Insulin Syringe Units to mL Conversion: 30U, 50U, and 100U Barrels

A unit on an insulin syringe is a volume mark. The scale was designed for U-100 insulin, which is made at 100 insulin units per millilitre, so the syringe puts 100 marks in every millilitre. One unit is therefore 0.01 mL, whatever liquid is in the barrel.

A 20 unit draw is 0.2 mL. A 0.35 mL dose is 35 units. The conversion never depends on the peptide or on how you mixed it.

A unit does not tell you how much peptide it holds. That depends on the concentration in your vial. A 10 mg vial mixed with 2 mL of bacteriostatic water holds 50 mcg in each unit, while the same vial mixed with 1 mL holds 100 mcg per unit. The reconstitution math blog covers how to get from a dose in milligrams to a unit count.

Insulin syringes come in three common sizes. Each one uses the same U-100 scale, so 10 units is 0.1 mL on all of them. They differ in how much they hold and how finely they are marked.

The smaller barrels are also narrower, so each unit takes up more length along the barrel. On a 30 unit syringe the gap between two marks is wide enough to see clearly. On a 1 mL syringe, ten units sit in a short stretch of barrel and the marks are close together.

A careful reader can usually place the plunger to within about half a mark. On a 1 mL syringe marked every 2 units, half a mark is 1 unit. On a 30 unit syringe marked every unit, it is half a unit. The error is fixed in size, so it matters more as the dose gets smaller.

• For a 5 unit draw, a 1 unit reading error is 20% of the dose.
• For the same draw on a 30 unit syringe, a half unit error is 10%.
• For a 50 unit draw on a 1 mL syringe, a 1 unit error is 2%, which rarely matters.

Odd numbers add a second problem. A 1 mL syringe has lines at 14 and 16 but none at 15, so a 15 unit draw means judging the halfway point by eye. On a 30 or 50 unit syringe, 15 has its own line.

Use the smallest barrel that holds the whole draw with some room to spare. Here is how four doses from a 10 mg vial mixed with 2 mL of BAC water read on each barrel size.

The 250 and 750 mcg doses read cleanly on a 30 unit syringe and land between lines on a 1 mL syringe. The 1.2 mg dose fits a 30 unit barrel, but only just, so a 50 unit barrel leaves more room for an air bubble or a slight overdraw. The 2.5 mg dose does not fit a 30 unit syringe at all. A 50 unit syringe holds it with no room left, which makes the 1 mL syringe the easier choice.

If your draw is under about 5 units, no syringe will measure it well, so change the mix: adding more water when you reconstitute spreads the same dose over more units. The BAC water volume blog shows how to pick a volume that lands your dose on a readable mark.

Read the syringe the same way every time:

1. Read at the edge of the rubber stopper closest to the needle. That flat edge is the top of the liquid column. The rest of the stopper and the plunger rod do not count.
2. Hold the syringe at eye level with the needle pointing up. Looking at the scale from above or below shifts where the stopper appears to sit.
3. Clear air bubbles before the final reading. Tap the barrel so bubbles rise, push them out, then top up to the mark. A bubble takes up space that reads as dose.
4. Count the marks between numbers before you start. If the numbers run 10, 20, 30 with four lines between them, each line is 2 units, not 1.

Dead space is the liquid left in the needle and hub after the plunger is pushed all the way in. Insulin syringes have the needle fixed permanently to the barrel, a design that leaves very little behind, so the marks are a close match for what you actually deliver.

Syringes with a detachable needle keep noticeably more liquid in the hub. The marks still measure what leaves the barrel, so the dose itself is close, but the leftover is drawn from the vial and thrown away with every injection. Over a vial of small doses that adds up to fewer doses than the math predicts. For peptide dosing, a fixed-needle insulin syringe is the better fit.

Everything above assumes a U-100 syringe. U-40 syringes, sold mainly for veterinary insulin, put 40 units in 1 mL, so each unit is 0.025 mL. The same number of units means two and a half times as much liquid.

A 20 unit draw is 0.2 mL on a U-100 syringe and 0.5 mL on a U-40 syringe. In the US, U-100 syringes usually have orange caps and U-40 syringes red ones, but packaging varies, so read the scale printed on the barrel or the box rather than going by color.