There’s no single required amount
How much bacteriostatic water to add to a 5mg retatrutide vial isn’t fixed by the vial itself; it’s a choice that determines the resulting concentration. More water means a more dilute solution and larger unit draws for the same dose; less water means a more concentrated solution and smaller draws.
Three common ratios for a 5mg vial
Mixing with 1mL gives 5mg/mL. Mixing with 2mL gives 2.5mg/mL. Mixing with 2.5mL gives 2mg/mL. Each is a valid choice; the difference is purely how many units a given mg dose works out to on the syringe.
Choosing a practical ratio
A concentration that produces round unit numbers for your intended doses is generally easier to work with day to day. If your typical dose is 0.5mg, a 5mg/mL concentration (1mL water) gives a clean 10-unit draw, while 2mg/mL (2.5mL water) gives 25 units for the same 0.5mg.
Why some people prefer more diluted mixes
A more dilute solution means larger unit draws, which some people find easier to measure precisely on a standard 100-unit insulin syringe, since small unit counts leave less room for a measurement error to matter proportionally.
After choosing
Whatever ratio is used, write it clearly on the vial, since that number is what every future dose calculation for that vial depends on.