Key takeaways
- Concentration = peptide ÷ water. 5 mg in 2 mL is 2.5 mg/mL.
- Units = dose ÷ concentration × 100 on a U-100 syringe, where 1 unit is 0.01 mL.
- Water changes the units you draw, not the dose. Recalculate every time you change it.
- mcg per unit = mg/mL × 10. It is the quickest way to check any result by hand.
- Most dosing errors come from mixing up mg, mcg, mL and units, or from using the wrong syringe.
How to use the peptide reconstitution calculator
The calculator needs four inputs, all of which you can read from your vial, your diluent and your plan. It updates as you type and keeps your numbers in the page link, so you can bookmark or share the exact result.
- Peptide in the vial (mg). Use the total amount printed on the label, for example 5 mg or 10 mg. If the label lists more than one peptide, use the peptide blend calculator instead.
- Bacteriostatic water added (mL). The volume of diluent you inject into the vial. If you have not mixed the vial yet, try a few volumes and see which one gives a convenient number of units.
- Dose per injection. Enter it in mcg or mg. The calculator flags values that look like a unit mix-up, such as 250 mg where 250 mcg was almost certainly meant.
- Syringe. Pick the barrel size you use. All three U-100 sizes share the same scale (100 units per mL); the choice changes the capacity and the graduation spacing used for rounding.
- Read the result. The large number is where to draw. Below it you will see the volume in mL, the concentration, the mcg in each unit and how many doses the vial holds. Open Show the math to see each step with your numbers.
The peptide reconstitution formula
Reconstitution is a dilution problem. A freeze-dried (lyophilised) peptide is a fixed mass of powder. Adding a known volume of liquid turns it into a solution with a known concentration, and every dose is then a volume of that solution. Three formulas cover it.
Multiplying by 1000 converts mg to mcg, because most peptide doses are written in micrograms.
U-100 means 100 units per mL, so 1 unit = 0.01 mL. The same rule holds for 0.3, 0.5 and 1 mL U-100 syringes.
Combined into a single line, the units for a U-100 syringe are:
Check: 250 × 2 ÷ (5 × 10) = 500 ÷ 50 = 10 units.
| Symbol | Meaning | Unit | Where to find it |
|---|---|---|---|
| Vial mass | Total peptide in the vial | mg | Vial or box label |
| Water | Diluent added to the vial | mL | The volume you inject into the vial |
| C | Concentration after mixing | mcg/mL or mg/mL | Calculated |
| Dose | Amount per injection | mcg or mg | Your prescription or protocol |
| V | Volume to draw | mL | Calculated |
| Units | Syringe reading | units | Calculated (100 per mL on U-100) |
What reconstitution does to a vial
The diagram follows one vial from powder to a single measured dose. The peptide mass never changes; only the liquid it is spread through does.
Worked examples
Example 1: 5 mg vial, 2 mL water, 250 mcg dose
Concentration is 5 × 1000 ÷ 2 = 2,500 mcg/mL. Volume is 250 ÷ 2,500 = 0.1 mL. On a U-100 syringe that is 10 units. The vial holds 5,000 ÷ 250 = 20 doses.
Example 2: 10 mg vial, 2 mL water, 2.5 mg dose
Concentration is 10 ÷ 2 = 5 mg/mL. Volume is 2.5 ÷ 5 = 0.5 mL, which is 50 units. That fills a 0.5 mL syringe completely, so a 1 mL syringe gives more room. The vial holds 4 doses.
Example 3: 2 mg vial, 1 mL water, 100 mcg dose
Concentration is 2,000 mcg/mL, so each unit is 20 mcg. 100 mcg is 0.05 mL, or 5 units. A 0.3 mL syringe, often marked in half units, makes this small volume much easier to read than a 1 mL syringe marked every 2 units.
Example 4: the same dose with a U-40 syringe
If Example 1 were drawn with a U-40 syringe, 0.1 mL would read 4 units, not 10. Someone who draws “10 units” on a U-40 syringe gets 0.25 mL, which is 625 mcg or 2.5 times the dose. This is why the calculator warns you whenever U-40 is selected.
How much bacteriostatic water should you add?
No volume is universally correct. The water decides how concentrated the solution is, and therefore how many units each dose takes up. The table keeps the vial (5 mg) and the dose (250 mcg) fixed and changes only the water.
| Water | Concentration | mcg per unit | Units per dose | Doses per vial |
|---|---|---|---|---|
| 0.5 mL | 10 mg/mL | 100 | 2.5 | 20 |
| 1 mL | 5 mg/mL | 50 | 5 | 20 |
| 1.5 mL | 3.33 mg/mL | 33.3 | 7.5 | 20 |
| 2 mL | 2.5 mg/mL | 25 | 10 | 20 |
| 2.5 mL | 2 mg/mL | 20 | 12.5 | 20 |
| 3 mL | 1.67 mg/mL | 16.7 | 15 | 20 |
Doses per vial does not change with water. Only the units per dose do.
