The Difference Between U-40 & U-100 Insulin Syringes

Both insulin and the syringes used to administer insulin are expressed in units of measure as a U. The syringes are calibrated to match the same unit on the insulin bottle for ease of use. When using a syringe with a different unit calibration, you must calculate how much insulin your pet needs.

Units of Insulin

The "U" on both the insulin bottle and the syringe designates the units of active insulin. A bottle of U-100 insulin has 100 units of active insulin in each mL of the liquid. A bottle of U-40 insulin contains 40 units of active insulin in each mL.

This information translates to the same volume of U-100 insulin having 2.5 more units in it than U-40 insulin.

Insulin Syringes

Each insulin syringe has the size stamped on it. Both U-100 and U-40 have the size on them. When you use the insulin and the syringe both in U-100 or U-40, you simply draw up the insulin into the syringe to the line of the appropriate units on the insulin bottle.

Some pet parents use a U-100 syringe with U-40 insulin, because the U-100 syringes are readily available at pharmacies and they have a very thin needle. The larger the number on the gauge of a needle, the thinner the needle. In this case, a conversion is necessary to supply the patient with the correct amount of insulin units.

Insulin Conversion

Calculate the amount of U-40 insulin administered in a U-100 syringe by multiplying the amount of units on the insulin bottle times 2.5. Draw that amount into the U-100 insulin syringe. For example, if your pet's prescription is for 6 units of U-40 insulin, 6 x 2.5= 15. Draw up the insulin in the U-100 syringe to the 15 unit marking.


    • Keep insulin refrigerated. 
    • Roll the bottle gently in your hands to mix the insulin hormone, but do not shake the bottle.
    • Use a new syringe and needle for each injection. 
    • Do not change your pet's dosage unless your veterinarian advises to do so.


Mary Lougee has been writing since 2004 and specializes in pets with publications in "Modern Dog" and "Pet Planet." Lougee gained extensive pet knowledge and expertise in care and rehabilitation, built a farm, and cares for rescue animals from small to large. She holds a bachelor's degree in management.