Editorial note: this article summarises published research on semaglutide and tirzepatide and explains reconstitution arithmetic. Baclabs does not sell semaglutide, tirzepatide or any peptide; we supply bacteriostatic water and reconstitution consumables. Nothing here is medical advice.
A semaglutide reconstitution calculator turns a vial mass in milligrams and a diluent volume in millilitres into a concentration, then turns a target dose into a syringe volume. The arithmetic is simple, but a misplaced decimal on a 10 mg vial is a ten-fold dosing error. This article shows the calculation in full, with tables for the common semaglutide and tirzepatide vial sizes, and explains what a calculator is doing behind the interface.
Context first. Every licensed GLP-1 product in the UK, including Wegovy, Ozempic, Mounjaro and Saxenda, is a solution in a pre-filled pen [1][2][3][8]; nothing needs reconstituting. Lyophilised (freeze-dried) semaglutide or tirzepatide vials exist only in the grey and research-chemical markets, where the Medicines and Healthcare products Regulatory Agency (MHRA) seized counterfeit and unlicensed products in 2025 and 2026 [4]. The maths below is for anyone handling such vials for legitimate research, and it is the same maths as for every peptide in the reconstitution hub. For clinical background, see the GLP-1 peptides guide.
The three formulas behind every calculator
Every reconstitution calculator, whatever its interface, runs the same three steps.
- Concentration. Concentration (mg/mL) = vial content (mg) ÷ diluent volume (mL). Multiply by 1,000 for mcg/mL.
- Volume per dose. Volume (mL) = target dose (mg) ÷ concentration (mg/mL).
- Syringe units. On a U-100 insulin syringe, 1 mL = 100 units, so units = volume (mL) × 100. Equivalently, one unit holds concentration (mg/mL) × 10 mcg.
Worked example: a 5 mg vial with 2 mL of bacteriostatic water gives 5 ÷ 2 = 2.5 mg/mL, or 2,500 mcg/mL. Each unit on a U-100 syringe holds 25 mcg. A 0.25 mg (250 mcg) draw is 0.25 ÷ 2.5 = 0.1 mL = 10 units. A 1 mg draw is 0.4 mL = 40 units.
The diluent volume, not the vial size, sets the concentration. More water means less peptide per unit and finer control of small doses; less water lets a large dose fit in one syringe.
Semaglutide reconstitution calculator: worked tables
Grey-market semaglutide vials are commonly labelled 3 mg, 5 mg or 10 mg. The licensed Wegovy schedule, reproduced for reference only, runs 0.25 mg weekly for four weeks, then 0.5 mg, 1 mg and 1.7 mg at four-week intervals, to a 2.4 mg maintenance dose [1]. The tables below show what those dose sizes look like in syringe units at each concentration.
| Vial | Bacteriostatic water | Concentration | Per unit (U-100) | 0.25 mg | 0.5 mg | 1 mg | 2.4 mg |
|---|---|---|---|---|---|---|---|
| 3 mg | 1 mL | 3 mg/mL (3,000 mcg/mL) | 30 mcg | 8.3 units | 16.7 units | 33.3 units | 80 units |
| 3 mg | 1.5 mL | 2 mg/mL (2,000 mcg/mL) | 20 mcg | 12.5 units | 25 units | 50 units | Exceeds one syringe |
| 5 mg | 1 mL | 5 mg/mL (5,000 mcg/mL) | 50 mcg | 5 units | 10 units | 20 units | 48 units |
| 5 mg | 2 mL | 2.5 mg/mL (2,500 mcg/mL) | 25 mcg | 10 units | 20 units | 40 units | 96 units |
| 10 mg | 2 mL | 5 mg/mL (5,000 mcg/mL) | 50 mcg | 5 units | 10 units | 20 units | 48 units |
| 10 mg | 3 mL | 3.33 mg/mL (3,333 mcg/mL) | 33.3 mcg | 7.5 units | 15 units | 30 units | 72 units |
Fractional units (8.3, 16.7) cannot be measured on a standard syringe, so the 3 mg vial with 1 mL is a poor choice for small doses; 1.5 mL gives cleaner numbers. A 2.4 mg dose from a 2.5 mg/mL solution nearly fills a 1 mL syringe.