Three practical rules help you choose:
- Aim for a whole-unit mark. A dose of 10 units is easier to measure than 7.3 units. The BAC water calculator works backwards from the number of units you want.
- Avoid very small draws. Under about 2 units, a small misreading is a large percentage error. More water makes the dose bigger in volume and easier to read.
- Respect the vial. Many peptide vials hold 2–3 mL. Leave headspace, and remember that more water also means a larger volume per injection.
Reconstitution chart: mcg per unit
Find your vial size on the left and your water volume across the top. The cell shows how many micrograms each U-100 unit contains. Divide your dose by that number to get units. For example, 10 mg in 2 mL gives 50 mcg per unit, so a 500 mcg dose is 10 units.
| Vial | 0.5 mL | 1 mL | 2 mL | 2.5 mL | 3 mL | 5 mL |
|---|---|---|---|---|---|---|
| 1 mg | 20 | 10 | 5 | 4 | 3.3 | 2 |
| 2 mg | 40 | 20 | 10 | 8 | 6.7 | 4 |
| 5 mg | 100 | 50 | 25 | 20 | 16.7 | 10 |
| 10 mg | 200 | 100 | 50 | 40 | 33.3 | 20 |
| 15 mg | 300 | 150 | 75 | 60 | 50 | 30 |
| 20 mg | 400 | 200 | 100 | 80 | 66.7 | 40 |
| 30 mg | 600 | 300 | 150 | 120 | 100 | 60 |
| 60 mg | 1,200 | 600 | 300 | 240 | 200 | 120 |
mcg per unit = vial (mg) × 1000 ÷ water (mL) ÷ 100. For mg per unit, divide by 1000.
Choosing a syringe for your dose
All U-100 insulin syringes use the same scale, 100 units per mL, so the unit count never changes with barrel size. What changes is how precisely you can read it. A smaller barrel spreads the same number of units over more length, which makes each mark wider apart.
- 0.3 mL (30 units): the most precise choice for small doses. Many brands print half-unit marks.
- 0.5 mL (50 units): marked every unit. A good match for doses between about 5 and 50 units.
- 1 mL (100 units): usually marked every 2 units. Best for larger volumes; odd numbers fall between marks.
If your dose lands on an odd number such as 7 units, a 1 mL syringe forces you to estimate between the 6 and 8 marks. Switching to a 0.5 mL syringe, or changing the water so the dose lands on an even number, removes the guesswork. To convert any mg or mL amount straight into units, use the mg to units calculator. If you already drew a dose and want to know what it contained, the reverse peptide calculator turns units back into mcg.
How to check any result by hand
You do not need to trust any calculator blindly, including this one. Two quick checks catch almost every error.
Check 1: mcg per unit
Multiply the concentration in mg/mL by 10 to get mcg per unit. At 2.5 mg/mL each unit holds 25 mcg. Then divide your dose by that number: 250 ÷ 25 = 10 units. If your answer is 100 units or 1 unit, you have slipped a factor of ten somewhere.
Check 2: the whole-vial sanity check
Multiply the units per dose by the number of doses per vial. The result should equal the water volume × 100. In Example 1, 10 units × 20 doses = 200 units = 2 mL, which matches the 2 mL of water added. If the numbers do not match, recheck the inputs.
These checks also work for the peptide-specific pages. The tirzepatide, semaglutide, retatrutide and BPC-157 calculators use exactly the same formula with presets for common vial sizes. For vials labelled in international units, such as somatropin, start with the HGH IU to mg calculator.
How to reconstitute a peptide vial, step by step
Always follow the instructions that come with your product and the advice of your pharmacist. The general sequence below reflects common aseptic technique for multi-dose vials.
- Wash your hands and work on a clean, dry surface.
- Let refrigerated vials reach room temperature if the product instructions say to.
- Wipe both rubber stoppers (peptide vial and water vial) with a fresh alcohol swab and let them dry.
- Draw the planned volume of water using a new sterile syringe.
- Add the water slowly down the inside wall of the peptide vial rather than spraying it directly onto the powder.
- Swirl or roll gently until dissolved. Avoid vigorous shaking unless the product label says otherwise.
- Inspect the solution. Do not use it if it is cloudy, discolored or has particles, unless the label says that is expected.
- Label the vial with the date mixed, the water volume and the concentration, then store as directed.
Storage and beyond-use dates
Stability depends on the specific peptide and product, so the manufacturer’s label always comes first. Some general principles apply widely:
| State | Typical storage | Notes |
|---|---|---|
| Lyophilised (powder) | Cold and dry, protected from light | Follow the label; many products specify refrigeration or freezing. |
| Reconstituted | Refrigerated at 2–8 °C (36–46 °F) | Do not freeze unless the label allows it. Keep away from light. |
| Multi-dose vial after first puncture | As labelled | For compounded vials without product data, USP <797> defaults to a 28-day beyond-use date. |
The doses per vial calculator tells you how many days your vial will last at your schedule and whether you will reach the end of the vial before its beyond-use date.
