Reconstitution calculator · water and strength math
Reconstitution Calculator: How Much Water to Add
How much water to add comes from your pharmacy's label or your prescriber. This page only shows the math for each amount. It does not choose a water amount or tell you how to prepare a vial.
Last updated Medical review in progress
Water and strength math
Check the strength or compare water amounts
How much water to add comes from your pharmacy's label or your prescriber. This calculator only shows the math for each amount.
Quick vial sizes
You can enter another vial amount in the field.
This page shows math only. Use the water amount and strength stated by your pharmacy or prescriber.
Strength for this mix
2.5 mg/mL
5 mg ÷ 2 mL = 2.5 mg/mL.
The answer in one minute
The strength depends on the water amount
Strength (mg/mL) = mg in the vial ÷ mL of water added. Units to draw = dose ÷ strength × 100.
Example: 5 mg in a vial with 2 mL of water is 5 ÷ 2 = 2.5 mg/mL. A prescribed 0.25 mg dose at that strength is 0.25 ÷ 2.5 × 100 = 10 units on a U-100 syringe.
How much water to add comes from your pharmacy's label or your prescriber. This page only shows the math for each amount.
Common vial sizes and strengths
These cells show the arithmetic for semaglutide and tirzepatide vial amounts. They are math only. The pharmacy label says how much to add.
| Product | Vial | 0.5 mL water | 1 mL water | 1.5 mL water | 2 mL water | 2.5 mL water | 3 mL water |
|---|---|---|---|---|---|---|---|
| Semaglutide | 5 mg | 10 mg/mL | 5 mg/mL | 3.333 mg/mL | 2.5 mg/mL | 2 mg/mL | 1.667 mg/mL |
| Semaglutide | 10 mg | 20 mg/mL | 10 mg/mL | 6.667 mg/mL | 5 mg/mL | 4 mg/mL | 3.333 mg/mL |
| Tirzepatide | 10 mg | 20 mg/mL | 10 mg/mL | 6.667 mg/mL | 5 mg/mL | 4 mg/mL | 3.333 mg/mL |
| Tirzepatide | 15 mg | 30 mg/mL | 15 mg/mL | 10 mg/mL | 7.5 mg/mL | 6 mg/mL | 5 mg/mL |
| Tirzepatide | 30 mg | 60 mg/mL | 30 mg/mL | 20 mg/mL | 15 mg/mL | 12 mg/mL | 10 mg/mL |
How much water to add comes from your pharmacy's label or your prescriber. This table only shows the math for each amount.
Read the label first
The exact product label and pharmacy instructions decide the liquid, amount, strength, storage, and dose details. This calculator helps you check the arithmetic after you have those instructions.
For preparation and handling information, read the existing guide to mixing a peptide vial. It explains why the label for your product comes first.
Common questions
How much bacteriostatic water should I add to 30 mg of tirzepatide?
Follow the amount on your pharmacy label or prescriber instructions. For math only, 30 mg with 1 mL is 30 mg/mL, with 2 mL is 15 mg/mL, and with 3 mL is 10 mg/mL. Those examples do not choose an amount for you.
Does more water change the dose?
No. The vial still contains the same total mg. More liquid makes a lower mg/mL strength, so the same prescribed dose takes more U-100 units.
Can I use this for retatrutide or other peptides?
The math works for any numbers you enter, but the examples here are for semaglutide and tirzepatide. Retatrutide is still being studied and isn't approved, and we don't publish mixing or water guidance for it.
Why do the units change when the water amount changes?
Units measure liquid volume on a U-100 syringe. When the same mg is spread through more mL, each unit contains less medicine, so a prescribed dose uses more units.
What should I do if my label is unclear?
Pause and ask the pharmacy or prescriber that gave you the vial. The label and their instructions decide the water amount, strength, storage, and dose details.
Sources
The medical and device facts on this page come from these sources. Each entry shows the date we accessed it.
- 01 Bacteriostatic Water for Injection, USP · DailyMed / Hospira, 08/2019 label. Accessed 2026-09-28.
- 02 USP <797> sterile compounding guidance · Florida Department of Health, Revision 1, 08/2024. Accessed 2026-09-28.
- 03 Zepbound single-dose vial Instructions for Use · Eli Lilly, 04/2026. Accessed 2026-09-28.
- 04 FDA clarifies policies for compounders as GLP-1 supply stabilizes · U.S. Food and Drug Administration, 04/01/2026. Accessed 2026-09-28.
- 05 FDA's concerns with unapproved GLP-1 drugs used for weight loss · U.S. Food and Drug Administration, 10/01/2026. Accessed 2026-10-01.