How to mix tirzepatide 10mg (and 5 mg, 15 mg) vials
Tirzepatide is dosed in larger milligram amounts than semaglutide, so the vials are larger and the concentration choice matters more. The licensed Mounjaro schedule starts at 2.5 mg weekly for four weeks, then rises in 2.5 mg steps at intervals of at least four weeks to a maintenance dose of 5, 10 or 15 mg [2]. Those figures are quoted for reference; they are not instructions for a research vial. The clinical differences between the two drugs are covered in the tirzepatide vs semaglutide article.
| Vial | Bacteriostatic water | Concentration | Per unit (U-100) | 2.5 mg | 5 mg | 7.5 mg | 10 mg |
|---|---|---|---|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL (5,000 mcg/mL) | 50 mcg | 50 units | 100 units | , | , |
| 5 mg | 2 mL | 2.5 mg/mL (2,500 mcg/mL) | 25 mcg | 100 units | , | , | , |
| 10 mg | 1 mL | 10 mg/mL (10,000 mcg/mL) | 100 mcg | 25 units | 50 units | 75 units | 100 units |
| 10 mg | 2 mL | 5 mg/mL (5,000 mcg/mL) | 50 mcg | 50 units | 100 units | , | , |
| 10 mg | 3 mL | 3.33 mg/mL (3,333 mcg/mL) | 33.3 mcg | 75 units | , | , | , |
| 15 mg | 1 mL | 15 mg/mL (15,000 mcg/mL) | 150 mcg | 16.7 units | 33.3 units | 50 units | 66.7 units |
| 15 mg | 2 mL | 7.5 mg/mL (7,500 mcg/mL) | 75 mcg | 33.3 units | 66.7 units | 100 units | , |
| 15 mg | 3 mL | 5 mg/mL (5,000 mcg/mL) | 50 mcg | 50 units | 100 units | , | , |
A dash means the dose exceeds 1 mL and cannot be drawn in one insulin syringe. For a 10 mg tirzepatide vial, 2 mL is the usual compromise: 5 mg/mL gives whole-number units for 2.5 mg and 5 mg draws. Using 1 mL keeps every dose under 100 units but halves measurement precision. The 15 mg vial with 3 mL lands on the same 5 mg/mL, convenient if both sizes are in use.
Planning around the 28-day window
An opened multi-dose vial is conventionally discarded 28 days after first puncture unless the manufacturer states otherwise, a rule set out in the US Centers for Disease Control and Prevention (CDC) injection-safety guidance and mirrored in pharmacy practice [5]. Peptide stability in solution is a separate question and often shorter. That window should inform vial choice: a 10 mg vial drawn at 2.5 mg weekly is four doses, or 28 days, which fits; a 15 mg vial at the same rate is six doses, the last on day 35, which does not. Reconstitute only what will be used within the window. The storage and shelf-life guide covers this in detail.
Mixing water for peptides: which diluent
The right mixing water for peptides in a multi-dose vial is bacteriostatic water: water for injection with 0.9% benzyl alcohol as a preservative to suppress bacterial growth between draws. The US Pharmacopeia (USP) product labelling carries two restrictions worth knowing: not for use in neonates, because of benzyl alcohol toxicity, and not for intravenous use without a solute, because it can cause haemolysis [6]. Sterile water for injection has no preservative; it suits a vial emptied in one draw and not one punctured repeatedly. The bacteriostatic water vs sterile water article sets out the difference. Benzyl alcohol is incompatible with some drug substances [6]; none is documented for the peptides discussed here.
Technique notes
- Swab both stoppers with 70% isopropyl alcohol and let them dry.
- Draw the intended volume of bacteriostatic water into a sterile syringe. Inject it slowly down the inside wall of the peptide vial rather than directly onto the powder.
- Do not shake. Swirl gently or leave the vial to stand until the solution is clear. Peptides are sensitive to shear and foaming.
- Label the vial with the date of reconstitution, the concentration (e.g. “5 mg/mL”) and the discard date.
- Store refrigerated at 2–8 °C, protected from light.
- Use a fresh needle for every draw; the Wegovy patient leaflet’s warning that reuse risks blockage, contamination and inaccurate dosing applies equally here [7].
