Initial Posting: 7/3/2016    Last Update: 7/30/2020
Question: Help! How do I know how much magnesium I am actually getting from my supplement? The label says it contains 500 mg of magnesium "as magnesium citrate" -- but how much of that is magnesium and how much is citrate?
Answer: This is a good question, since magnesium can make up less than half the weight of compounds that include it, such as magnesium citrate, magnesium chloride, and magnesium glycinate.
The amount of actual magnesium (sometimes listed as "elemental" magnesium) that you're getting from a supplement should be indicated on the label, but that's not always the case, so it's important to notice how the claim regarding magnesium is worded. For example, if your supplement label states: "Magnesium (as magnesium citrate)....500 mg" it should contain 500 mg of actual magnesium. However, if the label says "Magnesium citrate 500 mg," this means the entire compound is 500 mg — of which only a small percentage is magnesium (in fact, it's only 11.2% in magnesium citrate, or 16.2% in trimagnesium dicitrate, which may also labeled as magnesium citrate).
Keep in mind that even when a supplement appears to be properly labeled, the only way to know for sure whether it contains what it claims is if it has been tested by an independent group like ConsumerLab.com. Not all products contain what they claim. You can find out which magnesium supplements do or don't contain what they claim in our Magnesium Supplements Review. The Review also includes our Top Picks among magnesium supplements we consider best, as well as additional information about how to read magnesium supplement labels, including an example of a misleading label from a product reviewed by ConsumerLab.com. In the Review, you'll also get objective information about what magnesium does, proper dosage, side effects, and how the forms of magnesium differ.
Note that this labeling rule applies to other essential minerals, such as calcium, iron and zinc. See the "ConsumerTips" section of our Reviews of supplements of each type for more about the different forms and what to look for on labels.