Heads-up: this is general information for UK readers. Always follow your own prescriber's advice.
Why this matters: In the UK, Mounjaro (tirzepatide) pens deliver a fixed 0.6 mL per injection. Syringes often show units rather than mL on a U-100 scale. The tables and examples below show how units ↔ mL ↔ mg relate for each UK pen strength, so you can understand the maths.
UK quick facts
- Dose volume per injection: 0.6 mL for all strengths
- Doses per pen: 4 per pen → total 2.4 mL per pen
- Available strengths (per dose): 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, 15 mg
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 making squash in a measuring jug.
- The jug volume is in mL.
- The strength depends on how much concentrate you add. That's like mg per mL in a pen.
- The tick marks on the jug help you pour precisely. A U-100 syringe is like a jug with 100 tiny ticks per mL.
So 1 unit = 0.01 mL.
Same mL can contain different mg depending on the pen's strength. That is why pen strength (mg) changes how many units you draw to reach the same mg.
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
- 60 units = 0.60 mL
This aligns neatly with UK Mounjaro's per-injection volume of 0.6 mL.
Worked examples
Example 1: 40 units from a 5 mg UK pen
If you draw 40 units, that is 0.40 mL. A UK 5 mg pen delivers 5 mg in 0.6 mL, so 0.40 mL corresponds to about 3.33 mg. In words: forty units from a 5 mg pen gives roughly three and a third milligrams.
Example 2: aiming for about 3 mg at different strengths
From a 5 mg pen, about 3 mg is 36 units. From a 10 mg pen, about 3 mg is 18 units. Same mg, fewer units from the stronger pen.
Example 3: reading back from units to mg
If you took 24 units from a 7.5 mg pen, that is 0.24 mL. Because the 7.5 mg pen is stronger per mL, 0.24 mL equals about 3 mg.
Prefer a calculator over manual look-ups?
Skip the tables — enter your pen strength and units, mL or mg and get an instant conversion with our free UK calculator.
Open the calculator →Master conversion table (UK)
How to read this: If you want about 3 mg from a 5 mg pen, look down the 5 mg pen column. You'll see 36 units ≈ 3.000 mg and 40 units ≈ 3.333 mg. For the same ~3 mg from a 10 mg pen, the 10 mg pen column shows 18 units = 3.000 mg.
This shows why pen strength matters: stronger pens need fewer units for the same mg.
U-100 syringe; rows are units (5 → 60). "mL" is the same for all pens. Each pen column shows what that volume contains in mg for that UK strength (0.6 mL per injection).
| Units | mL | 2.5 mg pen | 5 mg pen | 7.5 mg pen | 10 mg pen | 12.5 mg pen | 15 mg pen |
|---|---|---|---|---|---|---|---|
| 5 | 0.05 | 0.208 | 0.417 | 0.625 | 0.833 | 1.042 | 1.250 |
| 10 | 0.10 | 0.417 | 0.833 | 1.250 | 1.667 | 2.083 | 2.500 |
| 15 | 0.15 | 0.625 | 1.250 | 1.875 | 2.500 | 3.125 | 3.750 |
| 20 | 0.20 | 0.833 | 1.667 | 2.500 | 3.333 | 4.167 | 5.000 |
| 25 | 0.25 | 1.042 | 2.083 | 3.125 | 4.167 | 5.208 | 6.250 |
| 30 | 0.30 | 1.250 | 2.500 | 3.750 | 5.000 | 6.250 | 7.500 |
| 35 | 0.35 | 1.458 | 2.917 | 4.375 | 5.833 | 7.292 | 8.750 |
| 40 | 0.40 | 1.667 | 3.333 | 5.000 | 6.667 | 8.333 | 10.000 |
| 45 | 0.45 | 1.875 | 3.750 | 5.625 | 7.500 | 9.375 | 11.250 |
| 50 | 0.50 | 2.083 | 4.167 | 6.250 | 8.333 | 10.417 | 12.500 |
| 55 | 0.55 | 2.292 | 4.583 | 6.875 | 9.167 | 11.458 | 13.750 |
| 60 | 0.60 | 2.500 | 5.000 | 7.500 | 10.000 | 12.500 | 15.000 |
Target dose look-up (by pen strength)
You can also read the maths this way: pick your target mg, then read across to find the units for each pen strength.
Using the same example: with a 5 mg pen and about 3 mg, the row shows 36 units. With a 10 mg pen for about 3 mg, the row shows 18 units.
| Pen strength | 0.5 mg | 1.0 mg | 2.5 mg | 3.0 mg | 5.0 mg |
|---|---|---|---|---|---|
| 2.5 mg | 12 u | 24 u | 60 u | 72 u* | — |
| 5 mg | 6 u | 12 u | 30 u | 36 u | 60 u |
| 7.5 mg | 4 u | 8 u | 20 u | 24 u | 40 u |
| 10 mg | 3 u | 6 u | 15 u | 18 u | 30 u |
| 12.5 mg | 2–3 u | 5 u | 12 u | 14–15 u | 24 u |
| 15 mg | 2 u | 4 u | 10 u | 12 u | 20 u |
*72 units exceeds 0.6 mL, which is more than one labelled UK dose volume.
Why "real-world use" can differ from the theoretical maths
The tables show pure arithmetic based on volume and strength. For example, a 5 mg pen contains about 20 mg in total, so you can calculate how many draws you'd get at a given mg.
In real-world use, the device and label rules limit that arithmetic:
- The UK pen is designed for four weekly injections of 0.6 mL each.
- Leftover liquid may remain after four doses to ensure accurate delivery, but you are instructed to discard the pen and not use any remainder.
- There is an in-use time limit after first use (for example, discard after 30 days), which can cap how much is usable from a pen.
So even if the maths suggests "more is left", the instructions for use take priority.
A question this page often raises
Working out the maths on a pen tends to lead straight to a second thought: am I paying a fair price for this in the first place? UK prices vary quite a lot between pharmacies for the same strength, and most people never check after their first order.
We keep two pages for exactly that. Neither is an advert, and we do not tell you which service to use.
- What GLP-1 medications cost in the UK — what we found when we checked, and what tends to be left out of a headline price.
- Compare UK weight-management providers — how the services differ on consultation, pharmacy registration and aftercare.
Getting the dose right is half the job
The other half is knowing what the weeks are adding up to. In the largest trial of its kind, people who kept daily food records lost twice as much weight as those who kept none. We've written up the evidence, and how little recording it actually takes.
Why writing it down works →References
European Medicines Agency
KDL Nursing Centre
