Read this first: legal statusNo FDA-approved tirzepatide is a powder that you mix with bacteriostatic water. Zepbound and Mounjaro come as ready-to-use solutions: single-dose pens and vials (0.5 mL), and multi-dose vials and KwikPens (four 0.6 mL doses).[1][2] The FDA declared the tirzepatide shortage resolved in December 2024, and the grace periods for compounders ended in early 2025.[4][5] The FDA has warned firms selling tirzepatide labeled "for research purposes" or "not for human consumption".[3] This page explains arithmetic only. It is not a guide to using unapproved products.
The math in three steps
Bac water math takes three steps: concentration, then volume, then units.
- Concentration. mg in the vial ÷ mL of water added. A 10 mg vial with 2 mL is 5 mg/mL.
- Volume of a dose. Dose ÷ concentration. 2.5 mg ÷ 5 mg/mL = 0.5 mL.
- Units. mL × 100 on a U-100 insulin syringe. 0.5 mL = 50 units.
Doubling the water halves the concentration and doubles the units for the same dose. The dose in milligrams is unchanged.
Concentration for each vial and water volume
Concentration is the vial mg divided by the mL of water: 10 mg in 2 mL is 5 mg/mL, and 60 mg in 3 mL is 20 mg/mL.
| Vial | 0.5 mL | 1 mL | 1.5 mL | 2 mL | 2.5 mL | 3 mL |
|---|---|---|---|---|---|---|
| 5 mg | 10 | 5 | 3.33 | 2.5 | 2 | 1.67 |
| 10 mg | 20 | 10 | 6.67 | 5 | 4 | 3.33 |
| 15 mg | 30 | 15 | 10 | 7.5 | 6 | 5 |
| 20 mg | 40 | 20 | 13.33 | 10 | 8 | 6.67 |
| 30 mg | 60 | 30 | 20 | 15 | 12 | 10 |
| 40 mg | 80 | 40 | 26.67 | 20 | 16 | 13.33 |
| 60 mg | 120 | 60 | 40 | 30 | 24 | 20 |
Units for each label dose, by vial size
Each grid shows the units to draw for the doses in the FDA labels (2.5 to 15 mg) at each water volume. Doses larger than the vial are left out. Cells over 100 units do not fit a 1 mL syringe.
| Dose | 0.5 mL | 1 mL | 1.5 mL | 2 mL | 2.5 mL | 3 mL |
|---|---|---|---|---|---|---|
| 2.5 mg | 12.5 | 25 | 37.5 | 50 | 62.5 | 75 |
| 5 mg | 25 | 50 | 75 | 100 | 125 | 150 |
| 7.5 mg | 37.5 | 75 | 112.5 | 150 | 187.5 | 225 |
| 10 mg | 50 | 100 | 150 | 200 | 250 | 300 |
| Dose | 0.5 mL | 1 mL | 1.5 mL | 2 mL | 2.5 mL | 3 mL |
|---|---|---|---|---|---|---|
| 2.5 mg | 6.3 | 12.5 | 18.8 | 25 | 31.3 | 37.5 |
| 5 mg | 12.5 | 25 | 37.5 | 50 | 62.5 | 75 |
| 7.5 mg | 18.8 | 37.5 | 56.3 | 75 | 93.8 | 112.5 |
| 10 mg | 25 | 50 | 75 | 100 | 125 | 150 |
| 12.5 mg | 31.3 | 62.5 | 93.8 | 125 | 156.3 | 187.5 |
| 15 mg | 37.5 | 75 | 112.5 | 150 | 187.5 | 225 |
| Dose | 0.5 mL | 1 mL | 1.5 mL | 2 mL | 2.5 mL | 3 mL |
|---|---|---|---|---|---|---|
| 2.5 mg | 4.2 | 8.3 | 12.5 | 16.7 | 20.8 | 25 |
| 5 mg | 8.3 | 16.7 | 25 | 33.3 | 41.7 | 50 |
| 7.5 mg | 12.5 | 25 | 37.5 | 50 | 62.5 | 75 |
| 10 mg | 16.7 | 33.3 | 50 | 66.7 | 83.3 | 100 |
| 12.5 mg | 20.8 | 41.7 | 62.5 | 83.3 | 104.2 | 125 |
| 15 mg | 25 | 50 | 75 | 100 | 125 | 150 |
| Dose | 0.5 mL | 1 mL | 1.5 mL | 2 mL | 2.5 mL | 3 mL |
|---|---|---|---|---|---|---|
| 2.5 mg | 2.1 | 4.2 | 6.3 | 8.3 | 10.4 | 12.5 |
| 5 mg | 4.2 | 8.3 | 12.5 | 16.7 | 20.8 | 25 |
| 7.5 mg | 6.3 | 12.5 | 18.8 | 25 | 31.3 | 37.5 |
| 10 mg | 8.3 | 16.7 | 25 | 33.3 | 41.7 | 50 |
| 12.5 mg | 10.4 | 20.8 | 31.3 | 41.7 | 52.1 | 62.5 |
| 15 mg | 12.5 | 25 | 37.5 | 50 | 62.5 | 75 |
Water that makes the numbers round
The same vial can be mixed so that whole units line up with whole milligrams. These are arithmetic options, not a recommendation for any product.
