To compare magnesium supplements, look for the elemental magnesium figure on the label, not the compound weight on the front. “500 mg magnesium oxide” contains about 300 mg of actual magnesium, while “500 mg magnesium glycinate” contains about 70 mg. Once you know the elemental amount per serve, the form and the price, any two products become comparable.
This guide covers the shelf: what each form is, how much magnesium each one really contains, what the absorption research can and cannot tell you, the reference numbers that put a dose in context, side effects and medicine interactions to check, and a step-by-step way to compare bottles. It does not cover magnesium in food, which is in our guide to getting more magnesium from your everyday diet, or the sleep research on one form, which is in magnesium glycinate and sleep.
Where this article links to a product, that link may be an affiliate link and this site may earn a commission at no extra cost to you. See our Affiliate Disclosure.
At a glance
- Magnesium is always sold bound to something else, such as oxygen, citric acid or glycine. That partner adds weight.
- Elemental magnesium is the number to compare. It ranges from about 60% of the compound for oxide to about 7% for orotate.
- More in the capsule is not more absorbed. The NIH notes that well-dissolving forms such as citrate, lactate, aspartate and chloride are absorbed more completely than oxide and sulfate.
- Supplement upper level: 350 mg a day of magnesium from supplements for adults in the US and in Australia and New Zealand. Food magnesium does not count towards it.
- Check medicines first. Magnesium can interact with some antibiotics, osteoporosis medicines and others. Talk to your doctor or pharmacist.
Why “500 mg” can mean very different things
Pure magnesium is a reactive metal and cannot go into a capsule. What you buy is a magnesium salt or chelate: magnesium joined to a partner molecule. Joined to oxygen it is magnesium oxide; to citric acid, magnesium citrate; to two molecules of the amino acid glycine, magnesium glycinate (also called bisglycinate).
The partner molecule has weight. The heavier it is, the smaller the share of the capsule that is magnesium. So the big number on the front, often the weight of the whole compound, can be four to eight times the amount of magnesium inside.
Elemental magnesium by form
The figures below are calculated from each compound’s molecular formula using standard atomic weights: the weight of the magnesium divided by the weight of the whole compound. They are approximate and refer to the dry (anhydrous) form unless stated.
| Form on the label | Approx. elemental magnesium | 500 mg of compound gives about |
|---|---|---|
| Magnesium oxide | 60% | 300 mg |
| Magnesium carbonate | 29% | 145 mg |
| Magnesium chloride (anhydrous) | 25% | 125 mg |
| Magnesium sulfate (anhydrous) | 20% | 100 mg |
| Magnesium citrate (trimagnesium citrate) | 16% | 80 mg |
| Magnesium malate | 16% | 80 mg |
| Magnesium glycinate / bisglycinate | 14% | 70 mg |
| Magnesium lactate | 12% | 60 mg |
| Magnesium chloride (hexahydrate) | 12% | 60 mg |
| Magnesium sulfate heptahydrate (Epsom salts) | 10% | 50 mg |
| Magnesium taurate | 9% | 45 mg |
| Magnesium aspartate | 8% | 40 mg |
| Magnesium L-threonate | 8% | 40 mg |
| Magnesium orotate | 7% | 35 mg |
Check the maths yourself: magnesium oxide is MgO. Magnesium weighs 24.305 and oxygen 15.999, so the compound weighs 40.304. 24.305 ÷ 40.304 = 0.603, or about 60%.
Two cautions. First, water locked into the crystal (hydrated forms) adds weight, which is why magnesium chloride appears at both 25% and 12%. Labels do not always say which was used. Second, branded or patented complexes and buffered chelates can be mixtures, so their content cannot be calculated. For these, the declared elemental figure on the panel is the only figure you have.
The same principle applies to calcium and iron. If you have read our guide to calcium citrate vs carbonate, it works the same way here.
Where to find the elemental figure on the label
Wording differs by country, but the elemental amount is usually there if you know where to look.
| Region | Typical wording | What to read |
|---|---|---|
| Australia (AUST L listed medicines) | “Magnesium glycinate 500 mg, equiv. magnesium 70 mg” | The “equivalent to magnesium” figure |
| United States (Supplement Facts) | “Magnesium (as magnesium glycinate) 70 mg” | The amount beside “Magnesium”; the form is in brackets |
| EU and UK (food supplements) | “Magnesium 70 mg (19% NRV)” | The milligrams; the % uses a 375 mg reference value |
If a front label shows only a compound weight and the panel does not give elemental magnesium, either calculate it from the table above or choose a product that states it clearly. For general panel-reading skills, see how to read a supplement label.
