Injectable peptides arrive as a tiny amount of powder in a glass vial labeled in milligrams. To draw an accurate dose you have to convert that into something a syringe can read: units on a U-100 insulin syringe. A peptide dose calculator handles the arithmetic so you stop second-guessing whether you’re about to draw 5 units or 50. Below you’ll get the formula, two worked examples, the mistakes that wreck people’s math, and a calculator plus a dosing log you can use right on this page.
Important: this is educational information, not medical advice. Prescription GLP-1 drugs like semaglutide and tirzepatide are prescription-only and must be supervised by a licensed prescriber who chooses your dose and titration. Many of the compounds people run dose math on—BPC-157, TB-500, and others—are sold “for research use only” and are not FDA-approved for human use; their safety and effective doses in people are not established. This article explains the math, not a green light. Talk to a doctor or pharmacist before you start, change, or stop any dose, and never use a calculator output as a substitute for a clinician’s prescription. For the bigger picture, see are peptides safe.
The quick answer: the one formula you need
Everything comes down to concentration. Once you know how many milligrams sit in each milliliter of liquid, the rest is division.
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Explore new cuisinesConcentration (mg/mL) = peptide in vial (mg) ÷ bacteriostatic water added (mL)
Then, because a U-100 insulin syringe reads 100 units per 1 mL:
Units to draw = (desired dose in mg ÷ concentration in mg/mL) × 100
That’s the whole engine behind any peptide dose calculator. The vial’s mg number is fixed by the manufacturer; the mL of water you add is the variable you control, and it sets how “strong” each unit is. Add less water and each unit carries more peptide. Add more water and the same dose spreads across more units, which is easier to measure precisely.
How to calculate a peptide dose, step by step
Here’s how to calculate a peptide dose without any tool, so you understand what the calculator is doing:
- Read the vial. Say it’s a 5 mg vial.
- Pick your water volume. You decide to add 2 mL of bacteriostatic water.
- Find concentration. 5 mg ÷ 2 mL = 2.5 mg/mL.
- Convert your dose to mg. If you want 250 mcg, that’s 0.25 mg (1 mg = 1000 mcg).
- Divide and scale to units. (0.25 ÷ 2.5) × 100 = 10 units on a U-100 syringe.
Notice mcg-to-mg in step 4. Mixing those up by a factor of 1000 is the single most common error in peptide math, and it’s why a calculator that handles the conversion for you is worth using. If the reconstitution step itself is new to you, read how to reconstitute peptides first—the math only works if the powder is dissolved correctly and the water volume is what you think it is.
Try the peptide dose calculator
Plug in your vial size, your water volume, and your target dose, and it returns the units to draw.
Peptide Reconstitution Calculator
If the number it gives you looks absurd—like 0.4 units, which no insulin syringe can measure—that’s a signal to change your water volume, not to eyeball it. More on that below.
A peptide dose chart for common BPC-157 setups
People searching for a peptide dose chart usually want to see how water volume changes the units for a fixed dose. This table uses a 5 mg vial and a 250 mcg (0.25 mg) target dose, a frequently discussed BPC-157 amount, to show how concentration shifts the syringe reading.
| BAC water added | Concentration | Units for a 250 mcg dose | Doses per 5 mg vial |
|---|---|---|---|
| 1 mL | 5.0 mg/mL | 5 units | 20 |
| 2 mL | 2.5 mg/mL | 10 units | 20 |
| 3 mL | 1.67 mg/mL | 15 units | 20 |
| 5 mL | 1.0 mg/mL | 25 units | 20 |
Two things to read off this chart. First, the number of doses per vial never changes—5 mg ÷ 0.25 mg is always 20 doses, no matter how much water you add. Water only changes how spread out the measurement is. Second, drawing 5 units leaves almost no room for error, while 25 units is easy to hit precisely. For small doses, more water usually means a more accurate draw. A BPC-157 dose, like any “research use only” compound, has no established human dosing standard, so treat any number you see online as illustrative, not prescriptive.
Suggested read: Liraglutide vs Semaglutide: Daily vs Weekly GLP-1
Common mistakes that throw the math off
- Confusing mcg and mg. 250 mcg is 0.25 mg, not 250 mg. A 1000× slip turns a microdose into a wildly oversized one. Always work in one unit.
- Assuming the vial volume equals the water you added. The freeze-dried powder adds negligible volume, so if you inject 2 mL of water, you have roughly 2 mL of solution. But if you only managed to push in 1.8 mL, your concentration is higher than you calculated.
