Free tool from The Complete Supplement Guide
Magnesium by form
How much magnesium is actually in magnesium citrate, oxide, glycinate, malate, threonate, chloride and the rest, a converter for the number on your bottle, and what the human trials say about which forms absorb.
Built by a private chef and strength coach, from molecular weights and the published trials.
Built October 2026 · Sources listed on the page
Type either number. The other side is calculated from the molecular weight of the form you pick. Daily need: 400 to 420 mg for men, 310 to 320 mg for women, food included. The upper limit for supplements alone is 350 mg a day, set by the point where loose stools begin.
Pick a form and enter a number.
How this is calculated
Elemental share = atomic weight of magnesium divided by the molecular weight of the compound, with the water of hydration included where the common commercial form is a hydrate. This is chemistry, not a measurement of absorption: it tells you how much magnesium is in the capsule, not how much reaches your blood. Absorption is covered further down the page.
Reading the label
The front of the bottle and the back disagree
Magnesium is the label people misread most, because the gap between the compound and the mineral is so large. "Magnesium glycinate 1,000 mg" is about 140 mg of magnesium. "Magnesium citrate 1,000 mg" is 113 to 162 mg depending on which citrate. "Magnesium oxide 500 mg" is about 300 mg of magnesium, which sounds like the best deal until you read the absorption section below.
The Supplement Facts panel on the back is required to give the elemental figure, written as Magnesium (as magnesium glycinate) 140 mg. When the front and the back disagree, the back is right. When a product lists only the compound weight, the calculator above gives you the magnesium.
Reference
Elemental magnesium in each form
Calculated from molecular weight, with water of hydration included where that is the form sold. The third column is how much of the compound you need to get 300 mg of magnesium, a common supplemental dose.
| Form | Magnesium by weight | Compound for 300 mg of magnesium | Notes |
|---|---|---|---|
| Magnesium oxide MgO | 60% | 497 mg | Highest magnesium content, lowest measured absorption. The usual form in cheap tablets and in laxatives. |
| Magnesium hydroxide Mg(OH)2 | 42% | 720 mg | Milk of magnesia. Antacid and laxative, not a repletion form. |
| Magnesium carbonate MgCO3 | 29% | 1,041 mg | Antacid; converts to chloride in stomach acid. |
| Magnesium chloride (anhydrous) MgCl2 | 26% | 1,175 mg | Rarely sold as such; most products are the hexahydrate below. |
| Magnesium chloride (hexahydrate) MgCl2·6H2O | 12.0% | 2,509 mg | Well absorbed in the Firoz 2001 comparison; the hexahydrate is what sells. |
| Magnesium citrate (trimagnesium dicitrate) Mg3(C6H5O7)2 | 16.2% | 1,856 mg | The citrate in most capsules. Best-studied organic form. |
| Magnesium citrate (1:1, magnesium hydrogen citrate) MgHC6H5O7 | 11.3% | 2,646 mg | The citrate in most powders and liquid laxatives. Lower magnesium share. |
| Magnesium malate MgC4H4O5 | 15.5% | 1,930 mg | Often sold for energy and muscle claims; absorption data is thin. |
| Magnesium bisglycinate (glycinate) Mg(C2H4NO2)2 | 14.1% | 2,128 mg | Chelated to glycine; gentle on the bowel. One human trial, in bowel-resection patients. |
| Magnesium lactate Mg(C3H5O3)2 | 12.0% | 2,499 mg | Equivalent to chloride and aspartate in Firoz 2001. |
| Magnesium aspartate Mg(C4H6NO4)2 | 8.4% | 3,561 mg | Equivalent to chloride and lactate in Firoz 2001. |
| Magnesium taurate Mg(C2H6NO3S)2 | 8.9% | 3,365 mg | Sold for blood pressure and heart claims; no human absorption trial found. |
| Magnesium L-threonate Mg(C4H7O5)2 | 8.3% | 3,635 mg | Sold for brain claims; the lowest magnesium share on this list and the highest price per milligram. |
| Magnesium sulfate (Epsom salt) MgSO4·7H2O | 9.9% | 3,042 mg | A laxative and bath salt, not a supplement form. |
| Magnesium orotate Mg(C5H3N2O4)2 | 7.3% | 4,129 mg | Sold for heart claims; expensive per milligram of magnesium. |
Magnesium 24.305; molecular weights from the formulas shown. Hydrated versions of malate and citrate carry a few percentage points less magnesium than the anhydrous figures here; the L-threonate ingredient sold as the monohydrate runs about 7.5 to 8%.
The evidence
What the human trials actually show
Oxide dissolves badly and absorbs badly. In 1990 a Dallas group tested magnesium oxide and magnesium citrate in solutions mimicking the stomach, from no acid to peak acid. Oxide was almost insoluble in water and only 43% soluble at peak acid; citrate was 55% soluble in plain water. In volunteers, the rise in urinary magnesium after a 25 mmol dose was many times higher after citrate than after oxide (Lindberg et al., J Am Coll Nutr 1990;9:48-55). A 2001 comparison of four US products put fractional absorption from magnesium oxide at about 4%, against significantly higher and equal absorption from magnesium chloride, lactate and aspartate (Firoz and Graber, Magnes Res 2001;14:257-62). Oxide is the form in most cheap tablets, and it is also what milk of magnesia and many laxatives rely on, for exactly this reason.
