Magnesium Intake & Elemental Content Calculator

Understanding the magnesium intake calculator — concept and principle

Magnesium supplement labels can be confusing: "500 mg magnesium gluconate" does not mean 500 mg of magnesium. This calculator converts the weight of a magnesium compound into elemental magnesium, the amount your body actually counts, and shows the adult RDA and the upper limit that applies to supplements.

The default gluconate pill gives only about 28 mg of magnesium — check the elemental amount on the label.

How it is calculated

Elemental magnesium (mg) = compound weight (mg) × magnesium fraction of the form

The calculator uses these approximate fractions: oxide 0.60, citrate 0.12, glycinate 0.14, gluconate 0.055 and L-threonate 0.075. Adult RDA: men 400–420 mg, women 310–320 mg (the lower figure for ages 19–30). Supplement upper limit: 350 mg a day.

The principle

A magnesium salt is a magnesium ion bound to another molecule, and only the magnesium part counts toward your intake. Its share depends on how heavy the partner molecule is. Magnesium oxide (MgO) has a very small partner, so magnesium is about 24.3 ÷ 40.3 ≈ 60% of its weight. Gluconate and threonate are large organic molecules, so magnesium is only about 5.5% and 7.5% of the weight. Exact figures depend on the salt's form and water content. Elemental content is not the same as absorption: organic forms such as citrate are generally absorbed better than oxide. The 350 mg limit applies only to supplements and medicines, not to food, because its main effect, diarrhoea, comes from unabsorbed magnesium salts in the gut.

Worked example

Default inputs: male, 500 mg of compound, gluconate form (0.055).
Elemental magnesium = 500 × 0.055 = 27.5, shown as 28 mg.
Adult RDA for a man = 400–420 mg; supplement upper limit = 350 mg/day.

As L-threonate (0.075) the same 500 mg would give 37.5 mg.

Each point is the calculator re-run with only “Supplement amount (mg of compound)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Supplement amount (mg of compound)	Elemental magnesium (mg)
125	7
250	14
375	21
500	28
625	34
750	41
1,000	55

How to read the result

28 mg is less than a tenth of the RDA, so a supplement like this adds relatively little; most magnesium should come from food such as nuts, seeds, legumes, wholegrains and leafy greens. If your label already states "elemental magnesium" or "magnesium (as …)", that number is the elemental amount and needs no conversion. When the elemental dose from supplements exceeds 350 mg a day, loose stools and cramps become more likely.

Common mistakes

Converting a figure that is already elemental, which understates the dose.
Assuming the form with the highest percentage is always the best absorbed.
Applying the 350 mg limit to food magnesium, which has no upper limit for healthy people.

Keep in mind: This is general information, not a diagnosis. People with kidney disease should not take magnesium supplements without medical advice, because impaired kidneys may not clear excess magnesium.

Frequently asked questions

Which magnesium form is best?

Citrate, glycinate and similar organic forms are generally better absorbed than oxide; oxide packs more magnesium per tablet but much of it is not absorbed.

Can magnesium help with muscle cramps or sleep?

Evidence is mixed; correcting a real deficiency helps, but extra magnesium in people with normal levels has shown small or inconsistent effects.

Sources: NIH Office of Dietary Supplements. Magnesium: Fact Sheet for Health Professionals. · Institute of Medicine. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. National Academies Press, 1997.

Checked 8 Nov 2026. NIH ODS Magnesium fact sheet.

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