Common reconstitution mistakes
The FDA has documented patients taking 5 to 20 times their intended dose of compounded semaglutide, most often because of confusion between units, millilitres and milligrams, or syringes far larger than the dose. The same mistakes apply to any reconstituted peptide.
- Mixing up mg and mcg. 1 mg is 1,000 mcg. Entering 250 mg instead of 250 mcg is a 1,000-fold error.
- Reading mL as units. 0.1 mL is 10 units on a U-100 syringe, not 0.1 units.
- Using a U-40 syringe for instructions written for U-100. Every unit is 2.5 times larger.
- Forgetting the water volume changed. The same “10 units” means a different dose after a different mix.
- Counting graduations wrong. 1 mL syringes are often marked every 2 units; 0.3 mL syringes may be marked every half unit.
- Assuming blend labels are per peptide. A vial labelled 10 mg that contains two peptides may hold 5 mg of each, or 10 mg of each. Check the label.
Key terms
- Reconstitution
- Adding a liquid diluent to a dry (lyophilised) drug to make a solution of known concentration.
- Lyophilised
- Freeze-dried. Most peptides are supplied this way because they are more stable as a powder.
- Diluent
- The liquid used for reconstitution, usually bacteriostatic water or sterile water.
- Bacteriostatic water
- Sterile water with 0.9% benzyl alcohol, which inhibits bacterial growth and allows repeated withdrawals from one vial.
- Concentration
- The amount of peptide per volume of solution, written as mg/mL or mcg/mL.
- U-100
- A syringe scale of 100 units per mL. One unit equals 0.01 mL.
- Beyond-use date (BUD)
- The date after which a compounded or opened preparation should not be used.
Frequently asked questions
How do you calculate peptide reconstitution?
Divide the peptide in the vial by the water you add to get the concentration (mg ÷ mL, or mg × 1000 ÷ mL for mcg/mL). Divide your dose by the concentration to get the volume in mL. On a U-100 syringe, multiply the mL by 100 to get units.
How many units is 250 mcg from a 5 mg vial?
It depends on the water. With 2 mL the concentration is 2,500 mcg/mL, so 250 mcg is 0.1 mL, or 10 units. With 1 mL it is 5 units. With 2.5 mL it is 12.5 units. With 3 mL it is 15 units.
How much bacteriostatic water do I mix with a 10 mg peptide vial?
Any volume the vial can hold works, as long as you recalculate. 1 mL gives 10 mg/mL (100 mcg per unit), 2 mL gives 5 mg/mL (50 mcg per unit) and 3 mL gives about 3.33 mg/mL (33.3 mcg per unit). Pick the volume that puts your dose on a readable whole-unit mark.
Does adding more water weaken the peptide?
It lowers the concentration, not the total amount. The vial still holds the same mg. You simply draw more units to get the same dose.
Can I use sterile water instead of bacteriostatic water?
Only if the product instructions allow it. Sterile water has no preservative, so it is generally intended for single use. Bacteriostatic water contains 0.9% benzyl alcohol, which is why it is used for vials that will be punctured several times. Benzyl alcohol should not be used in newborns.
Why does my dose fall between two syringe marks?
Your concentration does not divide evenly into the syringe’s graduations. You can round to the nearest mark (the calculator shows the dose that mark delivers) or change the water volume so the dose lands exactly on a mark.
What is the difference between mg/mL and mcg per unit?
Both describe the same concentration. mg/mL tells you how much peptide is in each millilitre. mcg per unit tells you how much is in each U-100 syringe unit, which is 0.01 mL. To convert, multiply mg/mL by 10: 2.5 mg/mL equals 25 mcg per unit.
Should I shake the vial after adding water?
Most product instructions say to swirl or roll the vial gently instead of shaking, because vigorous shaking can create foam and may damage some peptides. Follow the instructions for your specific product.
How long is a reconstituted peptide good for?
Follow the manufacturer’s labelling. When a compounded multiple-dose vial has no product-specific stability data, USP Chapter <797> sets a default beyond-use date of 28 days after first puncture, stored as labelled.
Is this calculator a dosing recommendation?
No. It converts the numbers you enter into volume and units. It does not tell you what dose to take. Always follow your prescription label and your clinician.
Sources
We check every formula and figure on this page against primary sources. See our editorial policy for how we review content.
Reviewed by the PeptideConvert Editorial Team
Last reviewed . PeptideConvert tools do arithmetic only. They do not recommend a dose, and nothing on this site is medical advice. Always follow the label and your prescriber. Read the full medical disclaimer.