How much bacteriostatic water do I need for a 5 mg peptide vial?

Most 5 mg vials are reconstituted with 1 mL, 2 mL, or 3 mL of bacteriostatic water depending on the prescribed dose volume. Adding 2 mL to a 5 mg vial gives you 2.5 mg/mL, so a 250 mcg dose is 0.1 mL — 10 units on a U-100 syringe. The peptide calculator above lets you set vial size and water volume independently and shows the resulting concentration in real time.

How do you calculate peptide dose in units on an insulin syringe?

A U-100 insulin syringe is graduated so that 100 units equals 1 milliliter. To convert a dose volume in milliliters to units, multiply by 100. A 0.25 mL dose is 25 units; a 0.10 mL dose is 10 units. The visual syringe meter on this page draws the exact fill line for any input combination so you can match the meter to the syringe in your hand.

What is the typical BPC-157 dose?

BPC-157 is most commonly dosed at 250 micrograms (mcg) once or twice daily for soft-tissue and gut-health protocols, with some protocols ranging from 200 to 500 mcg per dose. From a 5 mg vial reconstituted with 3 mL of bacteriostatic water (1.67 mg/mL), a 250 mcg dose is 0.15 mL — 15 units on a U-100 syringe. Always confirm dosing with a licensed provider.

What is a typical Semaglutide starting dose?

Semaglutide titration usually starts at 0.25 mg weekly for four weeks, then steps up to 0.5 mg, 1.0 mg, 1.7 mg, and finally 2.4 mg weekly as tolerated. From a 5 mg vial reconstituted with 3 mL of bacteriostatic water (1.67 mg/mL), a 0.25 mg starting dose is 0.15 mL — 15 units. Use the calculator above to plan each step of your titration.

Why does my U-100 syringe show 25 units when my dose is 0.25 mg?

It is a coincidence of the mass-to-volume math at common concentrations, not a one-to-one rule. At 1.0 mg/mL concentration, 0.25 mg = 0.25 mL = 25 units. At 1.67 mg/mL (5 mg vial in 3 mL), 0.25 mg = 0.15 mL = 15 units. Always run the math through a calculator — the milligram number and the unit number only match at exactly 1.0 mg/mL.

Peptide Calculator

Free reconstitution & dose tool with a visual U-100 syringe meter

Calculate the exact dose volume for any peptide vial in seconds. Enter your vial strength, bacteriostatic water volume, and prescribed dose — the calculator returns the precise milliliter and U-100 unit reading to draw on your insulin syringe. Built by licensed providers at CULTR Health for clinicians, patients, and self-administered peptide protocols.

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How do you reconstitute a peptide?

Reconstituting a lyophilized peptide is the process of dissolving the freeze-dried powder into bacteriostatic water so you can draw an accurate dose. Five steps, about five minutes, with sterile technique throughout.

1
Prep a clean workspace
Wash your hands with soap for at least 20 seconds and dry them with a clean towel. Set out your peptide vial, bacteriostatic water vial, an alcohol swab, a U-100 insulin syringe, and a sharps container on a flat surface that has been wiped clean. Letting the vials reach room temperature first reduces the chance of foaming when the diluent meets the powder.
2
Disinfect both vial septums
Wipe the rubber stopper of the peptide vial and the bacteriostatic water vial with a fresh alcohol swab and let each air-dry for ten seconds. Skipping this step is the most common source of contamination during reconstitution.
3
Draw the bacteriostatic water
Pull air into your syringe equal to the volume of water you plan to draw, insert the needle into the bacteriostatic water vial, push the air in, then invert the vial and slowly draw the exact volume the calculator above tells you to add. Tap to clear bubbles and push them out before withdrawing the needle.
4
Inject the diluent slowly
Insert the needle into the peptide vial and aim the bevel toward the inside wall of the glass — not at the powder directly. Depress the plunger slowly so the bacteriostatic water runs down the side and gently dissolves the lyophilized peptide. A fast injection can foam or denature the protein.
5
Swirl, inspect, verify
Swirl the vial in slow circles for thirty to sixty seconds — do not shake. The solution should look clear and colorless within a minute or two. If you see particulates, cloudiness, or persistent foam, do not inject; consult your provider. Once dissolved, label the vial with the reconstitution date and store in the refrigerator at 36–46°F.

How the peptide dose math works

Every peptide reconstitution and dose calculation reduces to three formulas. Each one uses the same five variables, and the calculator above runs them in real time as you change inputs.

