What is the difference between U-40 and U-100 syringes for dog and cat insulin?
1 unit on a U-40 syringe is 0.025 mL; 1 unit on a U-100 syringe is 0.01 mL. So the same printed number holds 2.5× more liquid on a U-40 barrel. Some insulins used for dogs and cats are U-40 (40 units per mL), and many human insulins are U-100, which is why the syringe has to match the insulin's U-number.
The math
U-40: 1 unit = 1 mL ÷ 40 = 0.025 mL U-100: 1 unit = 1 mL ÷ 100 = 0.01 mL Same volume: U-100 units = U-40 units × 2.5 Same volume: U-40 units = U-100 units ÷ 2.5 (× 0.4) Example: 4 U-40 units = 0.1 mL = 10 U-100 units
| Mark on barrel | mL on U-40 | mL on U-100 | U-40 holds |
|---|---|---|---|
| 1 units | 0.025 | 0.01 | 2.5× more |
| 2 units | 0.05 | 0.02 | 2.5× more |
| 4 units | 0.1 | 0.04 | 2.5× more |
| 5 units | 0.125 | 0.05 | 2.5× more |
| 10 units | 0.25 | 0.1 | 2.5× more |
| 20 units | 0.5 | 0.2 | 2.5× more |
| 40 units | 1 | 0.4 | 2.5× more |
Why the same mark means a different volume
A syringe's U-number says how many unit marks fit in 1 mL. A U-40 barrel spreads 40 marks over 1 mL, a U-100 barrel spreads 100. The "10" line on a U-40 syringe sits at 0.25 mL; the "10" line on a U-100 syringe sits at 0.10 mL. The insulin vial is labeled the same way (U-40 or U-100 insulin), and matching the two keeps the printed number equal to the insulin units.
The 2.5× rule, both directions
Reading a U-40 number off a U-100 syringe gives 2.5 times too little liquid: 10 U-40 units is 0.25 mL, but the U-100 "10" line is only 0.10 mL.
Reading a U-100 number off a U-40 syringe gives 2.5 times too much liquid: 10 U-100 units is 0.10 mL, but the U-40 "10" line is 0.25 mL.
Either way the error is large, which is why syringe and insulin U-numbers should match.
How to tell which syringe you have
The U-number is printed on the box and usually on the barrel. U-40 syringes often have a red cap and U-100 syringes an orange cap, but color coding is a convention, not a guarantee, so read the label. A 1 mL U-40 syringe tops out at 40 units; a 1 mL U-100 syringe tops out at 100.
Confirm with your veterinarian: These pages convert syringe marks and volumes only; the number of units your pet gets, and the syringe type to use, come from your vet.
Math only, not medical advice. This page converts numbers; it does not recommend any compound or amount.
Insulin units → mL and syringe mark (premixed insulin, no mixing step)
Enter the number of units to see the volume and where to draw on your syringe.
Math only, not medical advice. Confirm with your veterinarian.