Peptide Calculator

This is the one thing worth getting straight before anything else. An insulin syringe is graduated in units, not in millilitres, and on the U-100 calibration used throughout the US, 100 units is 1 mL. A unit is therefore a hundredth of a millilitre.

Every reconstitution calculation runs in millilitres right up until the last step, and that last step is where it becomes a number you can read off a barrel. Treating a figure in millilitres as units, or units as millilitres, is a hundredfold error in one direction or the other. Theinsulin syringe guide covers the barrels and their markings.

concentration = vial (mg) ÷ diluent (mL)  ·  volume = dose ÷ concentration  ·  units = volume × 100

A 5 mg vial with 5 mL of diluent is 1 mg/mL, which is 1000 mcg/mL, which means every unit on a U-100 syringe carries 10 mcg. A 250 mcg dose is therefore 25 units, and the vial holds 20 of them.

Put 2 mL into the same vial instead and the strength is 2.5 mg/mL. The same 250 mcg dose is now 10 units, and the vial still holds 20 doses. That is the part that surprises people: the diluent changes the number on the syringe and changes nothing about how much peptide you get or how long the vial lasts.

Since the diluent volume does not change the dose, it is free to be chosen for readability, and that is what it should be chosen for. The figure worth optimising is how many micrograms one unit carries, because that is the finest adjustment the syringe can make.

A low figure in that table is fine control and a larger injection volume. A high one is the opposite, and at the top end it starts costing you doses you cannot draw accurately: at 100 mcg per unit, nothing between 200 and 300 mcg exists on a barrel marked in whole units.

The diluent volume calculator runs this backwards, from the number you want to draw to the volume that produces it, and how much water to add goes through the trade in more detail.

One arrangement is worth remembering because it removes the arithmetic entirely. Add as many millilitres of diluent as the vial holds milligrams, and the strength is exactly 1 mg/mL, which on a U-100 syringe makes every unit exactly 10 mcg.

A 5 mg vial in 5 mL, a 10 mg vial in 10 mL, a 2 mg vial in 2 mL: in all of them the units you draw are the dose in micrograms divided by ten. It is the one setup where the reading can be done in your head and checked without a calculator, which is a reasonable argument for preferring it when the vial is big enough to hold the volume.

A syringe's calibration, not its size, sets how many units it calls a millilitre. U-100 is the standard and almost certainly what you have. U-40 still exists in veterinary insulin and in some markets, and on it the same millilitre reads as 40 units instead of 100.

An identical 5 mg vial in 5 mL with a 250 mcg dose draws 25 on a U-100 barrel and 10 on a U-40 one. Same vial, same dose, same volume of liquid in the syringe, two different numbers printed next to it. Drawing to the U-100 figure on a U-40 barrel delivers two and a half times the dose.

How many units is 250 mcg?

It depends entirely on how much diluent went into the vial. A 5 mg vial reconstituted with 5 mL gives 10 mcg per unit, so 250 mcg is 25 units. The same vial with 2 mL gives 25 mcg per unit, so the same 250 mcg is 10 units. The dose has not changed; the concentration has.

Are units the same as millilitres?

No, and this is the error worth being careful about. On a standard U-100 insulin syringe, 100 units is 1 mL, so a unit is a hundredth of a millilitre. Reading a figure in millilitres as though it were units, or the reverse, is a hundredfold error.

How much bacteriostatic water should I add?

There is no single correct volume. More diluent makes each unit worth less, which means finer control and an easier number to read on the barrel. Less diluent means a smaller injection volume. The amount of peptide you get per dose is identical either way.

Does adding more water make the dose weaker?

It makes the solution weaker but not the dose. You draw a proportionally larger volume to deliver the same amount of peptide, so the number of doses in the vial is unchanged. Only the syringe reading changes.

What is the easiest vial and water combination to read?

Adding as many millilitres of diluent as the vial holds milligrams gives exactly 10 mcg per unit on a U-100 syringe. A 5 mg vial in 5 mL, or a 10 mg vial in 10 mL: in both, the unit mark is the dose in micrograms divided by ten.