Answer: While different magnesium compounds vary in the amount of magnesium they contain (for example, just 16.2% of magnesium citrate is actual magnesium -- the rest is citric acid, while 14.1% of magnesium glycinate is magnesium), manufacturers are required to list the amount of actual, or "elemental" magnesium on labels so you can compare the actual amount of magnesium provided by supplements, even if they contain different forms. However, be aware that ConsumerLab.com has found some magnesium supplements in the past which were not labeled properly.
Absorption and even certain side-effects can vary by form, as can cost. You can read more about this in the "What to Consider When Buying" section of the Magnesium Supplements Review. In addition, we identify products which contain a form of magnesium with good absorption, and which specific product was the most economical way to get this type of magnesium.