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 U100 insulin has 100 units of active insulin in each mL of the liquid. A bottle of U40 insulin contains 40 units of active insulin in each mL.
This information translates to the same volume of U100 insulin having 2.5 more units in it than U40 insulin.
Insulin Syringes
Each insulin syringe has the size stamped on it. Both U100 and U40 have the size on them. When you use the insulin and the syringe both in U100 or U40, 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 U100 syringe with U40 insulin, because the U100 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 U40 insulin administered in a U100 syringe by multiplying the amount of units on the insulin bottle times 2.5. Draw that amount into the U100 insulin syringe. For example, if your pet's prescription is for 6 units of U40 insulin, 6 x 2.5= 15. Draw up the insulin in the U100 syringe to the 15 unit marking.
Tips

 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.
 Keep insulin refrigerated.