Citrate is the best-studied organic form. The longest trial compared citrate, an amino-acid chelate and oxide at 300 mg of magnesium a day for 60 days in 46 adults. The two organic forms were absorbed better than oxide by 24-hour urinary magnesium, citrate produced the highest serum magnesium after both the first dose and 60 days, and oxide was no different from placebo on the urinary measure (Walker et al., Magnes Res 2003;16:183-91). A 2017 single-dose crossover found higher urinary excretion and serum magnesium from citrate than from oxide at matched elemental doses, with no difference in the magnesium inside blood cells (Kappeler et al., BMC Nutr 2017;3:7).
Glycinate has one human trial, in a hard population. Twelve patients who had part of the small intestine removed took 100 mg of isotope-labeled magnesium as diglycinate or as oxide. Absorption was low and about equal for the group, 23.5% against 22.8%, but in the four patients who absorbed oxide worst, the glycinate did twice as well, 23.5% against 11.8%, peaked earlier, and was better tolerated by everyone (Schuette et al., JPEN 1994;18:430-5). That is the whole direct evidence for glycinate's absorption advantage. Its real, repeatable advantage is in the bowel: it is the form least likely to cause loose stools at a useful dose.
Threonate, taurate, malate and orotate are sold on claims, not on absorption data. Magnesium L-threonate is marketed for memory on the strength of a 2010 rat study and a few small human trials, most funded by the patent holder; no trial has shown it delivers more magnesium to the body than citrate, and it carries the least magnesium per gram on this list at the highest price. Taurate is sold for blood pressure, malate for energy and muscle pain, orotate for the heart; none has a human absorption comparison against citrate. They are not bad forms. They are expensive ways to buy magnesium plus a second molecule whose own evidence has to stand separately.
What this means in practice. Citrate and glycinate are the two forms worth buying for repletion. Choose citrate if you also want a mild laxative effect, glycinate if you do not. Chloride and lactate are fine and cheap. Treat oxide as a laxative that happens to contain magnesium. Split doses above 200 mg across the day, because absorption falls as the single dose rises, and remember that the 350 mg supplement limit exists because of diarrhea, not toxicity; people with normal kidneys excrete the surplus.
Worth knowing
Food first, and the laxative threshold
Pumpkin seeds carry about 150 mg of magnesium an ounce, cooked spinach about 160 mg a cup, black beans 120 mg a cup, almonds 80 mg an ounce, dark chocolate 65 mg an ounce. A person eating those regularly rarely needs a capsule at all, and food magnesium does not cause loose stools. See how much food matches your dose →
Anyone with reduced kidney function should not take magnesium supplements without medical supervision, because the kidney is the only exit. Magnesium also binds some antibiotics (tetracyclines, quinolones) and bisphosphonates in the gut; separate them by at least two hours. The full dosing, timing and stacking notes are in the magnesium entry.
Questions
Common questions
How much elemental magnesium is in magnesium citrate?
It depends which citrate. Trimagnesium dicitrate, the citrate in most capsules, is about 16% magnesium, so 1,000 mg holds about 162 mg of magnesium. The 1:1 salt, magnesium hydrogen citrate, used in many powders and liquid laxatives, is about 11% magnesium, so 1,000 mg holds about 113 mg. US Supplement Facts panels state the elemental amount, so a label reading "Magnesium (as magnesium citrate) 200 mg" means 200 mg of magnesium regardless of which citrate it is.
Which magnesium form has the most elemental magnesium?
Magnesium oxide, at about 60% magnesium by weight, followed by magnesium hydroxide at 42%, carbonate at 29% and anhydrous chloride at 26%. The organic forms are much lower: citrate 11 to 16%, malate 16%, glycinate 14%, lactate 12%, taurate 9%, threonate 8%. The ranking by magnesium content is close to the reverse of the ranking by absorption, which is why a 500 mg oxide tablet can deliver less usable magnesium than a 200 mg citrate capsule.
Is magnesium glycinate or magnesium citrate better absorbed?
Both are well absorbed and no direct head-to-head trial in healthy adults separates them. Citrate has the most evidence: it beat oxide on solubility and urinary magnesium in 1990, produced the highest serum magnesium of three forms over 60 days in a 46-person trial in 2003, and beat oxide again in a 2017 crossover. Glycinate's one human trial, in patients with bowel resection, found it absorbed about as well as oxide overall but better in the patients who absorbed oxide worst, and it was better tolerated. The practical difference is the bowel: citrate loosens stools and glycinate usually does not.
Why does magnesium oxide absorb so poorly?
Because it barely dissolves. In the 1990 Dallas study, magnesium oxide was almost insoluble in water and only 43% soluble even in simulated peak stomach acid, while magnesium citrate was 55% soluble in plain water. A 2001 comparison of four US products measured fractional absorption of magnesium oxide at about 4%, against significantly higher and equal absorption from chloride, lactate and aspartate. Oxide works as a laxative for the same reason: most of it stays in the gut and pulls water in with it.
About the author
Who built this
Daniel Edlund is a Le Cordon Bleu-trained private chef in Los Angeles, a certified personal trainer and strength coach with thirty years of training behind him, and the author of The Complete Supplement Guide, which grades 147 entries against the clinical literature. He has no financial interest in any supplement brand named on this page, receives no commission on anything sold here, and the forms are ranked by the trials, not by what sells.
This page is information, not medical advice. Supplements interact with prescription medication and with each other, and the right answer depends on your own bloodwork and history. Talk to your doctor before starting anything, particularly if you are pregnant, nursing, managing a health condition, or already taking medication.
Go further
The guide behind the tool
Magnesium has its own entry in The Complete Supplement Guide: the dose the research supports, which form for which purpose, what it does for sleep and blood pressure, and what to avoid stacking it with. 177 pages, 147 entries, graded against the clinical evidence.
Get The Complete Supplement Guide →
The magnesium entry → · Zinc by form → · Food vs. Capsule → · All 147 entries, graded →