ANSWER: This is a very common question and the answer is not a simple one. It depends on the person and what they are using it for. Are they dealing with an illness, an injury, or just looking for a daily magnesium supplement?
Using magnesium oil or bath salts as a daily magnesium supplement is the easiest to figure out but really you can determine how much you need by the changes you will see (better mood, better sleep, more energy, etc.) We recommend 1-2 sprays (1/2 of a teaspoon of balm) massaged into the tops of feet, behind knees, inside elbows and wrists, or 1-2 sprays on the chest area and the back of neck/shoulders every morning and 1-2 sprays (or 1/2 teaspoon of balm) on the tops or bottoms of your feet at night, 20 minutes before sleep.
As for how much of the bath salts you want to use, that will be determined by you. We recommend starting out with a ½ to 1 cup of salts each bath after your initial bath use, then more or less to get the results you desire.
Oral supplementation can be inefficient when compared to trans-dermal application (via the skin). Oral supplements which contain magnesium oxide mean absorption rates can be as low as 4%.
If the symptom persists then at least you will know that it is not a simple dropping of your Magnesium levels, so it might be time to visit the Doctor. Illnesses are the toughest to know how much magnesium oil or crystals to use and it is best to have a healthcare professional guide you in how much to use.
A suggested guide: