People search for this tool under a lot of names — peptide calc, pep calculator, pepcalc, peptide calculator online, free peptide calculator — and under all of them they are asking one of two questions. What does this thing actually compute? And can I trust the number it gives me? Both have short answers.
The whole calculation is three lines
A peptide arrives as a freeze-dried powder of a known mass. You add a known volume of liquid. Everything else falls out of that one division.
That is the entire mechanism, and it is why every honest calculator agrees with every other honest calculator to the decimal place. There is no proprietary model here and no algorithm worth branding. A 10 mg vial with 2 mL of water is 5 mg/mL; a 2 mg dose is 0.4 mL; 0.4 mL is 40 units. The same vial with 1 mL of water makes that same 2 mg dose read 20 units instead. The dose did not change. Only the mark on the barrel did.
Which also means the water volume is never a dosing decision. It is a readability decision, and the only thing it controls is where your dose lands on the syringe. The reconstitution guide walks through choosing it.
What no calculator can know
What is actually in your vial. The arithmetic takes the number printed on the label as true. For an approved product dispensed by a pharmacy that is a reasonable assumption. For a vial bought online from a seller who labels it “research use only”, the milligram figure is an unverified claim by someone with an interest in it, and no tool can audit it from your side of the screen.
Whether your dose is a dose. A calculator converts. It cannot tell you that a number is safe, effective or approved, and most compounds people bring to a tool like this have no approved human dose anywhere in the world.
What the powder displaced. Dissolved solids add volume. Measured displacement for lyophilised powders runs about 0.66 to 0.70 mL per gram of total solids, so 10 mg of solids displaces roughly 7 microlitres — under three quarters of one unit, finer than any barrel can be read. It only starts to matter when a vial carries a heavy bulking agent, and vendor vials almost never state total solids.
Six checks, about a minute
Run these against any calculator, including this one. Each has a right answer and none of them requires you to know anything about peptides. If you would rather see the list applied to specific named tools, the comparison pages do exactly that, on a stated date, and say what each one does better.
- Does it ask which syringe you are using? Every insulin syringe sold for human use is U-100: 100 units per millilitre, one unit per 0.01 mL. U-40 barrels exist, they are sold for veterinary insulin, and they are easy to end up with by accident. The same 0.4 mL reads 40 units on one and 16 on the other. A calculator that never asks has silently assumed U-100 — usually correctly, and catastrophically when not.
- Does it say where a dose figure came from? The arithmetic is the same for any substance in any vial, so nobody can get it wrong for long. Dose figures are where pages differ, and a “typical dose” with no label and no study behind it was written by somebody guessing. Look for a citation you can click.
- Does it distinguish approved drugs from unapproved compounds? These are not the same kind of claim. A dose from an FDA label is a fact about an approved product. A dose for a compound no regulator has approved is, at best, what one study gave to some rats. A tool that renders both as a tidy number in the same box has flattened the most important distinction on the page. The FDA has also said plainly that labelling a product “research use only” does not change its legal status when the seller's own marketing shows it is meant for people.
- Does it show its working? Concentration, draw volume, units — three numbers, and you should be able to see all three, not just the last one. If only the final unit count appears, you cannot tell whether the tool used your concentration or one it assumed.
- Does it handle international units honestly? An IU is defined separately for each substance against its own reference standard. Somatropin is 3.0 IU per mg because the WHO 2nd International Standard defines it that way; that factor means nothing for HCG, nothing for insulin, nothing for anything else. There is no general IU-to-mg conversion, and a calculator offering one is making it up.
- Can you reach the answer without an email address? The arithmetic costs nothing to give away, so when it is free but the “dosing chart” needs a sign-up, the chart is what is being sold. That chart is usually the part with no source behind it.
The one case where your own arithmetic is wrong
There is a real exception to “milligrams divided by millilitres”, and it is worth knowing because it is the one place a careful reader can out-think themselves. When a product has an FDA label, the labelled concentration governs, not the division you do in your head — because the vial contains a deliberate excess and the label accounts for it.
Tesamorelin is the clearest case: EGRIFTA WR is an 11.6 mg vial reconstituted with 1.3 mL, which is nominally 8.9 mg/mL, and the label declares 8 mg/mL. Somatropin does the same in the other direction: the Humatrope 12 mg cartridge is labelled 4.17 mg/mL after reconstitution, not the 4.00 mg/mL you get from dividing by the 3 mL of diluent in the accompanying syringe. Both are correct, and both are on the label precisely so that nobody has to guess. The FDA has its own guidance on allowable excess volume in injectable products, which is where the discrepancy comes from.
“Pepcalc” is a different tool entirely
A large share of people searching “pepcalc” are looking for PepCalc.com, which is run by Innovagen AB and has nothing to do with this site. We loaded it on 17 September 2026: you paste in an amino acid sequence, optionally specify terminal modifications and disulphide connectivity, and it returns molecular weight, isoelectric point, molar extinction coefficient, net charge at neutral pH, a titration plot and an estimate of water solubility. The underlying utilities were published in the Journal of Computer-Aided Molecular Design in 2016. It is free and it does not ask for an account.
What it does not do, anywhere on the page, is vials, bacteriostatic water, milligrams per millilitre, injection volume or syringe units. It is a bench tool for people synthesising peptides, not a dosing tool for people holding one. The two get conflated because the word “peptide calculator” fits both. If you want a molecular weight from a sequence, go there — this site does not compute that. If you want to know where to stop the plunger, that is what the tool here is for.
Free, simple, online — and what that should mean
The calculators on this site run in your browser, need no account, and put the result in the URL so you can bookmark or send it. Nothing is gated, because there is nothing here worth gating: the arithmetic is public property and the sourced dosing figures are more useful visible than hidden. Where a figure has no reliable source, the page says so instead of filling the gap, which is the one thing a calculator that wants your email address will never do.
Start from the general reconstitution calculator if you just have a vial and a dose, from a specific peptide page if you want the regulatory status and the evidence base alongside the maths, or from the blend calculator if your vial holds more than one peptide. The methodology page sets out how dosing figures get onto this site and what stops them.