There's no direct conversion between grains and milligrams because they are units of different systems of measurement.

Here's the breakdown:

* Grains (gr): A unit of mass used primarily in the apothecary system, historically used for medicines.

* Milligrams (mg): A unit of mass in the metric system.

To convert between them, you need a conversion factor:

* 1 grain = 64.79891 milligrams (approximately)

Therefore, there are approximately 64.8 milligrams in one grain.