Reconstitution Math
U40 vs U100 Insulin Syringes: Why the Type Changes Your Draw
Why U40 and U100 syringes produce different volumes for the same unit number, how to identify which syringe you have, the conversion table between types, and why this is the most common peptide dosing error.
Informational only. Not medical advice. Consult a licensed healthcare provider before starting, changing, or stopping any protocol.
The most common calculation error in peptide dosing
When someone draws the wrong volume for a peptide dose, the cause is almost always a syringe type mismatch — using a U40 syringe but calculating for U100, or vice versa. The two syringes have different scales, and the number marked on the barrel means different volumes depending on which type you're holding.
Getting this wrong doesn't produce an obviously wrong-looking draw. Both syringes look similar. The error shows up as under- or over-dosing — often silently.
What U100 and U40 mean
"U" stands for units per mL. The number tells you how many syringe graduation marks equal 1 mL:
| Syringe type | Units per mL | 1 mL = how many units? | Common in |
|---|---|---|---|
| U100 | 100 units/mL | 100 units = 1 mL | Standard US insulin syringes; most peptide protocols |
| U40 | 40 units/mL | 40 units = 1 mL | Some European/international markets; veterinary insulin |
| 1 mL (tuberculin) | No "units" scale | Graduated in mL/cc directly | Larger-volume injections; some pharmacy protocols |
Why the mismatch causes errors
Reconstitution calculators (including My Pep Calc) output a result like "draw to 10 units." That number is only correct for the syringe type specified. If the calculator assumes U100 but you're using a U40 syringe:
- Calculator says: 10 units (U100) = 0.10 mL
- You draw to "10" on a U40 syringe = 0.25 mL
- Result: you inject 2.5× the intended dose
The reverse error (U40 dose on a U100 syringe) produces under-dosing. Neither error is obvious from looking at the syringe — the liquid level looks plausible in both cases.
How to identify which syringe you have
Check the syringe barrel or packaging:
- "U-100" or "100 units" printed on the barrel → U100 syringe. In the US, the overwhelming majority of insulin syringes sold are U100.
- "U-40" or "40 units" → U40 syringe. Less common in the US but still sold. Frequently the syringe used for veterinary insulin (Vetsulin/Caninsulin).
- No unit marking, graduated in 0.1 mL increments → tuberculin or 1 mL syringe. Read directly in mL.
If you can't read the barrel, measure: draw 1 mL of water (stop exactly at the 1 mL mark if it has one, or fill to the top of a 1 mL syringe). Count how many graduation marks that spans. 100 marks = U100. 40 marks = U40.
Converting between syringe types
If your calculation was done for one syringe type but you have a different one, convert the volume:
| If your dose is | On U100 syringe | On U40 syringe | Volume (mL) — same either way |
|---|---|---|---|
| 0.10 mL | 10 units | 4 units | 0.10 mL |
| 0.20 mL | 20 units | 8 units | 0.20 mL |
| 0.25 mL | 25 units | 10 units | 0.25 mL |
| 0.50 mL | 50 units | 20 units | 0.50 mL |
The mL volume is always the same regardless of syringe type — only the unit number on the barrel differs. If you're unsure, convert everything to mL and draw to the mL mark.
Why the My Pep Calc calculator asks for syringe type
The reconstitution calculator asks for your syringe type specifically because the unit output depends on it. Select U100 if you're using a standard US insulin syringe (the most common setup). Select U40 if your syringe package says U-40. Select 1 mL if you're using a tuberculin syringe — the output will be in mL rather than units.
If you have multiple syringe types for different compounds, run each calculation with the correct syringe type for that compound's protocol.
The simplest rule
In the US, if you bought insulin syringes at a pharmacy or from an online medical supply retailer without specifying U40, you almost certainly have U100 syringes. U40 syringes are uncommon in the US human market. When in doubt, check the label before drawing anything.
Frequently asked questions
- What is the difference between U100 and U40 insulin syringes?
- U100 syringes have 100 graduation units per mL. U40 syringes have 40 units per mL. The same number on the barrel means different volumes: "10 units" on a U100 syringe = 0.10 mL; "10 units" on a U40 syringe = 0.25 mL. Using the wrong syringe type for your calculation produces significant dosing errors.
- How do I know if I have a U100 or U40 syringe?
- Check the barrel or packaging for "U-100" or "U-40" markings. In the US, standard pharmacy insulin syringes are almost always U100. U40 syringes are common in some international markets and for veterinary insulin. If unlabeled, count the graduation marks in 1 mL — 100 marks = U100, 40 marks = U40.
- What is a tuberculin syringe?
- A tuberculin (1 mL) syringe is graduated in mL directly rather than in insulin units. It's used when volume precision in mL is preferred over unit counting — some pharmacy protocols specify doses in mL. The My Pep Calc calculator outputs mL (not units) when you select the 1 mL syringe type.
- What happens if I use a U40 syringe with a U100 calculation?
- You will overdose. A calculation for "10 units on a U100 syringe" means 0.10 mL. Drawing to "10" on a U40 syringe draws 0.25 mL — 2.5× the intended dose. Always verify your syringe type before drawing.
Sources
- American Diabetes Association. Insulin Administration. Diabetes Care. 2004;27(Suppl 1):S106–S107.
- FDA. Types of Insulin and How They Work. Consumer guidance on insulin syringes and concentration.
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