Reconstitution Math, Without the Hand-Waving
How to work out what's actually in the syringe and how to check anyone else's numbers, including mine.
11 sections
Most reconstitution charts you'll find are just a table of numbers, with no indication as to how they were arrived at. This is backwards - the arithmetic is simple enough that anyone can do it, and once you can do it you don't need to take anyone else's word for it.
This page is arithmetic, and only arithmetic. It deliberately contains no doses, no suggestions, nothing about what to put in your body.
The whole thing in one line
Reconstitution is dilution, and dilution is division.
Concentration = amount in the vial ÷ volume of water added.
That's it.
A 10mg vial reconstituted with 2mL of bacteriostatic water is 10 ÷ 2 = 5mg/mL. There is no second step and no hidden factors.
Units are where people go wrong
There are two different "units" and they have nothing to do with each other.
Milligrams and micrograms are measures of mass. 1mg = 1,000mcg. This is the conversion that causes real trouble, because it's easy to make a typo or a visual mistake when reading numbers.
Syringe units are volume. On a standard U-100 insulin syringe, 100 units = 1mL. This means that 10 units = 0.1mL, always, regardless of what's in the syringe.
0.1 mL · 0 to 25 units
10 units is 0.1mL. The syringe measures volume - it has no idea what's in it. This is the whole point of the arithmetic.
The procedure, in five steps
Two divisions and a multiplication, in order.
Units = (target mass ÷ concentration) × 100
Vial to syringe units
Write down the amount in the vial, in milligrams.
Write down the volume of water you added, in millilitres.
The volume of the powder is considered negligible, and that's why nobody cares about it. Only the water counts.
Divide the first by the second. That is your concentration, in mg/mL.
Convert your target amount so it is in the same unit as the concentration.
Mass has to match mass before you divide, and 1mg = 1,000mcg.
Divide the target mass by the concentration for a volume in mL, then multiply that volume by 100 for syringe units.
The 100 comes from the syringe, not from what is in it: 100 units = 1mL.
The target amount in step four is yours. This page does not supply one.
A worked example
Say you have a 10mg vial reconstituted with 2mL, so 5mg/mL, and you want 250mcg.
| Step | This example |
|---|---|
| Amount in the vial | 10 mg |
| Water added | 2 mL |
| Concentration | 10 ÷ 2 = 5 mg/mL |
| Target, converted to mg | 250 mcg = 0.25 mg |
| Volume | 0.25 ÷ 5 = 0.05 mL |
| Syringe units | 0.05 × 100 = 5 units |
The step people forget is the conversion. Dividing 250 by 5 without converting to mg would give you 50 units - ten times too much, from a simple unit mistake.
Volume to syringe units
| Volume | Insulin syringe units |
|---|---|
| 0.05 mL | 5 units |
| 0.1 mL | 10 units |
| 0.2 mL | 20 units |
| 0.25 mL | 25 units |
| 0.5 mL | 50 units |
| 1.0 mL | 100 units |
Common vial and water combinations
Each row is simply (amount in vial ÷ water volume) × 0.1mL to get per-10-unit amounts. Check any of them for yourself.
| Vial | Water added | Concentration | Per 10 units (0.1 mL) |
|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL | 500 mcg |
| 5 mg | 2 mL | 2.5 mg/mL | 250 mcg |
| 10 mg | 1 mL | 10 mg/mL | 1,000 mcg (1 mg) |
| 10 mg | 2 mL | 5 mg/mL | 500 mcg |
| 50 mg | 2 mL | 25 mg/mL | 2.5 mg |
| 200 mg | 2 mL | 100 mg/mL | 10 mg |
| 500 mg | 5 mL | 100 mg/mL | 10 mg |
Note that rows 4 and 1 have the same amount per unit, just different starting vials. It's the concentration that determines your dose, not the vial.
Or let it do the division
The reconstitution calculator runs the same arithmetic and prints every step, so you can check it rather than trust it. It has presets for the common vial sizes and it will tell you when an amount lands between marks.