More in the capsule is not more absorbed
The amount in the capsule and the amount your gut takes up are different numbers, and the forms rank differently on each. The NIH Office of Dietary Supplements fact sheet states that forms that dissolve well in liquid are absorbed more completely than less soluble ones, and that magnesium aspartate, citrate, lactate and chloride are absorbed more completely than magnesium oxide and sulfate.
So the form with the most magnesium per capsule, oxide, is among the less well absorbed. But a well-absorbed form with very little magnesium per capsule is not automatically better value either. Comparative absorption studies are generally small and short, and measure things like urine or blood magnesium, so precise “times better” claims on packaging go further than the evidence. Our guide to supplement bioavailability explains why these studies are hard to compare.
The practical approach: compare elemental milligrams per serve, form and cost together, rather than chasing a single “best” form.
The forms you will see, compared
| Form | Elemental per weight | What the label and research suggest | Watch for |
|---|---|---|---|
| Oxide | High (60%) | Cheapest per mg; less soluble; often used as a laxative or antacid | More likely to cause loose stools |
| Citrate | Medium (16%) | Dissolves well; widely available and usually good value | Can loosen stools at higher amounts |
| Glycinate / bisglycinate | Low (14%) | Often marketed as gentler; large capsules or 2-3 per serve | “Buffered” versions may include oxide; check the panel |
| Malate | Medium (16%) | Similar magnesium content to citrate | Marketing claims run ahead of evidence |
| Chloride | Medium (12-25%) | Dissolves well; also sold as topical “magnesium oil” and flakes | Skin absorption is a separate, thinly researched question |
| Sulfate (Epsom salts) | Low-medium (10-20%) | Mainly sold as a bath product | Not a like-for-like swap for oral products |
| Taurate, orotate, L-threonate | Very low (7-9%) | Premium price per mg; early and limited human research | The capsule is mostly the partner molecule |
| Blends or “complex” | Varies | Several forms in one product | A proprietary blend may not show the split |
The reference numbers, and why the upper level looks lower
This confuses almost everyone. According to the NIH fact sheet, the US Recommended Dietary Allowance for adults is 310 to 420 mg of magnesium a day, depending on age and sex. Australia and New Zealand’s NHMRC Nutrient Reference Values set similar figures. That target covers magnesium from all sources, mainly food.
The upper level for magnesium from supplements and medicines is 350 mg a day for adults in the US, and also in Australia and New Zealand. In the UK, the NHS says taking 400 mg or less a day from supplements is unlikely to cause harm.
The upper level is lower than the recommended intake because they measure different things. The recommended intake counts everything you eat and swallow; the supplement upper level counts only concentrated doses, which affect the gut differently. A single high-strength serve of some products can reach or pass the supplement upper level on its own, and the packaging does not have to point that out. If a product is close to or above that figure, talk to your doctor or pharmacist first.
Side effects and medicine interactions to check
The most common effects of too much supplemental magnesium are diarrhoea, nausea and stomach cramps. The NIH fact sheet names magnesium carbonate, chloride, gluconate and oxide as the forms most often linked with diarrhoea. People with reduced kidney function are at greater risk of magnesium building up in the blood, so they should only use magnesium supplements under medical advice.
Documented interactions include:
- Bisphosphonates (osteoporosis medicines): magnesium can reduce their absorption, so doses are usually separated by at least two hours.
- Tetracycline and quinolone antibiotics: magnesium binds them in the gut, so timing needs to be separated as directed by a pharmacist.
- Diuretics: loop and thiazide types can increase magnesium loss; potassium-sparing types can reduce it.
- Proton pump inhibitors: long-term use has been linked with low blood magnesium.
This list is not complete. Take the bottle to your pharmacist before you start; it is a quick check. Some electrolyte tablets also contain magnesium, so count those too; see our electrolyte tablets guide.
How to compare two magnesium bottles: step by step
- Find the elemental magnesium per serve on the panel (“equivalent to magnesium” or “Magnesium (as …)”).
- Check the serving size: one capsule, two or three.
- If only the compound weight is given, multiply by the percentage in the table above.
- Work out cost per 100 mg of elemental magnesium: price ÷ (servings × elemental mg per serve) × 100.
- Note the form, and whether it is a blend or “buffered” product.
- Read the other ingredients for anything you avoid.
- Add any magnesium from multivitamins, electrolytes or antacids you already take.
