Magnesium oxide is 60.3% magnesium by weight and citrate 11.3–16.2%, so 200 mg of magnesium takes 330 mg of oxide or 1,240–1,770 mg of citrate. Citrate's range runs from trimagnesium dicitrate (16.2%) down to dibasic and hydrated forms (11.3%), and few labels say which. 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, oxide fits one size 00 capsule (660 mg) and citrate needs 4–5 (2,480–3,540 mg), by the formulas MgO and Mg₃(C₆H₅O₇)₂ at 0.8 g/mL.
- Fewest capsules. For 200 mg of magnesium: one oxide capsule of size 00 or 000, against 2–3 size 00 or 2 size 000 of citrate. For 400 mg: still one oxide capsule, against 4–5 size 00 or 3–4 size 000 of citrate. 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 citrate offer: $4.13. At the median ranked offer, oxide: $3.52; citrate: $13.90. Oxide carries 3.7–5.3 times more magnesium per gram than citrate, so a cross-salt price sort favours it by construction; this site ranks price only inside each salt.
- What studies measured. Nine studies in the site's PubMed-checked registry include both salts, more than any other pair:
- PMID 2407766 (1990): 25 mmol of each salt in acid solutions from no stomach acid to peak secretion. In that beaker test oxide stayed virtually insoluble across the range and citrate dissolved; solubility is not absorption in a person.
- PMID 2299712 (1990): seven volunteers and four patients with recurrent calcium oxalate stones took 486 mg of magnesium a day for two weeks. On an empty stomach urinary magnesium rose by 77–79 mg a day, and the rise changed when the dose was taken with food.
- PMID 22433473 (2012): magnesium inside cells, read by X-ray dispersion, after citrate or oxide; a different read-out from blood magnesium.
- PMID 35041150 (2022): 61 young women took oxide, citrate or carbonate for 10 days; ionized and total serum magnesium were measured before and after.
- PMID 14596323 (2003) and PMID 39770988 (2024) compared both with chelates in 46 and 40 adults; PMID 30761462 (2020) profiled absorption across doses.
- Two rat studies, PMID 16548135 (2005) and PMID 29679349 (2019), included both salts.
Side by side
For 400 mg of magnesium, citrate takes 2,480–3,540 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 citrate | Magnesium oxide |
|---|---|---|
| Magnesium by weight | 11.3–16.2% | 60.3% |
| Compound for 400 mg of magnesium | 2,480–3,540 mg | 660 mg |
| Size 00 capsules for 400 mg (salt alone) | 4–5 | 1 |
| Most magnesium one size 000 capsule can hold | 124–178 mg | 664 mg |
| Labels in NIH DSLD naming the salt | 212 | 124 |
| Median magnesium per serving on those labels | 235 mg | 321 mg |
| Ranked US price offers | 12 | 5 |
| Lowest price per 100 mg of magnesium, within the salt | $0.0344 | $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, 135 of 212 citrate labels and 61 of 124 oxide labels name that salt as the only magnesium source.
- Both salts, one total. A label reading "Magnesium (as magnesium oxide, magnesium citrate)" with one amount does not say how much comes from each salt.
- Magnesium or compound? Supplement Facts should state the magnesium itself. As compound weight, "Magnesium Citrate 1,000 mg" holds 113–162 mg of magnesium and "Magnesium Oxide 500 mg" holds 302 mg.
- Per-capsule ceiling. One size 000 capsule holds at most 124–178 mg of magnesium as citrate alone and 664 mg as oxide at 0.8 g/mL. 500 mg of magnesium as oxide is 830 mg of compound, which fits a size 000 capsule but not a size 00. The label audit lists labels above their capsule.
- Which citrate. Trimagnesium dicitrate is 16.2% magnesium; dibasic and hydrated forms go as low as 11.3%.
Where to go next
The citrate ranking and the oxide ranking order 12 and 5 ranked offers by price per 100 mg of magnesium, prices observed 2026-10-26 to 2026-11-02.