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Measuring accurately — units, bubbles and dead space

The mechanics of an accurate measurement: reading units, clearing bubbles, and where volume quietly goes missing.

Last reviewed 2026-09-02 · written by PepPrep · not medical advice

Nothing here is about how much of anything to use; that's between you and a clinician. This is purely the mechanics of getting a measurement right once you already know what it should be.

• Read units, not millilitres, on an insulin syringe. They're marked in units, and on any U-100 syringe 100 units = 1 mL. Converting in your head mid-task is how mistakes happen — use the marks that are printed.
• Line up the same part of the plunger every time. Read to the flat top edge of the rubber stopper, at eye level. Looking down at an angle makes the reading drift.
• Get the bubbles out. Draw slightly more than you need, hold the syringe upright, tap the barrel so bubbles rise, push them back into the vial, then bring it down to the mark. A bubble means part of what you're reading is air.
• Know about dead space. A little always stays behind in the needle and tip. It doesn't change what you measured, but it does mean a vial yields slightly less than the arithmetic promises. "Low dead space" designs waste less of it.
• More liquid makes small amounts easier to read. The same contents spread over more volume gives you more lines to work with, so a small measurement is less cramped and less error-prone.
• If a measurement keeps landing between two lines, the fix is different equipment, not a harder squint — a smaller barrel or a half-unit syringe gives you the resolution.
• If the markings are simply hard to see, a clip-on magnifier costs a few dollars and solves it permanently.
• Write it down. A logbook and a labeled vial answer next week's "what did I do?" far better than memory.

Supplies mentioned: Syringes & Needles · Measuring & Tracking