Read elemental magnesium per serving, compare bottle costs, and distinguish the Daily Value from the supplemental upper limit.
On a U.S. Supplement Facts panel, the amount beside Magnesium is the elemental magnesium per labeled serving. If the panel says 200 mg from magnesium glycinate, that is already 200 mg of magnesium. Do not multiply it by a glycinate percentage again. Check the serving size before treating that number as the amount in one tablet.
The source name, such as magnesium bisglycinate, describes the ingredient supplying the mineral. Its full compound weight includes more than magnesium. U.S. labeling rules distinguish the nutrient from its source and require a stated serving basis. See 21 CFR 101.36 and the FDA supplement-label guide.
Read the heading above the amounts. A serving may contain several capsules or tablets; a separate per-tablet column can also appear. A large number on the front of the package needs its accompanying description before you can compare it with the panel.
For a real example, the NOW Magnesium Glycinate Tablets page, checked October 31, 2026, lists:
That works out to 100 mg per tablet if the tablets contain equal amounts. Taking the panel's 200 mg and multiplying by two would count the serving twice. Applying a chemical percentage to the 200 mg would make the opposite error. This is a label-reading example, not an independent test of the product or a recommendation to take it. Check your own bottle because products and labels can change.
If the listing only says “magnesium glycinate 1,000 mg,” ask whether that means compound weight or elemental magnesium, and how many capsules make up that amount. Request a legible current Supplement Facts panel.
Do not fill the gap with a generic conversion percentage. Without the product's composition and reporting basis, that calculation cannot establish what the finished serving declares or contains. Likewise, the weight of an entire multi-ingredient blend does not tell you the magnesium contribution.
A useful manufacturer question is: “How many milligrams of elemental magnesium are declared per serving, what is the serving size, and which magnesium sources supply it?” If those answers remain unclear, the product cannot support a reliable quantity comparison.
For price comparison, calculate the total declared elemental amount in the bottle:
Servings per bottle × elemental milligrams per serving.
Then calculate:
Bottle price ÷ total elemental milligrams × 100 = cost per 100 mg.
Consider two hypothetical products, with shipping and discounts excluded:
Bottle B looks cheaper per serving because its serving contains less magnesium. Bottle A costs less per declared 100 mg. Neither calculation establishes better absorption, tolerability, clinical benefit or verified contents. The 100 mg comparison unit is arithmetic, not a suggested dose. Capsule burden and the other ingredients may also affect your choice.
The FDA's magnesium Daily Value is 420 mg. It is a labeling reference. A percentage on the panel does not tell you how much additional magnesium you personally need.
The NIH magnesium fact sheet gives a different figure: the adult tolerable upper intake level is 350 mg per day from supplements and medications, excluding magnesium naturally present in food. The dietary requirement and this upper limit count different sources. The upper limit is not a target; treatment above it needs appropriate clinical supervision.
Add the elemental amounts from the products you actually use, including combination supplements and magnesium-containing medicines. A smaller amount in each of several products can still produce a large combined intake. High supplemental intakes can cause diarrhea, nausea and cramping; impaired kidney function increases toxicity risk. Magnesium also interacts with some medicines, so a pharmacist should check product-specific timing rather than applying one spacing rule to every drug.
Before comparing bottles, record the serving size, elemental magnesium per serving, servings per bottle and price. Ask the manufacturer about missing or conflicting units. Take the combined supplement and medicine list to your clinician or pharmacist before changing intake.
Label interpretation tells you what a product declares. Establishing what a particular product contains requires separate evidence. For another example of that distinction, read the melatonin label-accuracy guide; its findings about melatonin do not measure magnesium products.
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