could be 100 mcg or 1,000 mcg, depending on how much BAC water was used to reconstitute the vial. Two people using the same syringe and drawing to the same unit mark could be drawing very different amounts if their vials were reconstituted with different volumes of water. This is why "how many units" only has an answer once you know the concentration, which is set when the vial is reconstituted, as covered in the reconstitution guide and the BAC water guide.
Converting units to mcg, and back
Once you know the concentration in mcg/mL, both directions of the conversion are direct:
mcg = units × concentration (mcg/mL) ÷ 100 units = mcg × 100 ÷ concentration (mcg/mL)Example. At a concentration of 2,500 mcg/mL, 10 units is 10 × 2,500 ÷ 100 = 250 mcg. Going the other way, 250 mcg at that same concentration is 250 × 100 ÷ 2,500 = 10 units, which checks out. The reconstitution calculator runs this conversion automatically once you enter your vial and water.
mcg per unit at common concentrations
The table below shows how many mcg are in 1, 5, 10, 20 and 50 units at five example concentrations. Figures are rounded to one decimal place. Use it to sanity-check a draw once you know your vial's concentration.
| Concentration | 1 unit | 5 units | 10 units | 20 units | 50 units |
|---|---|---|---|---|---|
| 1,000 mcg/mL | 10 mcg | 50 mcg | 100 mcg | 200 mcg | 500 mcg |
| 2,000 mcg/mL | 20 mcg | 100 mcg | 200 mcg | 400 mcg | 1,000 mcg |
| 2,500 mcg/mL | 25 mcg | 125 mcg | 250 mcg | 500 mcg | 1,250 mcg |
| 3,333 mcg/mL | 33.3 mcg | 166.7 mcg | 333.3 mcg | 666.6 mcg | 1,666.5 mcg |
| 5,000 mcg/mL | 50 mcg | 250 mcg | 500 mcg | 1,000 mcg | 2,500 mcg |
mg vs. mcg
1 milligram (mg) equals 1,000 micrograms (mcg). Vial labels are usually printed in mg (for example, "5 mg"), while individual draws are often discussed in mcg because the numbers are smaller and easier to compare to a syringe's unit markings. Convert a vial's mg amount to mcg before using either formula above, by multiplying by 1,000.
U-40 syringes: a different scale
All of the formulas and tables on this page assume a U-100 syringe, where 100 units equal 1 mL. A U-40 syringe (common for some veterinary insulin) is marked so that only 40 units equal 1 mL, which makes 1 unit on a U-40 barrel equal to 0.025 mL, two and a half times larger than 1 unit on a U-100 barrel. Reading a U-100 unit number off a U-40 barrel, or the reverse, gives you the wrong volume, because the same word "unit" means a different amount of liquid on each scale. Confirm which syringe you're holding before drawing anything, and don't mix the two conversions.
Frequently asked questions
How many mL is 1 unit on an insulin syringe?
On a U-100 syringe, 100 units equal 1 mL, so 1 unit equals 0.01 mL.
Does 1 unit always equal the same amount of peptide?
No. A unit is a fixed volume (0.01 mL on a U-100 syringe), but the amount of peptide in that volume depends entirely on the concentration of the solution, which depends on how much BAC water was added.
How do I convert units to mcg?
mcg = units × concentration (mcg/mL) ÷ 100. For example, 10 units at a concentration of 2,500 mcg/mL is 10 × 2,500 ÷ 100 = 250 mcg.
How do I convert mcg to units?
units = mcg × 100 ÷ concentration (mcg/mL). For example, 250 mcg at 2,500 mcg/mL is 250 × 100 ÷ 2,500 = 10 units.
What's the difference between mg and mcg?
1 milligram (mg) equals 1,000 micrograms (mcg). Vials are often labeled in mg while draws are often discussed in mcg, so converting everything to mcg first avoids a 1,000-fold error.
Can I use a U-40 syringe with these numbers?
Not directly. These tables and the formulas assume a U-100 syringe, where 100 units equal 1 mL. A U-40 syringe holds 40 units per mL, so the same unit number on a U-40 barrel is a different, larger volume.
Let the app do the conversion
PeptidesAI converts your vial and BAC water into a concentration once, then shows the units for any amount automatically — no manual unit math required. Free to download on iPhone.
Download on the App Store