On the subject in the title: Bug report: the reconstitution calculator and a rounding edge case Working notes rather than a conclusion.
Posting the calculation with the inputs, so that somebody can disagree with a step rather than with the answer.
Inputs: 10 mg in 2 mL, target 0.25 mg, U-100 barrel. Working shown below in the order I did it.
I have checked it twice by the same route, which is not the same as checking it twice.
I had written a reply contradicting the opening post and deleted it. Here is what survived.
Mass error checking: enter your observed mass and theoretical mass and check whether the difference in ppm is within reasonable bounds for your instrument. Useful sanity check on LC-MS results.
The claim is narrower than it sounds, and deliberately so.
Precision and significant figures: the calculators do not round to match your input precision. If you input values to one decimal place, the output might have more. Use significant figures appropriate to your input precision.
Adding it because I spent an afternoon working it out and nobody should have to twice.
Rounding behaviour is where two tools disagree. One rounding to the nearest graduation and one to two decimal places will produce different advice from identical inputs.
Post #2 is right about the mechanism and I think understates the practical bit.
Dose equivalence calculator: compounds have different potencies. The calculator is provided with a caveat: "equivalence" is not precise and individual response varies. It is a rough starting point, not a dose prescription.
Adding it in case it saves somebody the afternoon it cost me.
Nothing to add, except that this is the answer I would give if asked.
The most common input error is entering the vial mass in the concentration field. It produces a plausible number, which is why it survives.
Unit-conversion tools are only as good as the assumption they make about the syringe. Ask what barrel and what graduation interval it assumed before trusting the units figure.
Post #7 answers the question as asked. The question underneath it is different.
Tools that run entirely in the browser send nothing anywhere, which matters more to some people here than others and is worth stating either way.
Not a strong opinion, just a consistent one.
Do the arithmetic by hand the first few times, whatever tool you use afterwards. A calculator with a wrong input gives a confident wrong answer and there is nothing in the interface to tell you.
That is the shape of it. The detail is where I would expect to be corrected.
Picking up post #11: that is the part I would want checked first.
The reconstitution calculation is total mass divided by total volume for concentration, then dose divided by concentration for volume, then volume times one hundred for units on a U-100 barrel. Three steps, and every error is in one of them.
This has been discussed before and I could not find the thread, so, again.
Understood. Thank you for being specific about the limits of it.
Significant figures in the output should not exceed what the inputs support. A concentration calculated from a nominal vial mass is not known to four decimal places.
Somebody will have a better source than mine, and I hope they post it.
The most common input error is entering the vial mass in the concentration field. It produces a plausible number, which is why it survives.
Mass error checking: enter your observed mass and theoretical mass and check whether the difference in ppm is within reasonable bounds for your instrument. Useful sanity check on LC-MS results.
Coming back to post #15, because the follow-up matters more than the original answer.
Why calculators are not medical advice: they do arithmetic. Medical decisions are not arithmetic. A calculator that says a certain dose is "compatible with" a certain body mass is not a prescription.
It is worth checking rather than assuming, which costs nothing.
Post #15 is right about the mechanism and I think understates the practical bit.
Unit-conversion tools are only as good as the assumption they make about the syringe. Ask what barrel and what graduation interval it assumed before trusting the units figure.
Reading it again, the caveat matters more than the finding.
Precision and significant figures: the calculators do not round to match your input precision. If you input values to one decimal place, the output might have more. Use significant figures appropriate to your input precision.
Scoping that to what I have actually seen rather than what I have read.
Narrowing post #20, because the general version has more than one answer.
A saved calculation is worth more than a remembered one. Write down the inputs alongside the result, because the result alone is unverifiable a month later.
It is worth stating the boring hypothesis before the interesting one.
Everything in post #18 holds. The case it does not cover is the one I have.
The most common input error is entering the vial mass in the concentration field. It produces a plausible number, which is why it survives.
Not disagreeing with anyone above, just adding the bit I keep having to look up.
Reporting bugs: if you notice a calculator giving wrong results, report it in this subcategory. Bugs are fixed when identified.
Post #24 answers the question as asked. The question underneath it is different.
A calculator that shows its working is worth ten that give an answer. The whole value is being able to see which step you disagree with.
I have kept the units in throughout, for the obvious reason.
Precision and significant figures: the calculators do not round to match your input precision. If you input values to one decimal place, the output might have more. Use significant figures appropriate to your input precision.
That is a description of practice, not a recommendation of it.
Tools that run entirely in the browser send nothing anywhere, which matters more to some people here than others and is worth stating either way.
It is one reading of the data and not the only reasonable one.
No disagreement from me. Posting only so the question does not look ignored.
Significant figures in the output should not exceed what the inputs support. A concentration calculated from a nominal vial mass is not known to four decimal places.
Nothing a calculator produces is medical advice, and a correct calculation on inappropriate inputs is still an inappropriate answer.