The labels say: matching syringes onlyU-40 veterinary insulin labels warn that any syringe other than a U-40 syringe will result in incorrect dosing.[1][2] These charts explain the arithmetic and help you check a mistake. Do not use them to switch syringes unless your vet or prescriber has told you to and has given you the numbers.
The conversion
mL = units ÷ insulin strength (40 or 100)
marks on the syringe = mL × syringe scale (40 or 100)
U-40 insulin on a U-100 syringe: marks = units × 2.5
U-40 insulin has 40 units per mL and U-100 insulin has 100.[1][4] The syringe scale counts the same way, so each mark is 1/40 mL on a U-40 syringe and 1/100 mL on a U-100 syringe.
U-40 units as volume and U-100 marks
| U-40 units | mL | U-100 syringe marks |
|---|---|---|
| 1 units | 0.025 | 2.5 |
| 2 units | 0.05 | 5 |
| 3 units | 0.075 | 7.5 |
| 4 units | 0.1 | 10 |
| 5 units | 0.125 | 12.5 |
| 6 units | 0.15 | 15 |
| 8 units | 0.2 | 20 |
| 10 units | 0.25 | 25 |
| 12 units | 0.3 | 30 |
| 15 units | 0.375 | 37.5 |
| 20 units | 0.5 | 50 |
| 25 units | 0.625 | 62.5 |
| 30 units | 0.75 | 75 |
| 40 units | 1 | 100 |
Many results end in .5. A U-100 syringe with 1-unit or 2-unit lines cannot show them exactly, which is one more reason the labels call for U-40 syringes.
Mix-up check: what was actually given
If someone drew up to the dose number on the wrong syringe, these tables show the real dose.
| Line drawn to | mL | U-40 units given | Share of intended |
|---|---|---|---|
| 5 | 0.05 | 2 | 40% |
| 10 | 0.1 | 4 | 40% |
| 15 | 0.15 | 6 | 40% |
| 20 | 0.2 | 8 | 40% |
| 25 | 0.25 | 10 | 40% |
| 30 | 0.3 | 12 | 40% |
| 40 | 0.4 | 16 | 40% |
| 50 | 0.5 | 20 | 40% |
| Line drawn to | mL | U-100 units given | Times intended |
|---|---|---|---|
| 2 | 0.05 | 5 | 2.5× |
| 4 | 0.1 | 10 | 2.5× |
| 5 | 0.125 | 12.5 | 2.5× |
| 6 | 0.15 | 15 | 2.5× |
| 8 | 0.2 | 20 | 2.5× |
| 10 | 0.25 | 25 | 2.5× |
| 12 | 0.3 | 30 | 2.5× |
| 16 | 0.4 | 40 | 2.5× |
| 20 | 0.5 | 50 | 2.5× |
Merck Animal Health describes both errors: 2½ times less insulin with a U-100 syringe and Vetsulin, and 2½ times too much with a U-40 syringe and a U-100 insulin, which could be fatal.[3] If a mix-up has happened, contact your vet, doctor or poison control service straight away.
Related guides
The background, labels and colour codes are on U-40 vs U-100 syringes. If you are unsure which line the plunger is on, read how to read an insulin syringe. Volume equivalents for every scale are in 1 mL = how many units, and other errors of this size are listed in syringe measuring mistakes.
Frequently asked questions
How do I convert U-40 to U-100?
Multiply U-40 units by 2.5 to get the same volume in U-100 syringe marks. 10 units of U-40 insulin is 0.25 mL, which is the 25 mark on a U-100 syringe.
How many U-100 units is 1 unit of U-40 insulin?
One unit of U-40 insulin is 0.025 mL, which spans 2.5 marks on a U-100 syringe. It is still one unit of U-40 insulin.
What if I gave U-100 insulin with a U-40 syringe?
Drawing to the dose number gives 2.5 times the dose. Contact your doctor, vet or poison control service immediately.