- Compare the total with the 350 mg supplement upper level.
- Look for independent testing; see what supplement seals mean.
- Talk to your doctor or pharmacist before buying if you take any medicines or have kidney disease.
Worked example: Bottle A is 120 tablets of magnesium citrate, each giving 100 mg elemental magnesium, one per serve. Bottle B is 120 capsules of glycinate at 60 mg elemental each, two per serve (120 mg). Bottle A gives 12,000 mg of magnesium in total, Bottle B 7,200 mg. If both cost the same, Bottle A is roughly 40% cheaper per milligram of magnesium. Whether the form difference matters to you is a separate question to weigh up, ideally with a pharmacist.
Common mistakes
- Comparing compound weights. “1,000 mg magnesium complex” may contain far less magnesium than “400 mg magnesium oxide”.
- Missing the serving size. The elemental figure may be for three capsules, not one.
- Believing precise absorption claims. “Up to 10 times better absorbed” is marketing; the comparative research does not support exact multipliers.
- Counting only one product. Multivitamins, electrolytes and antacids can all add magnesium.
- Assuming topical products equal oral ones. Sprays and bath flakes are a different category with thin evidence for skin absorption.
- Skipping the medicine check. Interactions with antibiotics and osteoporosis medicines are well documented.
For how magnesium fits alongside every other mineral, our supplements evidence and label guide brings the label skills together in one place.
Many magnesium products also contain vitamin B6, so check the label and read how much vitamin B6 is safe.
Related guide: Is Magnesium L-Threonate Worth It? The Smart Label Check.
Related guide: Is Magnesium Oil Worth It? How to Avoid Costly Mistakes.
Sources this guide draws on: NIH Office of Dietary Supplements, Magnesium Fact Sheet for Health Professionals; NHMRC Nutrient Reference Values for Australia and New Zealand; NHS guidance on magnesium; US Food and Drug Administration, Dietary Supplement Labeling Guide; EU Regulation 1169/2011, Annex XIII; elemental percentages calculated from standard molecular formulas and IUPAC atomic weights.
Handy kit for this guide
These are practical tools, not supplements or medicines. As an Amazon Associate, michelle.net.au earns from qualifying purchases, at no extra cost to you. Prices and availability are set by Amazon and may vary by country.
- Ezy Dose Weekly AM/PM Pill Organiser (X-Large)
Seven days with separate morning and evening compartments, so you can see at a glance what has already been taken.
View on Amazon - Vaydeer Metal 7-Day Travel Pill Case
Slim metal case with a built-in desiccant, for keeping a week of tablets dry in a bag or suitcase.
View on Amazon - Bambaw Insulated Stainless Steel Water Bottle (32 oz)
Plastic-free and keeps water cold all day, useful when a label says to take a product with a full glass of water.
View on Amazon
Frequently asked questions
What is elemental magnesium?
It is the weight of the magnesium itself, not the whole compound it is bound to. A 500 mg capsule of magnesium glycinate contains about 70 mg of elemental magnesium. It is the only fair figure for comparing products.
Which magnesium form has the most magnesium?
Magnesium oxide, at about 60% by weight. However, the NIH notes it is absorbed less completely than forms that dissolve well, such as citrate, and it is more likely to cause loose stools.
How much magnesium is in 500 mg of magnesium citrate?
About 80 mg, based on trimagnesium citrate being roughly 16% magnesium. Some products use other citrate forms or hydrates, so use the declared elemental figure on the panel if it is given.
Is magnesium glycinate better than citrate?
The label difference is clear: glycinate has less magnesium per capsule. Direct comparisons of absorption between the two are limited, and claims of superiority go beyond the evidence. Our guide to magnesium glycinate and sleep looks at what trials on that form actually tested.
What is the upper limit for magnesium supplements?
In the US and in Australia and New Zealand, it is 350 mg a day of magnesium from supplements for adults. Magnesium from food does not count. Talk to your doctor or pharmacist before going near that amount.
Can magnesium interact with my medicines?
Yes. It can affect some antibiotics and osteoporosis medicines, and some diuretics and acid-reducing medicines change magnesium levels. A pharmacist can check your list in a few minutes.
The front of a magnesium bottle tells you the compound; the panel tells you the magnesium. Compare elemental milligrams, form and cost, add up every source, and check with your doctor or pharmacist before you start.
This guide is general education about reading labels. It is not medical advice and does not recommend a dose. Talk to your doctor or pharmacist before starting a supplement, particularly if you take medicines or have kidney disease.