| Vial | 5 mg/mL (20 units = 1 mg) | 10 mg/mL (10 units = 1 mg) | 20 mg/mL (5 units = 1 mg) |
|---|---|---|---|
| 5 mg | 1 mL | 0.5 mL | 0.25 mL |
| 10 mg | 2 mL | 1 mL | 0.5 mL |
| 15 mg | 3 mL | 1.5 mL | 0.75 mL |
| 20 mg | 4 mL | 2 mL | 1 mL |
| 30 mg | 6 mL | 3 mL | 1.5 mL |
| 40 mg | 8 mL | 4 mL | 2 mL |
| 60 mg | 12 mL | 6 mL | 3 mL |
Very small volumes are hard to measure. Under about 2 units, the width of a syringe line becomes a large share of the dose. Very large volumes may need more than one syringe for higher doses: at 2.5 mg/mL, a 15 mg dose would be 600 units.
How many doses a vial holds
A vial holds its mg divided by the dose: a 20 mg vial holds eight 2.5 mg doses or four 5 mg doses.
| Vial | 2.5 mg | 5 mg | 7.5 mg | 10 mg | 12.5 mg | 15 mg |
|---|---|---|---|---|---|---|
| 5 mg | 2 | 1 | – | – | – | – |
| 10 mg | 4 | 2 | 1 | 1 | – | – |
| 15 mg | 6 | 3 | 2 | 1 | 1 | 1 |
| 20 mg | 8 | 4 | 2 | 2 | 1 | 1 |
| 30 mg | 12 | 6 | 4 | 3 | 2 | 2 |
| 40 mg | 16 | 8 | 5 | 4 | 3 | 2 |
| 60 mg | 24 | 12 | 8 | 6 | 4 | 4 |
Why FDA status matters for this math
The math is only as good as the label on the vial, and for unapproved powder the FDA does not check that label.[3]
- The label on the vial is the only input. The calculator assumes the vial contains exactly the milligrams printed on it. The FDA says unapproved versions “do not undergo FDA's review for safety, effectiveness and quality,” and it has received more than 730 reports of adverse events linked to compounded tirzepatide.[3]
- Storage after mixing is unknown. There is no FDA-approved labeling for how long mixed tirzepatide powder stays stable. Ask a pharmacist.
- Compounding rules changed. Once tirzepatide left the shortage list, the FDA set end dates for compounders (February 18 and March 19, 2025), and in April 2026 it proposed excluding tirzepatide from the bulk substances outsourcing facilities may use.[5][6] Full timeline: compounded tirzepatide: FDA status.
Units for a single dose
For a liquid compounded vial (already mixed), see compounded tirzepatide dosage.
For a full table of one dose across every concentration, open: 2.5 mg · 5 mg · 7.5 mg · 10 mg · 12.5 mg · 15 mg.
Frequently asked questions
How much bac water do I add to 10 mg of tirzepatide?
There is no official answer, because no approved tirzepatide product is mixed with water. Arithmetically, 2 mL gives 5 mg/mL (2.5 mg = 50 units) and 1 mL gives 10 mg/mL (2.5 mg = 25 units). Ask the pharmacist who supplied the vial.
How many units is 2.5 mg from a 10 mg vial with 2 mL of water?
50 units. 10 mg ÷ 2 mL = 5 mg/mL; 2.5 mg ÷ 5 mg/mL = 0.5 mL; 0.5 mL × 100 = 50 units.
How many units is 5 mg from a 30 mg vial with 3 mL of water?
50 units. 30 ÷ 3 = 10 mg/mL; 5 ÷ 10 = 0.5 mL = 50 units.
Does more water make the dose weaker?
No. The dose in mg stays the same. More water spreads it through a bigger volume, so you draw more units to get the same milligrams.
Can I mix a Zepbound vial with bac water?
No. Zepbound vials are already a solution: 0.5 mL per single-dose vial, or four 0.6 mL doses per multi-dose vial. The label says, “Do not mix ZEPBOUND with any other medicine.”[1]
Sources
- Zepbound (tirzepatide) injection: prescribing information, Medication Guide and Instructions for Use
- Mounjaro (tirzepatide) injection: prescribing information and Instructions for Use
- FDA's concerns with unapproved GLP-1 drugs used for weight loss
- Declaratory order: resolution of shortages of tirzepatide injection products (Mounjaro and Zepbound)
- FDA clarifies policies for compounders as national GLP-1 supply begins to stabilize
- FDA proposes to exclude semaglutide, tirzepatide and liraglutide from the 503B bulks list