How many units do I draw?
Your vial is measured in milligrams. Your syringe is measured in units.
Those are different things, and the conversion changes every time the dose or the mixing
water changes. Three numbers, below.
Common setups:
Which number is which
| Box | Where it comes from |
|---|
| Vial strength | Printed on the vial label. The total milligrams in
there — not per shot. A 10 mg vial holds 10 mg no matter how you mix it. |
| Water you added | The bacteriostatic water you drew in when you
reconstituted it. If the pharmacy mixed it for you, the label says the concentration
instead — a vial marked 10 mg/mL is 10 mg vial with
1 mL of water. Put 10 and 1. |
| Your dose | What your prescriber told you, in mg. This is the number
that stays fixed while everything else moves. |
Four ways this goes wrong
1. Reading the vial's mg as the dose
A 10 mg vial is not a 10 mg shot. It's ten milligrams of powder sitting in a
glass bottle. How much liquid that becomes — and therefore how far up the barrel your dose
sits — depends entirely on how much water went in. Same vial, 1 mL vs 2 mL, and
your number doubles.
2. A U-40 syringe
U-40 syringes are sold for pet insulin. They're cheaper, they show up in the same search
results, and they look identical in the photo. They are not identical. A "20" on a U-40
barrel holds two and a half times the liquid of a "20" on a U-100 barrel. Look at the
barrel. It says U-100 or it says U-40.
Everything on this site assumes U-100.
3. Carrying last vial's number over
You worked out 25 units in March, you've drawn 25 units every Sunday since, and it's been
fine. Then a vial arrives at a different concentration and 25 units is now a different dose.
Your mg dose is the thing that stays the same. The units are just what that mg happens to
look like in this vial. Re-do it whenever a new vial arrives.
4. Pen clicks
If you're on a Mounjaro, Zepbound, Ozempic or Wegovy pen, none of this applies to you.
Pens are pre-measured — you dial a dose, not a volume. There's nothing to convert. This
page is for vials and syringes.
Answers to the things people email about
Does it matter how much water I use?
Not to the drug. 5 mg of tirzepatide is 5 mg whether it's floating in 1 mL
or 3 mL. It matters to you, because it decides whether your dose lands at
12 units (fiddly, easy to misread) or 60 units (comfortable). More water, bigger
and easier-to-hit number. Less water, smaller volume going under the skin. Most people find
somewhere around 25–60 units the sweet spot. Follow your pharmacy's instructions if
they gave any.
The calculator says 37.5 units. There's no 37.5 mark.
On a 100-unit barrel the marks are every 2, so 37.5 sits between 36 and 38. On a 30- or
50-unit barrel the marks are every 1, and half-marks are readable. Switch to the smaller
syringe if your dose fits — that's the whole reason "fit it for me" is the default.
Why does it warn me under 4 units?
Because at that size the plunger is barely off the hub and being one mark out is a
20 % dosing error. Nothing dangerous about small volumes as such — it's about how
accurately a human can hit a mark that small. Mixing with more water next time fixes it.
The arithmetic here is units = (dose ÷ (vial ÷ water)) × 100.
That's the whole thing. It's shown rather than hidden so you can check it against whatever
your pharmacy told you — and if the two disagree, go with the pharmacy and email me.
The charts
If you want the standard schedules laid out rather than one dose at a time: