Foundations
Dose-volume mathematics
The step-by-step arithmetic that turns a reported research amount into a syringe reading, and where the chain can silently fail.
- Last updated:
- 2026-07-22
Overview
The chain has three steps. First, compute concentration: vial mass in mg divided by diluent volume in mL. Second, compute volume: reported amount in mg divided by concentration in mg/mL. Third, translate volume to a syringe reading: volume in mL times 100 gives U-100 syringe units.
Every compound page's Preparation Calculator performs exactly this chain, in exactly this order.
Why it matters
This chain is where the risk of a wrong dose is highest. Because it links three units (mg, mL, units) across two multiplications and one division, a single error in the middle can compound into a dose that is off by a factor of ten.
Key concepts
Concentration first
Do the concentration calculation first, before you touch the volume calculation. If the concentration is right, the rest of the chain is straightforward.
Cross-check with the source
Every compound page cites the source (a peer-reviewed article or the current practitioner reference) that the calculator's example values come from. If your derived volume disagrees with the source's stated volume for the same amount and concentration, one of the inputs is wrong.
Sanity-check the syringe reading
Extremely small readings (under 2 units on a U-100 syringe) are hard to draw accurately. Extremely large readings (over about 40 units for a subcutaneous injection) may indicate the concentration is too dilute or the amount is out of range.
Common misconceptions
- The calculator's output does not validate the inputs. A believable number is not evidence that the inputs were correct.
- "It's just arithmetic" is true, but the arithmetic depends on units. Losing track of whether an input is in mg or mcg, or of whether a volume is in mL or units, is what makes the arithmetic go wrong.
- "I did it before" is not a check. Compounds vary in vial mass and diluent convention; re-do the arithmetic every time you open a new vial.
Practical interpretation
Do the calculation twice. Once mentally, before opening the calculator; once via the calculator itself. If the two disagree substantially, stop and find the discrepancy before proceeding.
Limitations
The arithmetic assumes the vial contains the mass on its label. If it does not, no amount of correct arithmetic will produce the intended amount of peptide.