Enter what you know, read what you need
Content of the vial, volume of water, and the measure your protocol names. Everything updates as you type.
The tool converts between milligrams, millilitres and syringe units. It performs arithmetic on the numbers you enter; it does not suggest, endorse or validate any figure. Research use only.
The arithmetic, once, slowly
Suppose a vial contains 10 mg of peptide and you add 2 ml of bacteriostatic water. Concentration is content divided by volume: 10 divided by 2 is 5 mg per ml, or 5000 mcg per ml. That number belongs on the vial label the moment you finish reconstituting.
Now suppose a protocol names a 250 mcg measure. Volume is measure divided by concentration: 250 divided by 5000 is 0.05 ml. On a U-100 insulin syringe, where 1 ml spans 100 units, 0.05 ml reads as 5 units. That is the whole calculation.
Run it backwards to plan a vial: 10 mg is 10,000 mcg, so at 250 mcg per measure the vial holds 40 measures. If that number looks inconvenient, change the water, not the arithmetic, the same vial at 3 ml gives easier syringe readings at the cost of drawing slightly more volume.
A peptide calculator is an arithmetic aid and nothing more; the working above is short enough to do on paper. Running it in a browser simply means every figure recalculates as you change one of them. What it cannot do is check the assumptions underneath, and what a purity figure leaves unanswered is the largest of those.
The one number you actually choose
Concentration is the only quantity on this page that you choose. The milligrams are fixed by whatever the vial holds, the measure is fixed by the protocol, and the millilitres of diluent are the single variable under your control. Choosing them deliberately is what makes every reading afterwards either easy or awkward.
The sensible approach is to pick a volume that puts the figures you use most often on marks you can actually read. Small volumes push readings towards the bottom of a syringe scale, where a fraction of a unit becomes a large proportional error; large volumes read more easily but use up capacity. There is no correct answer, only a convenient one, and the pen and vial comparison explains why a pre-filled format removes the decision entirely.
Whatever you settle on has to be written down at the time. Concentration cannot be recovered from a vial afterwards: the solution gives no clue how much liquid went into it, and the label prints only what was there beforehand. A vial with an unrecorded volume has lost its arithmetic, and the storage page is where the rest of that record belongs.
Four mistakes the tool cannot catch
A calculator is only as good as what is typed into it.
What the numbers refer to
The vials this arithmetic is usually done for, and the water it is done with.
Around the calculation
The pages that supply the numbers this tool consumes.
About the tool
Asked often enough to answer here.