How many milligrams in a millilitres?

Answer 1

None. Milligram is a unit of dry measure, and milliliter is a unit of liquid measure. That said, you could have a milliliter of fluid that contains (some number of) milligrams of a medication.

See the link for a metric equivalency table.

Edit:

You can do the conversion easily, if you have the density of the sustance.

Answer 2

It depends on the fluid being measured. One ml. of water weighs 1 000 mg. but 1 ml. of alcohol weight about 20% less.

The formula to use is:

1 ml. of liquid = 1000 mg. x Specific Gravity of the Liquid
milliliters measure volume and milligrams measure mass so you can't say how many milligrams (note spelling) are in a milliliter.

But you can ask how many milligrams is 10 milliliters of water / custard / coffee etc