A milligram, abbreviated mg, is a metric unit of mass or weight defined as one thousandth of a gram. A milliequivalent is a measure of the quantity of ions in an electrolyte fluid. One milliequivalent is one thousandth of one mole of charges and is represented by the symbol mEq. The ions of different elements vary in mass, so it is necessary to know the atomic or molecular weight of the ions and their valence before a you calculate a conversion.
Establish the valence of the relevant ions by consulting a table of valence values. Multiply this value by the mass expressed in milligrams. For example, 20 mg of Al +++, that has a valence of three, produces a result of 60: 3 x 20 = 60.
Look up the atomic or molecular mass of the ions, and then divide it by the result from the previous step. The result is the milliequivalent value of the ions.
Aluminum, used in the previous example, is a pure element so establish its atomic mass. This is 27. The valence multiplied by the example mass is 60, so divide 27 by 60. The result, 0.45 is the milliequivalent value of the example mass.
Check the result for errors by reversing the calculations. Divide the mEq value by the atomic or molecular mass multiplied by the valence. If the result is not the original mass in mg, then there was an error in your calculations. Repeat them until the answer is correct.
TL;DR (Too Long; Didn't Read)
To convert milligrams to milliequivalents use the formula: mEq = (mg x valence) / atomic or molecular weight.
One thousand milliequivalents equals one equivalent.
In the U.S. electrolyte concentration is measured in mEq. However, Europe and the rest of the world use millimoles per liter or micromoles per liter.