Insulin syringe unit conversion (revision 61)

An insulin syringe unit is a graduation on a syringe barrel calibrated for insulin of a stated concentration. On the U-100 scale, which is the international standard for human insulin, 100 units correspond to exactly 1 millilitre, so one unit is 0.01 mL.^[1] The unit is therefore a volume, and the mass of any substance it contains depends entirely on the concentration of the solution in the barrel.

This is the single most consequential fact on this page, and the source of the errors it exists to prevent. A "10 unit dose" is a statement about volume; it means 0.1 mL, and it says nothing about milligrams until a concentration is supplied. Where a solution has been reconstituted by the user rather than filled by a manufacturer, no syringe marking can carry information about its strength.

Two other scales remain in circulation. U-50 syringes use a 0.5 mL barrel with the same 0.01 mL graduation, giving finer visual resolution for small volumes. U-40 syringes, still used for veterinary insulin in some jurisdictions, place 40 units in 1 mL, so one unit is 0.025 mL — two and a half times the U-100 unit.^[1][2]

Enter any one of units, volume or mass together with the solution concentration, and the tool reports the other two on the selected scale.

The U-100 and U-50 columns are identical because both use a 0.01 mL graduation; they differ in barrel capacity and therefore in the physical spacing of the marks, which is the point of the U-50 syringe. The U-40 column is the hazardous one: an identical reading corresponds to two and a half times the volume.

A reader who transfers a "20 unit" figure from a U-100 context to a U-40 syringe delivers 0.5 mL instead of 0.2 mL. Reports of this error in the veterinary and human literature are consistent enough that colour-coded caps — orange for U-100, red for U-40 — are conventional.^[2]

Because a unit is a fixed volume, the mass it contains scales linearly with concentration. The table below is the whole of the arithmetic, on the U-100 scale.

Two observations follow. A tenfold error in assumed concentration produces a tenfold error in delivered mass with no change in the syringe reading and nothing visibly wrong. And at high concentrations a single graduation carries a large mass, so the resolution of the syringe becomes the limiting factor on dose precision: at 20 mg/mL, one unit is 200 µg, and half a graduation cannot be read reliably.^[3]

Scale confusion

Transferring a unit figure between U-100 and U-40 scales, as above.

Unit-as-mass

Treating "units" as a quantity of drug. This is correct only for insulin, for which the international unit is defined by biological activity and for which the syringe scale was designed.^[4]

Graduation reading

Estimating between graduations. ISO 7886 permits a volume tolerance of a few percent of nominal capacity, which at the bottom of a 1 mL barrel is a substantial fraction of the reading.^[3]

Air and dead space

A bubble displaces solution; dead space retains it. Neither changes the barrel reading.

Rounding

A computed 12.4 units cannot be drawn on a barrel graduated in whole units. Rounding to 12 is a 3% dose reduction, which matters more at low doses than high ones.

• Insulin syringe
• Reconstitution calculator
• Syringe dead space
• Subcutaneous injection
• Reconstitution of lyophilised peptides