Question:   How can I convert between kilograms and milliliters?

Answer:

Unfortunately, kilograms are units of mass and milliliters are units of volume, so you can't really convert between them.

However, when people talk about making conversions like this, they're usually talking about converting kilograms of water into mL, or vice-versa.  For pure water (and water only!), one gram is equal to one milliliter.  As a result, one liter of water weighs one kilogram.  The reason for this is that the density of water is one gram per cubic centimeter, and cubic centimeters are equivalent to milliliters.