1 mg equals how many grains?

1 grain (gr) = 64.79891 mg (milligrams) = 0.06479891 g (grams)

According to Wikipedia, this is the metric (SI) definition since 1958.

1 gram = 15.432 358 353 grain

In the US, in medicine, the use of grains is disappearing and seems to be somewhat approximate: A 60 mg dose may be described as "1 grain", but you should expect 1 grain to be about 65 mg, or 0.065 grams.

There are also two specialized units called the 'pearl grain', used for weighing pearls, and the 'metric grain' used for diamonds, and these are 1/4 carat or 50 mg.