Heads-up: this is general information for US readers. Always follow your prescriber's advice and the FDA-approved Instructions for Use that come with your medication.
Why this matters: if you get Zepbound (tirzepatide) as single-dose vials through LillyDirect, you draw each dose yourself with a syringe. Most people use a U-100 insulin syringe, and those syringes are marked in units, not mL. The good news is that the US conversion is unusually simple: every full vial dose is 0.5 mL, which is 50 units. The tables below show how units, mL, and mg relate for each vial strength, so the numbers on the syringe actually make sense.
US quick facts
- Volume per dose: 0.5 mL for every strength, in both the single-dose pen and the single-dose vial
- Full vial draw on a U-100 syringe: 50 units, always
- Available strengths: 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, 15 mg (2.5 mg is a starter dose, not maintenance)
- Vials: single-dose only. One month's supply is 4 vials, one per week
- Brand note: in the US, tirzepatide for weight loss is Zepbound. Mounjaro is the same molecule branded for type 2 diabetes
Pens, vials, and the KwikPen
Zepbound comes in three formats in the US, and only one of them involves any measuring on your part:
- Single-dose autoinjector pen. The classic retail-pharmacy format. Each pen holds one preset 0.5 mL dose. You press it against your skin and it injects everything. No units, no drawing, nothing to convert.
- Single-dose vial. Sold through LillyDirect's self-pay program in all six strengths. You draw the dose out of the vial with a syringe and inject it yourself. This is the format the rest of this article is about, because it's the only one where syringe units matter.
- Zepbound KwikPen. Launched in the US in February 2026. It's a multi-dose pen holding four weekly doses, and you dial and inject rather than draw anything. One thing worth knowing: each KwikPen dose is 0.6 mL, not 0.5 mL. UK patients have used this format since early 2024 (tirzepatide for weight loss is sold as Mounjaro there), which is why unit charts written for UK readers say 60 units per dose. Those charts do not apply to US vials.
On cost, since vials are why most people end up doing draws: LillyDirect's self-pay prices run from $299 to $449 a month depending on dose (as of July 2026), against a list price of roughly $1,086. On the 7.5 mg dose and above, the discounted price depends on refilling within a 45-day window, so check the current terms on lilly.com before you rely on a number.
The three quantities: units, mL, mg
- mL is the volume of liquid.
- mg is the amount of medicine in that liquid.
- Units on a U-100 insulin syringe are just another way to show volume: 100 units = 1.00 mL.
A simple analogy:
Think of mixing lemonade from a powdered drink mix.
- The water you pour is measured in mL.
- How strong it tastes depends on how much mix went in. That's mg per mL in a vial.
- The tick marks on your measuring cup help you pour precisely. A U-100 syringe is a measuring cup with 100 tiny ticks per mL.
So 1 unit = 0.01 mL.
Every Zepbound vial holds the same 0.5 mL of liquid. What changes between strengths is how much medicine is dissolved in it. A 15 mg vial isn't fuller than a 5 mg vial; it's three times more concentrated.
What is a U-100 syringe?
A U-100 syringe shows volume in units where:
- 100 units = 1.00 mL
- 10 units = 0.10 mL
- 50 units = 0.50 mL
That last line is the one to remember. Lilly's own Instructions for Use for the vial tell you to use a syringe that can measure a 0.5 mL dose, and a standard 1 mL (100-unit) insulin syringe does exactly that. Use a new syringe and needle for every injection.
One caution: these are volume units, not insulin. Zepbound is not insulin, and the "units" on the syringe don't measure medicine strength. They only measure liquid.
The 50-unit rule for vials
Here's the part that surprises people coming from forums full of UK screenshots. In the US there's exactly one number to know for a full vial dose:
Full dose = 0.5 mL = 50 units. Every strength. Every time.
Your prescribed mg is set by which vial you were sent, not by how far you pull the plunger. On a 2.5 mg vial, 50 units delivers 2.5 mg. On a 10 mg vial, the same 50 units delivers 10 mg. The draw never changes; the concentration does.
Where the tables below earn their keep is reading things back the other way. People want to know what 20 units from a 5 mg vial works out to in mg, usually because a draw came up short, a syringe had dead space, or they're double-checking something they read online. That's arithmetic worth understanding even though the label only ever asks you for the full 50-unit draw.
Worked examples
Example 1: the full draw
You're on 7.5 mg and your LillyDirect box has four 7.5 mg vials. Each week you draw to the 50-unit line, which is 0.5 mL, and that delivers the full 7.5 mg. Next month your prescriber moves you to 10 mg. The draw is still 50 units. Only the vial label changed.
Example 2: reading back from units to mg
Say you only got 40 units out before the vial ran dry against the stopper. 40 units is 0.40 mL. A 5 mg vial holds 5 mg in 0.5 mL, so 0.40 mL is 4 mg, and you'd know you were about 1 mg short. (If that happens regularly, mention it to your pharmacist. Technique and syringe choice both play a part.)
