Because they are two different kinds of tools and only one of them is doing arithmetic.
A reconstitution calculator works out how many units to draw. That is one formula with no assumptions in it, so two of them should agree exactly. When they do not, it is almost always units, syringe type, rounding, or a vial size one of them assumed.
A "level in your body" calculator is a model. It has to assume a half-life, and for most compounds there is no reliable figure to assume. Two of those tools can disagree by a wide margin and both be working correctly.
So the first thing to establish is which kind you are comparing.
The reconstitution formula
One line. That is the whole thing.
units to draw = (dose in mg ÷ vial size in mg) × (volume added in mL × 100)
The last part assumes a U-100 syringe, where 100 units is 1 mL. That is the standard insulin syringe and the one most people have.
Worked example. A 10 mg vial, 2 mL of water added, 0.5 mg dose.
0.5 ÷ 10 = 0.05, so the dose is one twentieth of the vial. 2 mL × 100 = 200 units in the vial. 200 × 0.05 = 10 units.
| Step | Arithmetic | Result |
|---|---|---|
| Dose as a share of the vial | 0.5 mg ÷ 10 mg | 0.05 (one twentieth) |
| Total units in the vial | 2 mL × 100 | 200 units |
| Units to draw | 200 × 0.05 | 10 units |
| Doses per vial | 10 mg ÷ 0.5 mg | 20 doses |
Any calculator that does not give you 10 units for those inputs is either using different inputs or is not a reconstitution calculator.
Why two of them disagree
Four reasons. The first causes most of it.
Units. One tool wants your dose in milligrams and the other in micrograms, and the field label is small. A dose entered as 500 when the tool wanted 0.5 is out by a factor of a thousand. Our guide on mcg versus mg covers the conversion and the three ways the error happens.
Syringe type. The formula above assumes U-100. A U-40 syringe has 40 units per mL, so the same draw is a different number. Some calculators ask, some assume.
Rounding. 10.4 units is not drawable on a syringe marked in whole units. One tool rounds to 10, another shows 10.4, a third rounds up. None is wrong, and they do not match.
An assumed vial size. Some tools prefill a common vial size for the compound you picked. If you did not notice and your vial is a different size, everything downstream changes.
If two reconstitution calculators disagree, work the formula by hand once. It will show you which input the tools read differently.
The other kind of calculator
A model, not arithmetic. That distinction is the point of this page.
A tool that tells you the level in your body at any given moment has to assume three things: how much of the dose reaches the bloodstream, how fast it is cleared, and how doses stack when you give the next one before the last has gone.
The second of those is the half-life, and it is where these tools diverge. Change the assumed half-life by a day and the predicted level after a few weeks changes substantially, because the effect compounds with every dose.
Where the half-life numbers come from
No calculator tells you this, and it is the reason to distrust the output.
Our half-life index grades every figure by where it came from. More than half the compounds in the library have no numeric half-life recorded. 78 have a numeric figure recorded as human data, including 37 from an approved drug label, the strongest grade we record. 3 numeric figures rest on community practice, meaning forums and group chats rather than any study, and 9 are recorded for an animal species.1
So for most compounds, a tool that models your blood level is using a number that does not exist, or one that came from a rat, or one somebody posted.
Examples from our half-life index, which records the species, evidence grade and source for every figure.
| Compound | Figure | Where it comes from |
|---|---|---|
| Semaglutide | About 1 week | Ozempic label, section 12.3 |
| Tirzepatide | About 5 days | Mounjaro label, section 12.3 |
| Retatrutide | About 6 days | Trial protocol; recorded as unverified, source link not stored |
| BPC-157 | About 8 to 30 minutes in rats | Animal studies; recorded as unverified, no human figure published |
The index flags retatrutide and BPC-157 as unverified.1
Retatrutide is the compound in most of the threads that prompted this page, and its figure is the weakest of the three approved-adjacent ones. BPC-157 has no human figure at all, so any tool showing you a BPC-157 blood level is extrapolating from rats.
Two tools using 5 days and 7 days for the same compound will diverge steadily, and neither is lying to you. They are making different guesses about a number nobody has pinned down.
What our calculator does
Ours is the first kind. It does the arithmetic on the numbers you type in and it does not model anything.
In the words of the calculator's own page: “It turns three numbers you enter, the milligrams in the vial, the water you add, and the dose you want, into the concentration, the syringe draw in units, and the cost per dose. It does arithmetic. It does not recommend a dose.”
What that means in practice. It will tell you the units to draw, the concentration, how many doses are in the vial and what each one costs. It will not tell you the level in your bloodstream, how long until the drug is cleared, or what dose you should take.
