The formula
A retatrutide dose in units comes from three numbers: the vial’s total mg, the volume of bacteriostatic water used to reconstitute it, and the target mg dose. Concentration (mg per mL) is vial mg divided by water mL. Units per dose is the target mg divided by that concentration, multiplied by 100 (since a standard insulin syringe reads 100 units per mL).
Worked example
A 10mg vial reconstituted with 2mL gives a concentration of 5mg/mL. To draw a 2mg dose: 2 ÷ 5 × 100 = 40 units. To draw a 4mg dose from the same vial: 4 ÷ 5 × 100 = 80 units.
Where people make mistakes
The most common error is using a chart built for a different water volume. If the same 10mg vial were mixed with 1mL instead of 2mL, the concentration doubles to 10mg/mL, and the exact same 2mg dose would only need 20 units, not 40. Always confirm the water volume that was actually used before applying any chart.
Rounding
Insulin syringes are usually marked in single-unit increments. A calculated dose that lands on a fraction, like 42.5 units, typically gets rounded to the nearest marked line. A small, consistent rounding choice matters less than picking a water volume that gives round numbers in the first place, which is why many people choose a reconstitution ratio deliberately to make the maths easier.
Double-checking before drawing up
Recalculate from the vial label every time a new vial is opened, rather than reusing a number from memory. A vial’s actual mg content, confirmed by its COA, is the only number this calculation should ever start from.