The
milligram is a measure of weight or mass, whereas the
tablespoon is a measure of volume.
You
can convert milligrams to tablespoons, but
only for a particular substance, material, food item, etc...
Also, please note that a metric tablespoon differs in volume from a tablespoon used in the US.
For example, 1 US tablespoon of table salt weighs 18.25 grams, and 1 metric tablespoon of that same table salt weighs 18.51 grams.
Note: I've used this
volume to weight calculator for table salt to calculate how much a tablespoon of table salt weighs:
http://www.aqua-calc.com/calculate/food-volume-to-weight/substance/Salt-coma-and-blank-table