If you want the half-life figure for a compound and the grade attached to it, that lives on the half-life page rather than inside the calculator, because the figure and its provenance belong together.
Blends are the exception that breaks the formula
The one case where the arithmetic itself changes, and the threads do not mention it.
A blend vial contains two or more compounds. A vial sold as 10 mg of a BPC-157 and TB-500 blend may hold 5 mg of each, or 10 mg of each for 20 mg total, and the listing often does not say which.
The formula still works, but the vial size you put into it has to be the amount of the compound you are dosing, not the number on the label. Put in the total and every answer downstream is wrong, usually by a factor of two.
Two calculators can disagree here because one asked you for the blend total and the other for the per-compound amount. Neither did anything wrong.
If your vial is a blend and the seller states only one number, you do not have the input the formula needs. That is a question for the seller rather than for a calculator.
When the answer is not a number you can draw
A practical problem the tools rarely flag, and the fix is upstream.
If the arithmetic gives you 3.2 units, no syringe marked in whole units can show that. Our insulin syringe guide explains the markings. The answer is not to round and hope, it is to change the amount of water you add, because that is the one input that moves the draw without changing the dose.
More water spreads the same drug through more volume, so the same dose becomes a larger number on the syringe. Less water does the reverse. Our guide on how much water to add works through choosing a volume that puts your dose on a readable mark.
This is also why two calculators can look like they disagree when they do not. One may suggest a water volume and another may take yours, and the units differ because the dilution differs rather than because the arithmetic does.
Checking your own answer
Three things to do before trusting any calculator, including ours.
Work the formula by hand once, with your own vial, the way the example above does. It takes a minute and it tells you whether the tool and you agree.
Check the four numbers that give away a wrong dose: under 2 units, over 50 units, fewer than three doses per vial, or a blend where you used the label total. Our guide on spotting a wrong dose calculation covers why each one is a signal.
And if the tool shows you a blood level, find out what half-life it assumed before you believe it. If it will not tell you, that is the answer.
Common questions
Why do two peptide calculators give different numbers?
If both are reconstitution calculators, the arithmetic is identical and the difference is in the inputs: milligrams against micrograms, U-100 against U-40 syringes, rounding, or a vial size one of them assumed. If one of them models the level in your body, it is a different kind of tool and will not match.
What is the reconstitution formula?
Units to draw equals your dose divided by the vial size, multiplied by the volume of water added in millilitres times 100. The times 100 assumes a U-100 insulin syringe.
Which peptide calculator is most accurate?
For reconstitution, accuracy is not the variable: every correct tool gives the same answer, so the question is which one reads your inputs the way you meant them. For blood level tools, accuracy depends on a half-life figure that for most compounds has never been measured in humans.
Why do blood concentration calculators disagree so much?
Because they assume different half-lives, and the difference compounds with every dose. Two tools using 5 days and 7 days for the same compound will drift apart steadily, and neither is doing anything wrong.
Can I trust a calculator that shows my peptide level over time?
Treat it as an illustration of a pattern rather than a measurement. More than half the compounds in our library have no numeric half-life recorded; only a minority have one from an approved drug label.
Does your calculator show blood levels?
No. It does arithmetic on the numbers you enter: units to draw, concentration, doses per vial and cost per dose. It does not model what happens after the injection.
My vial is a blend. Which number do I enter?
The amount of the compound you are dosing, not the total on the label. A 10 mg blend vial may hold 5 mg of each component or 10 mg of each. If the seller states only one figure, you do not have the input the formula needs, and that is a question for the seller.
My answer is 10.4 units. What do I draw?
That is a rounding question rather than a calculator question, and it is one reason two tools disagree. A syringe marked in whole units cannot show tenths, which is why the volume of water you add matters: it decides whether your dose lands on a number you can actually draw.
Sources
- Peptide Decoding half-life index, dated compound-library snapshot (2026-10-05): 245 compounds; 158 without a numeric half-life recorded, 78 with a numeric figure recorded as human data, and 9 with a numeric animal-species figure. Numeric figures by evidence grade: fda-label: 37; trial-protocol: 17; peer-reviewed-animal: 5; community-practice: 3; peer-reviewed-human: 20; discontinued: 3; peer-reviewed: 2. These counts include only usable numeric figures, not blank entries that carry an evidence label. The dated snapshot is retained separately from the live index.
Citing this page. Peptide Decoding. Why Two Peptide Calculators Give Different Answers. Half-life coverage from our compound library. https://peptidedecoding.com/guides/why-calculators-disagree