Guide
Reconstitution math, explained
Three conversions stand between a vial of powder and a number on a syringe. None of them are hard, but each one has a classic way to go wrong. This page walks through the arithmetic. It is not guidance on what to take or how much.
What reconstitution changes
A lyophilised vial holds a known amount of compound as dry powder. Adding diluent turns it into a solution with a known concentration: the amount of compound in every millilitre. The amount in the vial does not change. Only how spread out it is.
concentration = amount in vial ÷ diluent volume (ml)
Add more diluent and the concentration drops. That is the only lever you have over how big each draw will be, which is why it matters later.
From a dose to a volume
Once you know the concentration, the volume that holds a given dose is a straight division.
volume (ml) = dose ÷ concentration
The units must match. If the vial amount is in milligrams, the dose must be in milligrams too. Mixing milligrams and micrograms here is the single most common error, and it is off by a factor of a thousand.
From a volume to syringe units
Insulin-style syringes are not marked in millilitres. They are marked in units, and the scale depends on the syringe.
- U-100
- 100 units per ml. Each unit is 0.01 ml. 10 units is 0.1 ml.
- U-40
- 40 units per ml. Each unit is 0.025 ml. 10 units is 0.25 ml.
- Other
- Some syringes use different scales. Check the barrel: it states how many units make up a millilitre.
syringe units = volume (ml) × units per ml
The same 0.3 ml is 30 units on a U-100 syringe and 12 units on a U-40. Draw 30 units on a U-40 and you have pulled two and a half times the intended volume. The syringe scale is not a detail.
A worked example
Say a vial contains 10 mg of powder and you add 3 ml of diluent.
- Concentration
- 10 mg ÷ 3 ml = 3.33 mg per ml
- Dose
- 1 mg
- Volume
- 1 mg ÷ 3.33 mg/ml = 0.3 ml
- U-100
- 0.3 ml × 100 = 30 units
- U-40
- 0.3 ml × 40 = 12 units
Try the same numbers in the calculator and change the diluent to see the draw move.
Why diluent volume matters
A draw of 0.03 ml is 3 units on a U-100 syringe. That is a hair's width on the barrel, and the reading error is a large fraction of the dose. Adding more diluent makes the same dose a bigger, more readable draw. The trade-off is that a vial then holds fewer doses per fill and takes longer to use up.
Solum flags draws by volume, purely on how measurable they are:
- 0.05 ml or less
- Too small to measure reliably.
- Under 0.10 ml
- Small. Readable, but the tolerance is tight.
- 0.10 to 0.50 ml
- Comfortable.
- 0.50 to 1.00 ml
- Large draw. Fine, but the barrel is filling up.
- Over 1.00 ml
- More than a 1 ml syringe holds.
Milligrams, micrograms and IU
1 mg is 1000 mcg. That conversion is exact and safe to do. IU (international units) is different: it is defined per substance by biological activity, and there is no general conversion between IU and milligrams. Solum treats the unit as a label you choose per compound. It never converts IU to mass, because there is no honest way to do it generically.
The mistakes that actually happen
- mg where mcg was meant, or the reverse. A thousandfold error that looks plausible on the syringe.
- Wrong syringe scale. Calculating for U-100 and drawing on U-40, or vice versa.
- Using the vial's nominal volume instead of the diluent you actually added. The concentration depends on what went in, not what the label says the vial holds.
- Topping up a vial and keeping the old concentration. Any extra diluent changes the number for every draw after it.
- Rounding early. Round the final units, not the concentration.
Solum does this once per vial. Enter the amount and diluent when you reconstitute, and every logged dose shows the draw for the syringe you use. Remaining volume ticks down with each log so you know when the vial will run out.