Which syringe, and what the ticks mean.

The three U-100 sizes

• 0.3 mL — 30 units. Marked every half unit. The precision syringe: best for anything under about 15 units.
• 0.5 mL — 50 units. Marked every unit. The everyday middle.
• 1 mL — 100 units. Marked every 2 units. Needed when a dose runs over 50 units, but 25 units on it is a between-the-marks read.

Match the syringe to the number

The calculator's tip line does this automatically. If your dose is 25 units it will say a 1 mL syringe reads in 2-unit steps and a 0.3 or 0.5 mL reads 25 exactly. If your dose is 60 units it will say a 0.5 mL can't hold it. If your dose is 3 units it will say to add more water rather than trust a 3-unit read on anything.

Reading the barrel

Read at the top edge of the plunger's black stopper, at eye level, with the syringe horizontal. Air bubbles displace liquid, so a bubble at the 20-unit line means less than 20 units of solution. Draw a little past the mark, tap, push back to the line.

Needle length and gauge

Insulin syringes come with fixed needles in the 29–31 gauge range and 6–13 mm lengths. Gauge and length don't change the unit math at all. Which to use is a technique and comfort question that isn't this page's business.

U-40 and U-500 warning

Everything here assumes U-100. U-40 syringes (veterinary) and U-500 syringes exist and their unit marks mean different volumes. If a syringe doesn't say U-100 on the barrel, the calculator's answer does not apply to it.