Why Your Syringe Measurement Is Off
The most likely cause is a U-40 syringe, whose lines are worth two and a half times more liquid than a U-100's; next is a 1 mL barrel marked every 2 units rather than every unit. Dead space, a bubble, the wrong water volume and reading at an angle are the other four.
Last edited · written by PepPrep · not medical advice
You drew to the line, the amount came out different, or two syringes of the "same" size read differently. Nearly always it is one of these six, in this order of likelihood. This page is about the equipment and the arithmetic; it is not advice about what amount is right.
1. The syringe is a different scale
U-100 has 100 units to the millilitre. U-40, sold for animals and easy to buy by mistake, has 40, so every line is worth two and a half times more liquid. Check the barrel; the U-100 term page shows what to look for. The listing translator flags U-40 in a title.
2. The lines are worth different amounts
A 0.3 or 0.5 mL syringe is marked every unit; most 1 mL syringes are marked every two. "Draw to the third line" is 3 units on one and 6 on the other. The 0.5 vs 1 mL comparison shows the difference side by side.
3. Dead space
A syringe with a detachable needle leaves 0.05–0.1 mL in the hub after the plunger bottoms out; a fixed-needle insulin syringe leaves almost nothing. Drawing twice with a detachable-needle syringe pays that cost twice. See dead space.
4. A bubble
A bubble takes up space on the scale, so the draw is short by its size. The air-bubble guide has the five-step fix.
5. The water volume was wrong at mixing
Units measure liquid, not what is dissolved in it. If a vial was mixed with 2 mL instead of 1, every unit afterwards contains half as much. Redo the calculator with the volume actually added, and write it on the label next time.
6. Reading the plunger from an angle
Hold the syringe at eye level and read the flat edge of the plunger's black tip, not the curved top. A clip-on magnifier or a lamp makes a 0.3 mL barrel readable.
Supplies mentioned: Syringes & Needles · Syringe finder
Equipment only: nothing on this page is advice about what to inject, how much, or whether a product is fit to use. That comes from the product's labelling and whoever supplied it.