More water is not weaker, it is easier to measure
A common mistake is to think that adding more water makes the solution weaker. It doesn't - it makes it more dilute, but the amount of substance is the same. The same 10mg vial has 10mg of peptide regardless of how much water you add.
What you do by adding more water is make it easier to measure a small amount. With 1mL, a target of 250mcg is 2.5 units - a fraction you can't really measure on the syringe. With 3mL, the same target is 7.5 units, which is right between two marks. With 2mL it's 5 units, which is a whole number.
Dilute to make your target amount land on a mark you can actually measure. That's the only reason to pick one volume of water over another.
Storage
These are facts, not suggestions:
What to check
Reconstituted solution goes in the refrigerator, not the freezer.
Lyophilized (freeze-dried) powder is stable at refrigerator temperatures and generally tolerant of shipping at room temperature. Once reconstituted, solutions are stored in the refrigerator.
Read the label on the water: bacteriostatic and sterile are not the same thing.
Bacteriostatic water contains benzyl alcohol, which inhibits bacterial growth. Sterile water does not, and the two are not interchangeable once a vial is opened more than once.
Water goes down the side of the vial, not directly onto the powder.
Peptides are fragile in a mechanical sense - shaking or vigorous movement can damage them.
What this math doesn't tell you
The arithmetic is simple, but everything it's built on may not be.
It assumes that the vial actually contains what it's supposed to. This is an assumption about the manufacturer, not the chemistry, and it's the only factor that can make all of this math incorrect. A purity or fill-weight issue would make every calculation on this page wrong in a way no amount of careful division will fix.
It doesn't say anything about whether a particular amount is appropriate, safe, or effective for any reason. Those are entirely separate discussions, and the confidence with which a dosing chart presents numbers has a way of making people think that the chart answers those questions, too.
A number being calculable is not evidence that it's a good idea.
My take
I would rather you did the division by hand once before trusting the calculator, or me. A chart hands you a number with nothing attached to check it against, and this is one of the very few things in this field you can verify completely on your own, with no equipment and nobody's permission.
If you have used a vendor's chart to work out a volume, I want to know whether it showed you where its numbers came from, or only what they were.
Frequently asked
How do you calculate reconstitution concentration?
Concentration is the amount in the vial divided by the volume of water added. A 10mg vial reconstituted with 2mL of bacteriostatic water is 10 divided by 2, or 5mg/mL. There is no second step and no hidden factors, and the volume of the powder is treated as negligible.
How many units is 0.1mL on an insulin syringe?
On a standard U-100 insulin syringe, 100 units is 1mL, so 0.1mL is 10 units. That holds regardless of what is in the syringe, because the syringe measures a volume and has no idea what it contains.
Why did my calculation come out ten times too high?
Usually because the mass units were not matched before dividing. For a 5mg/mL solution and a target of 250mcg, the target has to become 0.25mg first. Dividing 250 by 5 instead of 0.25 by 5 returns 50 units rather than 5 units, ten times too much, from a simple unit mistake. The figures here are an example of the conversion, not a recommendation of an amount.
Does adding more water make a solution weaker?
It makes it more dilute, but the amount of substance is the same. A 10mg vial has 10mg of peptide in it regardless of how much water is added. What changes is where a target lands on the syringe: 250mcg is 2.5 units at 1mL, 5 units at 2mL, and 7.5 units at 3mL, between two marks. That figure is an example of the arithmetic, not a recommendation of an amount.
Is bacteriostatic water the same as sterile water?
No. Bacteriostatic water contains benzyl alcohol, which inhibits bacterial growth, and sterile water does not. The two are not interchangeable once a vial is opened more than once.
Can this arithmetic be wrong?
The division cannot, but what it rests on can. It assumes the vial actually contains what it is supposed to, which is an assumption about the manufacturer rather than the chemistry, and a purity or fill-weight issue would make every calculation wrong. It also says nothing about whether an amount is appropriate, safe or effective.
This page only discusses dilution arithmetic. It is not medical advice, and it is not a recommendation to use any particular substance. Nothing on this page has been evaluated by the FDA.
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