Magnesium oxide is 60.3% magnesium by weight and glycinate 11.8–14.1%, so 400 mg of magnesium takes 660 mg of oxide, one size 00 capsule, or 2,840–3,380 mg of glycinate, four to five. Oxide is the densest of the 15 salts in the site's chemistry table. Label counts come from the NIH Dietary Supplement Label Database (DSLD) snapshot of 2026-08-10; prices come from Amazon offers observed 2026-10-26 to 2026-11-02 and are compared only inside one salt.
Which to pick for what
At 400 mg of magnesium the gap is one size 00 capsule of oxide against 4–5 of glycinate, computed from the formulas MgO and Mg(C₂H₄NO₂)₂ at 0.8 g/mL.
- Fewest capsules. For 200 mg of magnesium: 330 mg of oxide, one capsule of size 00 or 000, against 1,420–1,690 mg of glycinate, 2–3 size 00 or 2 size 000. For 400 mg: 660 mg of oxide, still one capsule, against 2,840–3,380 mg of glycinate, 4–5 size 00 or 3–4 size 000. Counts assume salt alone at 0.8 g/mL, the top of the usual powder density: 730 mg per size 00, 1,100 mg per size 000.
- Lowest cost inside each salt. 30 days at 400 mg a day from the cheapest ranked oxide offer: $1.78. From the cheapest ranked glycinate offer: $7.99. At the median ranked offer, oxide: $3.52; glycinate: $22.67. Oxide carries 4.3–5.1 times more magnesium per gram than glycinate, so a cross-salt price sort favours it by construction. This site ranks price only inside each salt.
- What studies measured. Five studies in the site's PubMed-checked registry include both salts, and one more measured oxide alone:
- PMID 14596323 (2003): in a randomised, double-blind trial, 46 adults took 300 mg of magnesium a day as an amino-acid chelate, citrate or oxide. The chelate arm was a class of compounds, not one glycinate.
- PMID 7815675 (1995): magnesium diglycinate against oxide in patients after ileal resection, a group with poor absorption to begin with. Results do not transfer to people with an intact gut.
- PMID 39770988 (2024): 40 women and men took single doses of oxide, bisglycinate, citrate and a microencapsulated form; plasma magnesium was measured at 0, 1, 4 and 6 hours, with gut side effects.
- PMID 30761462 (2020): absorption of oxide, citrate, glycinate and malate was profiled across several doses.
- PMID 16548135 (2005): ten salts, both of these included, in 80 magnesium-depleted rats, traced with stable isotopes.
- PMID 42091979 (2026): 24 men aged 18–45 took one dose of 750 mg or 2,000 mg of oxide; serum magnesium was followed for 12 hours.
Side by side
For 400 mg of magnesium, glycinate takes 2,840–3,380 mg of compound and oxide 660 mg; label counts are from the DSLD snapshot of 2026-08-10 and prices were observed 2026-10-26 to 2026-11-02.
| Measure | Magnesium glycinate | Magnesium oxide |
|---|---|---|
| Magnesium by weight | 11.8–14.1% | 60.3% |
| Compound for 400 mg of magnesium | 2,840–3,380 mg | 660 mg |
| Size 00 capsules for 400 mg (salt alone) | 4–5 | 1 |
| Most magnesium one size 000 capsule can hold | 130–155 mg | 664 mg |
| Labels in NIH DSLD naming the salt | 126 | 124 |
| Median magnesium per serving on those labels | 238 mg | 321 mg |
| Ranked US price offers | 17 | 5 |
| Lowest price per 100 mg of magnesium, within the salt | $0.0666 | $0.0148 |
Compound masses are rounded to the nearest 10 mg.
What to check on the label
In the DSLD snapshot of 2026-08-10, 73 of 126 glycinate labels name glycinate as the only magnesium source; the rest name another salt too or use "buffered", "complex" or "blend" wording.
- Oxide inside glycinate. "Buffered" glycinate means an alkaline magnesium salt, usually oxide, is mixed in, and the label rarely says how much.
- Per-capsule ceiling. One size 000 capsule holds at most 155 mg of magnesium as glycinate alone at 0.8 g/mL. A glycinate capsule declaring 200 mg either holds a denser salt as well or carries a figure that needs checking; the label audit lists such labels.
- Magnesium or compound? Supplement Facts should state the magnesium itself, with the salt in brackets. As compound weight, "Magnesium Oxide 500 mg" holds 302 mg of magnesium and "Magnesium Glycinate 400 mg" holds 47–56 mg.
- Big oxide doses. 500 mg of magnesium as oxide is 830 mg of compound: it fits a size 000 capsule, not a size 00.
Where to go next
The oxide ranking and the glycinate ranking order 5 and 17 ranked offers by price per 100 mg of magnesium, prices observed 2026-10-26 to 2026-11-02.