How the calculator works: building your own
A reconstitution calculator takes three inputs (vial mg, diluent mL, target dose) and returns three outputs (concentration, volume, units). In a spreadsheet, with vial mass in cell A1, diluent volume in B1 and target dose in mg in C1:
- Concentration in mg/mL: =A1/B1
- Concentration in mcg/mL: =A1/B1*1000
- Draw volume in mL: =C1/(A1/B1), which simplifies to =C1*B1/A1
- Syringe units: =C1*B1/A1*100
- Doses per vial: =A1/C1
Check any calculator against a mental estimate. If a 10 mg vial with 2 mL gives anything other than 5 mg/mL, or a 2.5 mg draw from it anything other than 50 units, an input is wrong. The commonest error is entering mcg into a field expecting mg, a thousand-fold discrepancy a sanity check catches instantly.
Multi-dose vials need a preserved diluent. Baclabs supplies UK-stocked bacteriostatic water in 30 mL vials, which is enough to reconstitute ten to fifteen typical peptide vials; the reconstitution guide covers the general method for any peptide.
Frequently asked questions
How much bacteriostatic water do I add to a 5 mg semaglutide vial?
There is no single correct volume; it depends on the dose sizes to be measured. With 2 mL, the vial becomes 2.5 mg/mL and each unit on a U-100 insulin syringe holds 25 mcg, so 0.25 mg is 10 units and 1 mg is 40 units. With 1 mL, the concentration doubles to 5 mg/mL and the unit counts halve.
How do you mix a 10 mg tirzepatide vial?
The common choice is 2 mL of bacteriostatic water, giving 5 mg/mL; 2.5 mg is then 50 units and 5 mg is 100 units on a U-100 syringe. With 1 mL the vial is 10 mg/mL and 2.5 mg is 25 units. Add the water slowly down the vial wall, swirl rather than shake, and label with the date and concentration.
Can I use sterile water instead of bacteriostatic water?
For a single-use vial emptied in one draw, yes. For a multi-dose vial punctured over days or weeks, no: sterile water has no preservative, so nothing suppresses bacterial growth between draws. Bacteriostatic water contains 0.9% benzyl alcohol for exactly that purpose [6]. See the comparison article.
How long does reconstituted semaglutide last?
The multi-dose vial convention is to discard 28 days after first puncture unless the manufacturer states otherwise [5]. That is a contamination limit, not a stability guarantee; peptide degradation in solution depends on temperature, light and pH, and grey-market vials come with no stability data. Store refrigerated and reconstitute only what will be used inside the window.
Why do licensed semaglutide and tirzepatide not need reconstituting?
Because Wegovy, Ozempic and Mounjaro are supplied as ready-to-use solutions in pre-filled pens with the dose pre-set [1][2]. Lyophilised vials are not a licensed format for either drug in the UK. Anything sold in that form is outside the licensed supply chain, and the MHRA has seized such products in enforcement operations [4].
References
- Wegovy FlexTouch solution for injection in pre-filled pen, Summary of Product Characteristics. Novo Nordisk, emc, 2026
- Mounjaro KwikPen 2.5 mg solution for injection in pre-filled pen, Summary of Product Characteristics. Eli Lilly, emc, 2026
- Saxenda 6 mg/mL solution for injection in pre-filled pen, Summary of Product Characteristics. Novo Nordisk, emc
- Two arrested during the MHRA’s largest ever seizure of unlicensed weight loss medicines. MHRA / GOV.UK, May 2026
- Preventing Unsafe Injection Practices. US Centers for Disease Control and Prevention
- Bacteriostatic Water for Injection, USP, Prescribing Information. Pfizer (Hospira)
- Wegovy 2.4 mg FlexTouch, Patient Information Leaflet. Novo Nordisk, emc
- Ozempic 0.25 mg solution for injection in pre-filled pen, Summary of Product Characteristics. Novo Nordisk, emc
Disclaimer: This content is for educational purposes only and does not constitute medical advice. Bacteriostatic water is supplied for reconstitution of substances intended for research and for use as directed by a healthcare professional. Peptides and medicines discussed here may be unlicensed in the UK or prescription-only; consult a qualified clinician before using any medicine. Baclabs does not sell peptides.