You need to specify what kind of "units" you're asking about! 0.5 ml could be equal to:

* 0.5 milliliters (the most straightforward answer)

* 500 microliters (since 1 ml = 1000 microliters)

* A specific number of units on a syringe (depending on the syringe's markings)

Please provide more context about what kind of units you're looking for.