Variables

Variable	Unit	Definition
vial_mg	milligrams	Total peptide mass in the vial as printed on the pharmacy label.
water_ml	milliliters	Bacteriostatic water added to reconstitute the vial.
concentration_mg_per_ml	mg/mL	Peptide mass per milliliter of solution after reconstitution.
dose_mcg	micrograms	Prescribed dose. 1 mg = 1,000 mcg.
dose_ml	milliliters	Volume to draw on the syringe for one dose.
syringe_units	units (U-100)	Reading on the U-100 insulin syringe barrel. 100 units = 1 mL.

Formulas

// 1. Concentration after reconstitution
concentration_mg_per_ml = vial_mg / water_ml

// 2. Dose volume on the syringe
dose_ml = (dose_mcg / 1000) / concentration_mg_per_ml

// 3. U-100 insulin syringe units (100 units per 1 mL)
syringe_units = dose_ml * 100

Worked example: a 5 mg vial reconstituted with 3 mL of bacteriostatic water gives concentration_mg_per_ml = 5 / 3 = 1.67. A 250 mcg dose is dose_ml = (250 / 1000) / 1.67 = 0.15 mL, which reads syringe_units = 0.15 * 100 = 15 units on a U-100 insulin syringe.

Three peptide calculator modes

Reconstitution math has three common shapes depending on which value you already know and which one you need to solve for. The calculator above handles all three.

Mode 1 — Solve for dose volume

Use when: you know the vial strength, how much bacteriostatic water you added, and the dose your provider prescribed. Solve for the milliliters and U-100 units to draw on the syringe. This is the default mode of the calculator above and the most common day-to-day calculation.

// Inputs:  vial_mg, water_ml, dose_mcg
// Output:  dose_ml, syringe_units

concentration_mg_per_ml = vial_mg / water_ml
dose_ml                 = (dose_mcg / 1000) / concentration_mg_per_ml
syringe_units           = dose_ml * 100

Worked example: 5 mg vial, 3 mL water, 250 mcg dose → concentration is 1.67 mg/mL, dose volume is 0.15 mL, syringe reading is 15 units.

Mode 2 — Solve for bacteriostatic water volume

Use when: you know the vial strength and the concentration you want. Solve for how much bacteriostatic water to add. This is useful when you want a specific dose volume to land on a clean syringe line — for example, sizing a Tirzepatide vial so a 5 mg dose draws to exactly 0.5 mL.

// Inputs:  vial_mg, target_concentration_mg_per_ml
// Output:  water_ml

water_ml = vial_mg / target_concentration_mg_per_ml

Worked example: a 20 mg Tirzepatide vial at a target 10 mg/mL needs water_ml = 20 / 10 = 2 mL of bacteriostatic water. A 5 mg dose then draws to 0.5 mL — 50 units on a U-100 syringe.

Mode 3 — Solve for vial strength (back-calculate concentration)

Use when: you know the dose you need and the volume you want to draw, and you want to back-solve for the concentration the vial must be reconstituted to. This is useful when planning a protocol against a fixed syringe size — for example, sizing every dose to land below 0.3 mL on a 0.3 mL syringe.

// Inputs:  dose_mcg, target_dose_ml
// Output:  required_concentration_mg_per_ml

required_concentration_mg_per_ml = (dose_mcg / 1000) / target_dose_ml

// Then size the water volume against your vial:
water_ml = vial_mg / required_concentration_mg_per_ml

Worked example: you want a 250 mcg dose to draw to 0.10 mL (10 units) on a 0.3 mL syringe. Required concentration is (250 / 1000) / 0.10 = 2.5 mg/mL. For a 5 mg vial that means water_ml = 5 / 2.5 = 2 mL of bacteriostatic water.

What is a peptide calculator and why does dosing accuracy matter?

A peptide calculator translates the milligrams of compound in a freeze-dried vial into the exact volume to draw on an insulin syringe. The math is simple in principle — concentration equals mass divided by volume — but a single misplaced decimal turns a 250-microgram dose into a 2,500-microgram dose. Therapeutic peptides have narrow effective ranges; underdosing wastes compound and overdosing risks side effects. The visual syringe meter on this peptide calculator draws the fill line for you, so the number on the page matches the line on the barrel in your hand.

How much bacteriostatic water do I add to a peptide vial?

Bacteriostatic water volume is a choice, not a fixed answer. Adding more water lowers the concentration and forces a larger dose volume; adding less raises the concentration and shrinks the dose volume. Most patients pick the volume that puts a typical dose between 0.10 mL and 0.25 mL on a 0.5 mL or 1.0 mL U-100 syringe — easy to read, easy to draw. For a 5 mg vial dosed at 250 mcg, 2 mL of bacteriostatic water gives a clean 0.10 mL (10 unit) dose. The reconstitution calculator above shows the trade-off in real time as you adjust the water slider.