Example 3: same mg, different vials
About 3 mg is 30 units from a 5 mg vial but only 15 units from a 10 mg vial. Same medicine, half the volume, because the stronger vial packs twice the mg into every unit. This is why you should never borrow someone else's units number without knowing their vial strength.
Prefer a calculator over manual look-ups?
Enter your vial strength and units, mL, or mg and get an instant conversion with our free US calculator.
Open the calculator →Master conversion table (US vials)
How to read this: find your units in the left column, then read across to your vial strength. For example, 20 units from a 5 mg vial is 2.00 mg, while the same 20 units from a 12.5 mg vial is 5.00 mg.
U-100 syringe; rows are units (5 to 50). "mL" is the same for every vial. Each column shows what that volume contains in mg for that strength (0.5 mL per vial). 50 units is the full labeled dose.
| Units | mL | 2.5 mg vial | 5 mg vial | 7.5 mg vial | 10 mg vial | 12.5 mg vial | 15 mg vial |
|---|---|---|---|---|---|---|---|
| 5 | 0.05 | 0.25 | 0.50 | 0.75 | 1.00 | 1.25 | 1.50 |
| 10 | 0.10 | 0.50 | 1.00 | 1.50 | 2.00 | 2.50 | 3.00 |
| 15 | 0.15 | 0.75 | 1.50 | 2.25 | 3.00 | 3.75 | 4.50 |
| 20 | 0.20 | 1.00 | 2.00 | 3.00 | 4.00 | 5.00 | 6.00 |
| 25 | 0.25 | 1.25 | 2.50 | 3.75 | 5.00 | 6.25 | 7.50 |
| 30 | 0.30 | 1.50 | 3.00 | 4.50 | 6.00 | 7.50 | 9.00 |
| 35 | 0.35 | 1.75 | 3.50 | 5.25 | 7.00 | 8.75 | 10.50 |
| 40 | 0.40 | 2.00 | 4.00 | 6.00 | 8.00 | 10.00 | 12.00 |
| 45 | 0.45 | 2.25 | 4.50 | 6.75 | 9.00 | 11.25 | 13.50 |
| 50 | 0.50 | 2.50 | 5.00 | 7.50 | 10.00 | 12.50 | 15.00 |
Notice the bottom row: 50 units delivers exactly the labeled strength of whichever vial you're holding. That's the whole system.
Target dose look-up (by vial strength)
You can also read the math the other way: pick a target mg, then read across to find the units for each vial strength. Useful for sanity-checking a number you saw in a Facebook group before you take it at face value.
| Vial strength | 1.0 mg | 2.5 mg | 3.0 mg | 5.0 mg | 7.5 mg |
|---|---|---|---|---|---|
| 2.5 mg | 20 u | 50 u | 60 u* | n/a | n/a |
| 5 mg | 10 u | 25 u | 30 u | 50 u | n/a |
| 7.5 mg | 6-7 u | 16-17 u | 20 u | 33-34 u | 50 u |
| 10 mg | 5 u | 12-13 u | 15 u | 25 u | 37-38 u |
| 12.5 mg | 4 u | 10 u | 12 u | 20 u | 30 u |
| 15 mg | 3-4 u | 8-9 u | 10 u | 16-17 u | 25 u |
*60 units is 0.6 mL, which is more than a 0.5 mL vial holds. You cannot get 3 mg out of a 2.5 mg vial.
Track your doses free in Healthcount. Log units, mL, and mg with automatic conversions, alongside weight, food, and side effects.
Get StartedWhy the label limits the math
The tables above are pure arithmetic based on volume and concentration. Real-world use has rules the arithmetic doesn't know about:
- Vials are single-dose and preservative-free. Lilly's Instructions for Use say to throw away an opened vial after your injection, even if liquid is left. Splitting one vial across multiple injections is off-label and raises sterility risk.
- The 2.5 mg strength is for starting, not staying. The prescribing information is explicit that 2.5 mg is a treatment-initiation dose, not approved maintenance.
- Partial draws aren't a dosing method. If you and your prescriber decide a lower dose makes sense, the clean route is a prescription for a lower-strength vial or pen, not drawing less from a stronger one.
- Fresh syringe every time. New syringe and needle for each injection, per the Instructions for Use.
So use the math the way it's meant to be used: to understand what the 50-unit line means, to sanity-check numbers you see online, and to catch a short draw. The label decides the rest.
Sources
Zepbound US Prescribing Information (FDA label)
Strengths, 0.5 mL dose volume, and single-dose pen/vial presentations
Zepbound single-dose vial Instructions for Use (Lilly)
Syringe requirements, drawing the dose, and discarding opened vials
Eli Lilly announcement: all approved Zepbound doses in single-dose vials
LillyDirect Zepbound self-pay terms
Zepbound KwikPen US launch
HCPLive reporting on the multi-dose KwikPen (four 0.6 mL doses per pen)
Zepbound list price context
