Converting Millilitres to Syringe Units
Most dosing references state volume in millilitres, while insulin syringes are graduated in units. The conversion is fixed and depends only on the syringe scale:
So 0.4 mL is 40 units, 0.15 mL is 15 units, and 0.07 mL is 7 units. The relationship is linear with no rounding built in, which means very small volumes fall between the printed graduations and cannot be read precisely on a 1 mL barrel.
When the Number Falls Between Marks
A 1 mL U-100 barrel is typically printed with a mark every 2 units. A calculated value of 7 units therefore sits between graduations. Two options exist: move to a 0.3 mL barrel, which is marked in single units, or reconstitute at a lower concentration so the same mass occupies a larger, more readable volume.
Reverse Conversion and Cross-Checking
Running the conversion in both directions is a cheap way to catch a decimal error. If you calculate 0.25 mL → 25 units, converting back should return 0.25 mL. A mismatch by a factor of ten almost always means a decimal place moved during the original calculation.