How do I read a U-100 insulin syringe?

U-100 means 100 units per 1 milliliter. The barrel is graduated in units, not milliliters, so a 0.25 mL dose reads as 25 units, and a 0.10 mL dose reads as 10 units. Common syringe sizes are 0.3 mL (max 30 units), 0.5 mL (max 50 units), and 1.0 mL (max 100 units). The syringe meter on this peptide calculator visually fills the chosen syringe size up to the dose line — match the line to the same line on the barrel as you draw, and you have the right volume.

What is the difference between mcg, mg, and units?

Milligrams (mg) and micrograms (mcg) are units of mass: 1 mg equals 1,000 mcg. Most peptide protocols are specified in either mg (Semaglutide 2.5 mg, Tirzepatide 5 mg) or mcg (BPC-157 250 mcg, Sermorelin 250 mcg). Insulin syringe units are units of volume: 100 units equal 1 mL. The calculator handles all three so you can enter your dose in the unit your protocol uses and see the equivalent in milliliters and syringe units instantly.

How is bacteriostatic water different from sterile water or saline?

Bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative. The benzyl alcohol suppresses bacterial growth across the multi-day life of a reconstituted peptide vial. Plain sterile water for injection has no preservative and is intended for single-use applications only. Saline contains sodium chloride and is also non-preserved. Always reconstitute multi-dose peptide vials with bacteriostatic water from a licensed pharmacy — never tap water, distilled water, or any non-sterile source.

Calculate dose for popular peptides

Pre-fill the peptide calculator with typical reconstitution and dosing values for the most commonly prescribed compounds. Each link opens the calculator with the preset loaded — adjust to match your prescription.

Frequently asked questions

Answers to the questions patients and clinicians ask most often about peptide reconstitution, dosing, and the math behind the syringe meter.

Milligrams (mg) and micrograms (mcg) are units of mass and convert directly: 1 mg equals 1,000 mcg. International Units (IU) are a measure of biological activity and only apply to certain peptides — for most research peptides, IU is not used. The calculator supports mg and mcg input and converts between them automatically; if your prescription is in IU, talk to your provider for the mass-equivalent.

Most reconstituted peptides remain stable for 28 to 60 days when refrigerated at 36–46°F (2–8°C) in their original sterile vial. Stability varies by compound — Semaglutide and Tirzepatide are typically stable for at least 28 days, while BPC-157 and TB-500 can be stable longer when kept cold. Always follow your pharmacy or provider beyond-use date and inspect for cloudiness or particulates before each injection.

No. Bacteriostatic water contains 0.9% benzyl alcohol that suppresses microbial growth across the multi-day life of the reconstituted vial. Sterile water without a preservative is intended for single-dose use only and will support bacterial contamination once punctured. Tap water is non-sterile and can introduce endotoxins. Always use bacteriostatic water from a licensed pharmacy for any multi-dose peptide vial.

Subcutaneous injection of properly compounded peptides under provider supervision is the most common administration route for self-injected protocols. Risks include injection-site reactions, infection if sterile technique is broken, and adverse responses to specific compounds. Peptides should only be sourced from licensed compounding pharmacies, dosed within prescriber guidance, and stored per pharmacy instructions. This calculator is an educational dosing tool and does not replace medical supervision.

In the United States, GLP-1 agonists like Semaglutide and Tirzepatide and most therapeutic peptides require a prescription from a licensed provider. Compounded peptides are dispensed by 503A or 503B compounding pharmacies. Research-grade peptides sold "for research use only" are not regulated for human use and should not be self-administered. CULTR Health connects you with licensed providers who can review your goals, prescribe when appropriate, and supervise your protocol.

CULTR Health offers telehealth consultations for eligible patients physically located in Florida for GLP-1 weight management, longevity peptides, and hormone optimization protocols. Take the assessment quiz to share your goals, lab history, and current medications, then book a video visit. Your provider reviews the calculator output you bring in and adjusts vial strength, water volume, and titration cadence based on your response.

References & further reading

Authoritative sources on compounding standards, peptide pharmacology, and safe injection practice.

Need a provider-built peptide protocol?

The calculator handles the math. A licensed CULTR Health provider handles the medicine — reviewing your goals, lab history, and current medications before prescribing a peptide protocol that fits you. Telehealth visits are available only to eligible patients physically located in Florida.

This peptide calculator is an educational tool. It does not replace medical advice, prescription, or supervision. Compounded peptides are dispensed only by licensed 503A and 503B pharmacies under a valid prescription.