You can't directly convert between milliliters (ml) and milligrams (mg) because they measure different things:

* Milliliters (ml): Measure volume, or the amount of space a substance occupies.

* Milligrams (mg): Measure mass, or the amount of matter in a substance.

To convert between ml and mg, you need to know the density of the substance.

Density = Mass / Volume

Example:

* The density of water is approximately 1 gram per milliliter (g/ml).

* 1 gram is equal to 1000 milligrams.

* Therefore, 1 ml of water would weigh approximately 1000 mg.

So, there's no fixed number of milligrams in 1 ml. It depends on the substance's density.