# U-100 vs U-40 Syringes — How to Read Them

Source: https://www.trackpep.com/blog/u100-vs-u40-syringes

> Educational information only. Not medical advice, and not a dosing recommendation. TrackPep records what you enter and converts a dose you were prescribed into a syringe reading; it never decides a dose for you. Talk to a licensed clinician.

The most common dosing error in peptide protocols is reading a U-100 scale as if it were U-40 (or vice versa). Here's how to never make it.

<Disclaimer />

The most common dosing mistake in self-administered peptide protocols isn't math — it's **reading the wrong syringe scale**. This post explains the difference and how to never confuse the two.

## The short version

- **U-100** = 100 units per 1 mL
- **U-40** = 40 units per 1 mL
- The "unit" is a scale convention, not a fixed amount of drug.

A 0.5 mL injection reads as **50 units** on a U-100, **20 units** on a U-40. Same volume, same drug, different number on the barrel.

## How to identify which one you have

- **U-100 1 mL syringe** — barrel says "U-100", max is 100.
- **U-100 0.5 mL syringe** — barrel says "U-100", max is 50.
- **U-100 0.3 mL syringe** — max is 30.
- **U-40 1 mL syringe** — barrel says "U-40", max is 40.

**Default to caution: if you can't confirm, don't use it for peptides.**

## The math

| Inject | U-100 units | U-40 units |
|---|---|---|
| 0.10 mL | 10 | 4 |
| 0.20 mL | 20 | 8 |
| 0.25 mL | 25 | 10 |
| 0.50 mL | 50 | 20 |
| 1.00 mL | 100 | 40 |

## Worked example

5 mg vial + 2 mL BAC water → 2.5 mg/mL. Target 0.5 mg → volume 0.2 mL.

- U-100 syringe: draw to **20 units** ✓
- U-40 syringe: draw to **8 units** ✓
- **Wrong:** "20 units" on U-40 = 0.5 mL = **1.25 mg** — 2.5× the intended dose.

<Callout>
**Say the syringe type out loud before drawing.** "U-100, twenty units." Catches errors.
</Callout>

## Which should you use?

For most subcutaneous peptide protocols (GLP-1s, BPC-157, ipamorelin) where typical doses are below 1 mL, **U-100** has finer gradations (1-unit lines) for better readability.

<FAQ>
  <div>
    <dt>**Can I use a "tuberculin" syringe with mL marks?**</dt>
    <dd>Yes. Inject the same volume; read mL directly.</dd>
  </div>
  <div>
    <dt>**What if I only have U-40 but my protocol assumes U-100?**</dt>
    <dd>Compute volume in mL first, then multiply by 40. Don't try to convert unit-to-unit.</dd>
  </div>
</FAQ>

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*Informational only. Not medical advice.*