- Reading a U-40 syringe as if it were U-100. Most peptide work uses U-100 (100 units per mL). A U-40 syringe reads 40 units per mL, and using the wrong one silently multiplies your dose by 2.5.
- Foaming the peptide. Shooting water hard against the powder or shaking the vial can denature fragile peptides. Aim the stream at the glass wall and swirl gently. This doesn’t change the math, but it changes how much intact peptide you actually draw.
- Trusting a draw smaller than 2 units. Insulin syringes can’t reliably measure half a unit. If your math lands under ~2 units, add more water to spread the dose out.
Where prescription drugs differ from research peptides
The reconstitution math is identical for any lyophilized powder, but the regulatory and safety reality is not. This matters because the doses are not interchangeable, and the consequences of a math error scale with the drug.
| Prescription GLP-1 (semaglutide, tirzepatide) | Research peptides (BPC-157, TB-500) | |
|---|---|---|
| Approval | FDA-approved for obesity/diabetes | “Research use only,” not approved for human use |
| Who sets the dose | Licensed prescriber, with titration | No established human dose |
| Typical form | Pre-filled pen or pharmacy-compounded vial | Lyophilized vial you reconstitute |
| Evidence base | Large randomized trials | Mostly preclinical/animal |
For context on how strong the prescription side is: in the STEP 1 trial, weekly semaglutide produced about 14.9% mean body-weight loss over 68 weeks,1 and tirzepatide reached roughly 15% to 20.9% across doses in SURMOUNT-1 at 72 weeks.2 The newer triple agonist retatrutide hit up to about 24% at 48 weeks in a phase 2 trial.3 Those results come from supervised dosing in monitored trials—not from someone reconstituting a vial at home. If you’re curious about the very low end of GLP-1 dosing that some clinics use, see microdosing GLP-1, and for the compound itself, retatrutide. GLP-1 drugs also carry a known side-effect and contraindication profile that a calculator can’t flag for you.4
Suggested read: Tirzepatide Dosage Chart: Titration & Units Guide
A note on muscle, dose, and why supervision matters
Dose accuracy isn’t only about avoiding too much. Rapid weight loss on GLP-1 therapy can strip lean mass along with fat, and research points to protein intake and resistance training as the main ways to blunt that.5 Some experimental peptides are being studied specifically to preserve muscle during GLP-1 use.6 If muscle preservation is your goal, the lever is training and protein, not a clever injection schedule you worked out from a chart—see peptides for muscle growth for what the evidence does and doesn’t support. The broader category, including what peptides are and how they’re regulated, is covered in peptides.
Keep a dosing log
A calculator gives you a number once. A log tells you what you actually did over weeks—dates, doses, the vial, side effects, and how much solution is left. That record is what a prescriber or pharmacist can actually review, and it’s how you catch a drift in your routine before it becomes a problem.
Peptide & GLP-1 Dose Log
Log the real values, not your intended ones. If you meant to draw 10 units and drew 12, write 12.
Bottom line
A peptide dose calculator turns one formula—mg in the vial divided by mL of water, then scaled to U-100 units—into a number you can trust, as long as you feed it the right inputs and keep mcg and mg straight. The math is the easy part. The hard part is everything the math can’t tell you: whether a compound is approved, whether it’s right for you, what dose is safe, and how to titrate. Those answers come from a licensed prescriber or pharmacist, not a web page. Use the calculator to remove arithmetic errors, use the log to stay honest, and let a clinician own the dose itself.
Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med. 2021;384(11):989-1002. PubMed ↩︎
Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022;387(3):205-216. PubMed ↩︎
Jastreboff AM, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 Trial. N Engl J Med. 2023;389(6):514-526. PubMed ↩︎
Ghusn W, Hurtado MD. Glucagon-like Receptor-1 agonists for obesity: Weight loss outcomes, tolerability, side effects, and risks. Obes Pillars. 2024;12:100127. PubMed ↩︎
Neeland IJ, Linge J, Birkenfeld AL. Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies. Diabetes Obes Metab. 2024;26 Suppl 4:16-27. PubMed ↩︎
Nunn E, et al. Antibody blockade of activin type II receptors preserves skeletal muscle mass and enhances fat loss during GLP-1 receptor agonism. Mol Metab. 2024;80:101880. PubMed ↩︎
